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The position of the macula in comparison to the blind spot

The position of the macula in comparison to the blind spot



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Which is located in a higher postion? Macula or blind spot?


The position of the macula in comparison to the blind spot - Biology

The physiological blind spot refers to a zone of functional blindness all normally sighted people have in each eye, due to an absence of photoreceptors where the optic nerve passes through the surface of the retina. Here we report that the functional size of the physiological blind spot can be shrunk through training to distinguish direction signals at the blind spot periphery. Training on twenty successive weekdays improved sensitivity to both direction and colour, suggesting a generalizable benefit. Training on one blind spot, however, did not transfer to the blind spot in the untrained eye, ruling out mediation via a generic practice effect nor could training benefits be attributed to eye movements, which were monitored to ensure stable fixation. These data suggest that training enhances the response gains of neurons with receptive fields that partially overlap, or abut, the physiological blind spot, thereby enhancing sensitivity to weak signals originating primarily from within the functionally-defined region of blindness 1, 2, 3. Our results have important implications for situations where localised blindness has been acquired through damage to components of the visual system 4, 5, and support proposals that these situations might be improved through perceptual training 5, 6, 7.


Wash your hands and don PPE if appropriate.

Introduce yourself to the patient including your name and role.

Confirm the patient’s name and date of birth.

Briefly explain what the examination will involve using patient-friendly language.

Gain consent to proceed with the examination.

Position the patient sitting on a chair.

Ask if the patient has any pain before proceeding.


What causes a macular pucker?

Most of the eye’s interior is filled with vitreous, a gel-like substance that fills about 80 percent of the eye and helps it maintain a round shape. The vitreous contains millions of fine fibers that are attached to the surface of the retina. As we age, the vitreous slowly shrinks and pulls away from the retinal surface. This is called a vitreous detachment, and is normal. In most cases, there are no adverse effects, except for a small increase in floaters, which are little “cobwebs” or specks that seem to float about in your field of vision.

However, sometimes when the vitreous pulls away from the retina, there is microscopic damage to the retina’s surface (Note: This is not a macular hole). When this happens, the retina begins a healing process to the damaged area and forms scar tissue, or an epiretinal membrane, on the surface of the retina. This scar tissue is firmly attached to the retina surface. When the scar tissue contracts, it causes the retina to wrinkle, or pucker, usually without any effect on central vision. However, if the scar tissue has formed over the macula, our sharp, central vision becomes blurred and distorted.


How will my eye doctor check for Stargardt disease?

An eye care professional can make a positive diagnosis of Stargardt disease by examining the retina. Lipofuscin deposits can be seen as yellowish flecks in the macula. The flecks are irregular in shape and usually extend outward from the macula in a ring-like pattern. The number, size, color, and appearance of these flecks are widely variable.

A standard eye chart and other tests may be used to assess symptoms of vision loss in Stargardt disease, including:

  • Visual field testing. Visual fields testing attempts to measure distribution and sensitivity of field of vision. Multiple methods are available for testing none is painful and most share a requirement for the patient to indicate ability to see a stimulus / target. This process results in a map of the person’s visual field, and can point to a loss of central vision or peripheral vision.
  • Color Testing: There are several tests that can be used to detect loss of color vision, which can occur late in Stargardt disease. Three tests are often used to get additional information: fundus photography combined with autofluorescence, electroretinography, and optical coherence tomography.
  • A fundus photo is a picture of the retina. These photos may reveal the presence of lipofuscin deposits. In fundus autofluorescence (FAF), a special filter is used to detect lipofuscin. Lipofuscin is naturally fluorescent (it glows in the dark) when a specific wavelength of light is shined into the eye. This test can detect lipofuscin that might not be visible with standard fundus photography, making it possible to diagnose Stargardt disease earlier.
  • Electroretinography (ERG) measures the electrical response of rods and cones to light. During the test, an electrode is placed on the cornea and light is flashed into the eye. The electrical responses are viewed and recorded on a monitor. Abnormal patterns of light response suggest the presence of Stargardt disease or other diseases that involve retinal degeneration.
  • Optical coherence tomography (OCT) is a scanning device that works a little like ultrasound. While ultrasound captures images by bouncing sound waves off of living tissues, OCT does it with light waves. The patient places his or her head on a chin rest while invisible, near-infrared light is focused on the retina. Because the eye is designed to allow light in, it’s possible to get detailed pictures deep within the retina. These pictures are then analyzed for any abnormalities in the thickness of the retinal layers, which could indicate retinal degeneration. OCT is sometimes combined with infrared scanning laser ophthalmoscope (ISLO) to provide additional surface images of the retina.

Anatomy

Structure

The macula is an oval-shaped area near the center of the retina. The retina is a light-sensitive layer that lines the back of the eye. It is made up of 200 million neurons, but is only about 0.2 millimeters thick. The retina contains photoreceptors that absorb light and then transmit those light signals through the optic nerve to the brain. Much like film in a camera, images come through the eye's lens and are focused on the retina. The retina then converts these images to electric signals and sends them to the brain.

The macula has a diameter of about 5 mm.   The macula can be seen with the use of an ophthalmoscope or a retinal camera. It has six clear subdivisions, including the umbo, foveola, foveal avascular zone, fovea, parafovea, and perifovea areas.

Location

The macula is the pigmented part of the retina located in the very center of the retina. In the center of the macula is the fovea, perhaps the most important part of the eye. The fovea is the area of best visual acuity. It contains a large amount of cones—nerve cells that are photoreceptors with high acuity.

Color

The macula is yellow in color. The yellow color is derived from lutein and zeaxanthin in the diet, both yellow xanthophyllcarotenoids contained within the macula. Because of its yellow color, the macula absorbs excess blue and ultraviolet light that enter into the eye, acting as sunblock to protect the retinal area.


Most doctors will give you numbing eye drops, then clean your eye, and perhaps eyelids, with a yellow iodine solution. They will position an eyelid holder, so you don&rsquot have to worry that you will blink at the wrong time. Then, they will numb the eye with drops, gel, a medicated Q-tip, or a superficial injection of anesthetic. Many will measure the position of injection, which is often placed in the lower, outer (toward your ear) aspect of the white part of the eye. The eye doctor will ask you to look up, and will perform the injection through a tiny needle. You may feel nothing, a little pressure, or, in some cases, some moderate discomfort lasting a few seconds. Some people see a web of lines as the medicine mixes with the fluids inside the eye.

After the injection, many doctors will examine your eye with a light and clean around your eye. Most will ask you to use antibiotic eye drops for a day or two.

Your eye will probably be sore and your vision somewhat foggy for a day or two, and then should improve. Any discomfort can often be relieved with Tylenol or Advil. A cool, clean washcloth held gently on the closed eye (for no more than 10 minutes every half hour) can also provide relief.

Sometimes the needle breaks a blood vessel on the surface of the eye at the time of injection. This can cause the white of the eye (sclera) to look red for as long as two weeks. If the eye is red, but is painless and the vision is good, then it is most likely harmless.

There is a low risk of serious complications caused by the injections (about 0.1% chance per injection). These are retinal detachment or infection in your eye (endophthalmitis). The symptoms of retinal detachment are an arc of flashing light in your peripheral vision, floating spots or lines in your vision that appear to move with your eye, or a &ldquocurtain&rdquo coming across part of your vision and blocking it. The symptoms of endophthalmitis are often blurry vision and pain (lasting more than just overnight after the injection). If you have symptoms of retinal detachment or endophthalmitis, call your ophthalmologist right away.

After the first injection, patients learn what to expect and it becomes less scary. Some patients switch doctors at some point, and are surprised that the new doctor&rsquos technique is a little different from the previous one. This is to be expected.

Sometimes patients will experience improved vision (better than before the injection) within a week of the procedure. Most will have their vision stabilized.


Blind spot (vision)

A blind spot, scotoma, is an obscuration of the visual field. A particular blind spot known as the physiological blind spot, "blind point", or punctum caecum in medical literature, is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on the optic disc of the retina where the optic nerve passes through the optic disc. [2] Because there are no cells to detect light on the optic disc, the corresponding part of the field of vision is invisible. Processes in the brain interpolate the blind spot based on surrounding detail and information from the other eye, so it is not normally perceived.

Although all vertebrates have this blind spot, cephalopod eyes, which are only superficially similar, do not. In them, the optic nerve approaches the receptors from behind, so it does not create a break in the retina.

The first documented observation of the phenomenon was in the 1660s by Edme Mariotte in France. At the time it was generally thought that the point at which the optic nerve entered the eye should actually be the most sensitive portion of the retina however, Mariotte's discovery disproved this theory.

The blind spot is located about 12–15° temporally and 1.5° below the horizontal and is roughly 7.5° high and 5.5° wide. [3]


Simple Anatomy of the Retina by Helga Kolb

When an ophthalmologist uses an ophthalmoscope to look into your eye he sees the following view of the retina (Fig. 1).

In the center of the retina is the optic nerve, a circular to oval white area measuring about 2 x 1.5 mm across. From the center of the optic nerve radiates the major blood vessels of the retina. Approximately 17 degrees (4.5-5 mm), or two and half disc diameters to the left of the disc, can be seen the slightly oval-shaped, blood vessel-free reddish spot, the fovea, which is at the center of the area known as the macula by ophthalmologists.

Fig. 1. Retina as seen through an opthalmoscope
CLICK HERE to see an animation (from the iris to the retina) (Quicktime movie)

A circular field of approximately 6 mm around the fovea is considered the central retina while beyond this is peripheral retina stretching to the ora serrata, 21 mm from the center of the retina (fovea). The total retina is a circular disc of between 30 and 40 mm in diameter (Polyak, 1941 Van Buren, 1963 Kolb, 1991).

Fig. 1.1. A schematic section through the human eye with a schematic enlargement of the retina

The retina is approximately 0.5 mm thick and lines the back of the eye. The optic nerve contains the ganglion cell axons running to the brain and, additionally, incoming blood vessels that open into the retina to vascularize the retinal layers and neurons (Fig. 1.1). A radial section of a portion of the retina reveals that the ganglion cells (the output neurons of the retina) lie innermost in the retina closest to the lens and front of the eye, and the photosensors (the rods and cones) lie outermost in the retina against the pigment epithelium and choroid. Light must, therefore, travel through the thickness of the retina before striking and activating the rods and cones (Fig. 1.1). Subsequently the absorbtion of photons by the visual pigment of the photoreceptors is translated into first a biochemical message and then an electrical message that can stimulate all the succeeding neurons of the retina. The retinal message concerning the photic input and some preliminary organization of the visual image into several forms of sensation are transmitted to the brain from the spiking discharge pattern of the ganglion cells.

A simplistic wiring diagram of the retina emphasizes only the sensory photoreceptors and the ganglion cells with a few interneurons connecting the two cell types such as seen in Figure 2.

When an anatomist takes a vertical section of the retina and processes it for microscopic examination it becomes obvious that the retina is much more complex and contains many more nerve cell types than the simplistic scheme (above) had indicated. It is immediately obvious that there are many interneurons packed into the central part of the section of retina intervening between the photoreceptors and the ganglion cells (Fig 3).

All vertebrate retinas are composed of three layers of nerve cell bodies and two layers of synapses (Fig. 4). The outer nuclear layer contains cell bodies of the rods and cones, the inner nuclear layer contains cell bodies of the bipolar, horizontal and amacrine cells and the ganglion cell layer contains cell bodies of ganglion cells and displaced amacrine cells. Dividing these nerve cell layers are two neuropils where synaptic contacts occur (Fig. 4).

The first area of neuropil is the outer plexiform layer (OPL) where connections between rod and cones, and vertically running bipolar cells and horizontally oriented horizontal cells occur (Figs. 5 and 6).

Fig. 5. 3-D block of retina with OPL highlighted
Fig. 6. Light micrograph of a vertical section through the OPL

The second neuropil of the retina, is the inner plexiform layer (IPL), and it functions as a relay station for the vertical-information-carrying nerve cells, the bipolar cells, to connect to ganglion cells (Figs. 7 and 8). In addition, different varieties of horizontally- and vertically-directed amacrine cells, somehow interact in further networks to influence and integrate the ganglion cell signals. It is at the culmination of all this neural processing in the inner plexiform layer that the message concerning the visual image is transmitted to the brain along the optic nerve.

Fig. 7. 3-D block of retina with IPL highlighted
Fig. 8. Light micrograph of a vertical section through the IPL

2. Central and peripheral retina compared.

Central retina close to the fovea is considerably thicker than peripheral retina (compare Figs. 9 and 10). This is due to the increased packing density of photoreceptors, particularly the cones, and their associated bipolar and ganglion cells in central retina compared with peripheral retina.

Fig. 9. Light micrograph of a vertical section through human central retina
Fig. 10. Light micrograph of a vertical section through human peripheral retina
  • Central retina is cone-dominated retina whereas peripheral retina is rod-dominated. Thus in central retina the cones are closely spaced and the rods fewer in number between the cones (Figs. 9 and 10).
  • The outer nuclear layer (ONL), composed of the cell bodies of the rods and cones is about the same thickness in central and peripheral retina. However in the peripheral the rod cell bodies outnumber the cone cell bodies while the reverse is true for central retina. In central retina, the cones have oblique axons displacing their cell bodies from their synaptic pedicles in the outer plexiform layer (OPL). These oblique axons with accompanying Muller cell processes form a pale-staining fibrous-looking area known as the Henle fibre layer. The latter layer is absent in peripheral retina.
  • The inner nuclear layer (INL) is thicker in the central area of the retina compared with peripheral retina, due to a greater density of cone-connecting second-order neurons (cone bipolar cells) and smaller-field and more closely-spaced horizontal cells and amacrine cells concerned with the cone pathways (Fig. 9). As we shall see later, cone-connected circuits of neurons are less convergent in that fewer cones impinge on second order neurons, than rods do in rod-connected pathways.
  • A remarkable difference between central and peripheral retina can be seen in the relative thicknesses of the inner plexiform layers (IPL), ganglion cell layers (GCL) and nerve fibre layer (NFL) (Figs. 9 and 10). This is again due to the greater numbers and increased packing-density of ganglion cells needed for the cone pathways in the cone-dominant foveal retina as compared the rod-dominant peripheral retina. The greater number of ganglion cells means more synaptic interaction in a thicker IPL and greater numbers of ganglion cell axons coursing to the optic nerve in the nerve fibre layer (Fig. 9).

3. Muller glial cells.

Fig. 11. Vertical view of Golgi stained Muller glial cells

Muller cells are the radial glial cells of the retina (Fig. 11). The outer limiting membrane (OLM) of the retina is formed from adherens junctions between Muller cells and photoreceptor cell inner segments. The inner limiting membrane (ILM) of the retina is likewise composed of laterally contacting Muller cell end feet and associated basement membrane constituents.

The OLM forms a barrier between the subretinal space, into which the inner and outer segments of the photoreceptors project to be in close association with the pigment epithelial layer behind the retina, and the neural retina proper. The ILM is the inner surface of the retina bordering the vitreous humor and thereby forming a diffusion barrier between neural retina and vitreous humor (Fig. 11).

Throughout the retina the major blood vessels of the retinal vasculature supply the capillaries that run into the neural tissue. Capillaries are found running through all parts of the retina from the nerve fibre layer to the outer plexiform layer and even occasionally as high as in the outer nuclear layer. Nutrients from the vasculature of the choriocapillaris (cc) behind the pigment epithelium layer supply the delicate photoreceptor layer.

4. Foveal structure.

The center of the fovea is known as the foveal pit (Polyak, 1941) and is a highly specialized region of the retina different again from central and peripheral retina we have considered so far. Radial sections of this small circular region of retina measuring less than a quarter of a millimeter (200 microns) across is shown below for human (Fig. 12a) and for monkey (Fig.12b).

Fig. 12a. Vertical section of the human fovea from Yamada (1969)
Fig. 12b. Vertical section of the monkey fovea from Hageman and Johnson (1991)

The fovea lies in the middle of the macula area of the retina to the temporal side of the optic nerve head (Fig. 13a, A, B). It is an area where cone photoreceptors are concentrated at maximum density, with exclusion of the rods, and arranged at their most efficient packing density which is in a hexagonal mosaic. This is more clearly seen in a tangential section through the foveal cone inner segments (Fig. 13b).

Fig. 13. Tangential section through the human fovea

Below this central 200 micron diameter central foveal pit, the other layers of the retina are displaced concentrically leaving only the thinnest sheet of retina consisting of the cone cells and some of their cell bodies (right and left sides of Figs. 12a and 12b). This is particularly well seen in optical coherence tomography (OCT) images of the living eye and retina (Fig. 13a, B). Radially distorted but complete layering of the retina then appears gradually along the foveal slope until the rim of the fovea is made up of the displaced second- and third-order neurons related to the central cones. Here the ganglion cells are piled into six layers so making this area, called the foveal rim or parafovea (Polyak, 1941), the thickest portion of the entire retina.

5. Macula lutea.

The whole foveal area including foveal pit, foveal slope, parafovea and perifovea is considered the macula of the human eye. Familiar to ophthalmologists is a yellow pigmentation to the macular area known as the macula lutea (Fig. 14).

This pigmentation is the reflection from yellow screening pigments, the xanthophyll carotenoids zeaxanthin and lutein (Balashov and Bernstein, 1998), present in the cone axons of the Henle fibre layer. The macula lutea is thought to act as a short wavelength filter, additional to that provided by the lens (Rodieck, 1973). As the fovea is the most essential part of the retina for human vision, protective mechanisms for avoiding bright light and especially ultraviolet irradiation damage are essential. For, if the delicate cones of our fovea are destroyed we become blind.

Fig. 14. Ophthalmoscopic appearance of the retina to show macula lutea
Fig. 15. Vertical section through the monkey fovea to show the distribution of the macula lutea. From Snodderly et al., 1984

The yellow pigment that forms the macula lutea in the fovea can be clearly demonstrated by viewing a section of the fovea in the microscope with blue light (Fig. 15). The dark pattern in the foveal pit extending out to the edge of the foveal slope is caused by the macular pigment distribution (Snodderly et al., 1984).

If one were to visualize the foveal photoreceptor mosaic as though the visual pigments in the individual cones were not bleached, one would see the picture shown in Figure 16 (lower frame) (picture from Lall and Cone, 1996). The short-wavelength sensitive cones on the foveal slope look pale yellow green, the middle wavelength cones, pink and the long wavelength sensitive cones, purple. If we now add the effect of the yellow screening pigment of the macula lutea we see the appearance of the cone mosaic in Figure 16 (upper frame). The macula lutea helps enhance achromatic resolution of the foveal cones and blocks out harmful UV light irradiation (Fig. 16 from Abner Lall and Richard Cone, unpublished data).

6. Ganglion cell fiber layer.

The ganglion cell axons run in the nerve fiber layer above the inner limiting membrane towards the optic nerve head in a arcuate form (Fig. 00, streaming pink fibers). The fovea is, of course, free of a nerve fiber layer as the inner retina and ganglion cells are pushed away to the foveal slope. The central ganglion cell fibers run around the foveal slope and sweep in the direction of the optic nerve. Peripheral ganglion cell axons continue this arcing course to the optic nerve with a dorso/ventral split along the horizontal meridian (Fig. 00). Retinal topography is maintained in the optic nerve, through the lateral geniculate to the visual cortex.

7. Blood supply to the retina.

There are two sources of blood supply to the mammalian retina: the central retinal artery and the choroidal blood vessels. The choroid receives the greatest blood flow (65-85%) (Henkind et al., 1979) and is vital for the maintainance of the outer retina (particularly the photoreceptors) and the remaining 20-30% flows to the retina through the central retinal artery from the optic nerve head to nourish the inner retinal layers. The central retinal artery has 4 main branches in the human retina (Fig. 17).

Fig. 17. Fundus photograph showing flourescein imaging of the major arteries and veins in a normal human right eye retina. The vessels emerge from the optic nerve head and run in a radial fashion curving towards and around the fovea (asterisk in photograph) (Image courtesy of Isabel Pinilla, Spain)

The arterial intraretinal branches then supply three layers of capillary networks i.e. 1) the radial peripapillary capillaries (RPCs) and 2) an inner and 3) an outer layer of capillaries (Fig. 18a). The precapillary venules drain into venules and through the corresponding venous system to the central retinal vein (Fig. 18b).

Fig. 18a. Flatmount view of a rat retina stained with NADPH-diaphorase at the level of focus of a major artery and arterioles. (Courtesy of Toby Holmes, Moran Eye Center)
Fig. 18b. Flatmount view of a rat retina stained with NADPH-diaphorase at the level of focus of a major vein and venules. (Courtesy of Toby Holmes, Moran Eye Center)

The radial peripapillary capillaries (RPCs) are the most superfical layer of capillaries lying in the inner part of the nerve fiber layer, and run along the paths of the major superotemporal and inferotemporal vessels 4-5 mm from the optic disk (Zhang, 1994). The RPCs anatomose with each other and the deeper capillaries. The inner capillaries lie in the ganglion cell layers under and parallel to the RPCs. The outer capillary network runs from the inner plexiform layer to the outer plexiform layer thought the inner nuclear layer (Zhang, 1974).

As will be noticed from the flourescein angiography of Figure 17, there as a ring of blood vessels in the macular area around a blood vessel- and capillary-free zone 450-600 um in diameter, denoting the fovea. The macular vessels arise from branches of the superior temporal and inferotemporal arteries. At the border of the avascular zone the capillaries become two layered and finally join as a single layered ring. The collecting venules are more deep (posterior) to the arterioles and drain blood flow back into the main veins (Fig. 19, from Zhang, 1974). In the rhesus monkey this perimacular ring and blood vessel free fovea is clearly seen in the beautiful drawings made by Max Snodderly’s group (Fig. 20, Sodderly et al., 1992.)

Fig. 19. The macular vessels of the monkey eye form a ring around the avascular fovea (star)(From Zhang, 1994)
Fig. 20. Diagram of the retinal vasculature around the fovea in the rhesus monkey derived from more than 80 microscope fields. (From Snodderly et al., 1992)

The choroidal arteries arise from long and short posterior ciliary arteries and branches of Zinn’s circle (around the optic disc). Each of the posterior ciliary arteries break up into fan-shaped lobules of capillaries that supply localized regions of the choroid (Hayreh, 1975). The macular area of the choroidal vessels are not specialized like the retinal blood supply is (Zhang, 1994). The arteries pierce the sclera around the optic nerve and fan out to form the three vascular layers in the choroid: outer (most scleral), medial and inner (nearest Bruchs membrane of the pigment epithelium) layers of blood vessels. This is clearly shown in the corrosion cast of a cut face of the human choroid in Figure 21a (Zhang, 1974). The corresponding venous lobules drain into the venules and veins that run anterior towards the equator of the eyeball to enter the vortex veins (Fig. 21b). One or two vortex veins drain each of the 4 quadrants of the eyeball. The vortex veins penetrate the sclera and merge into the ophthalmic vein as shown in the corrosion cast of Figure 21b (Zhang. 1994).

Fig. 21a. The three vascular layers in the choroid: outer arteries and veins(red/blue arrow), medial arterioles and venules(red arrow) and inner capillary bed (yellow star. Corrosion cast of a cut face of the human choroid (From Zhang, 1994)
Fig. 21b. Corrosion cast of the upper back of the human eye with the sclera removed. The vortex veins collect the blood from the equator of the eye and merge with the ophthalmic vein. (From Zhang, 1994).

8. Degenerative diseases of the human retina.

The human retina is a delicate organization of neurons, glia and nourishing blood vessels. In some eye diseases, the retina becomes damaged or compromised, and degenerative changes set in that eventally lead to serious damage to the nerve cells that carry the vital mesages about the visual image to the brain. We indicate four different conditions where the retina is diseased and blindness may be the end result. Much more information concerning pathology of the whole eye and retina can be found in a website made by eye pathologist Dr. Nick Mamalis, Moran Eye Center.

Fig. 22. A view of the fundus of the eye and of the retina in a patient who has age-related macular degeneration.
Fig. 23. A view of the fundus of the eye and of the retina in a patient who has advanced glaucoma.

Age related macular degeneration is a common retinal problem of the aging eye and a leading cause of blindness in the world. The macular area and fovea become compromised due to the pigment epithelium behind the retina degenerating and forming drusen (white spots, Fig. 22) and allowing leakage of fluid behind the fovea. The cones of the fovea die causing central visual loss so we cannot read or see fine detail.

Glaucoma (Fig. 23) is also a common problem in aging, where the pressure within the eye becomes elevated. The pressure rises because the anterior chamber of the eye cannot exchange fluid properly by the normal aqueous outflow methods. The pressure within the vitreous chamber rises and compromises the blood vessels of the optic nerve head and eventually the axons of the ganglion cells so that these vital cells die. Treatment to reduce the intraocular pressure is essential in glaucoma.

Fig. 24. A view of the fundus of the eye and of the retina in a patient who has retinitis pigmentosa
Fig. 25. A view of the fundus of the eye and of the retina in a patient who has advanced diabetic retinopathy

Retinits pigmentosa (Fig. 24) is a nasty hereditary disease of the retina for which there is no cure at present. It comes in many forms and consists of large numbers of genetic mutations presently being analysed. Most of the faulty genes that have been discoverd concern the rod photoreceptors. The rods of the peripheral retina begin to degenerate in early stages of the disease. Patients become night blind gradually as more and more of the peripheral retina (where the rods reside) becomes damaged. Eventally patients are reduced to tunnel vision with only the fovea spared the disease process. Characteristic pathology is the occurence of black pigment in the peripheral retina and thinned blood vessels at the optic nerve head (Fig. 24).

Diabetic retinopathy is a side effect of diabetes that affects the retina and can cause blindness (Fig. 25). The vital nourishing blood vessels of the eye become compromised, distorted and multiply in uncontrollable ways. Laser treatment for stopping blood vessel proliferation and leakage of fluid into the retina, is the commonest treatment at present.

9. References.

Balashov NA, Bernstein PS. Purification and identification of the components of the human macular carotenoid metabolism pathways. Invest Ophthal Vis Sci.199839:s38.

Hageman GS, Johnson LV. The photoreceptor-retinal pigmented epithelium interface. In: Heckenlively JR, Arden GB, editors. Principles and practice of clinical electrophysiology of vision. St. Louis: Mosby Year Book 1991. p. 53-68.

Harrington, D.O. and Drake, M.V. (1990) The Visual Fields, 6th ed. Mosby. St. Louis.

Hayreh SS. Segmental nature of the choroidal vasculature. Br J Ophthal. 197559:631–648. [PubMed] [Free Full text in PMC]

Henkind P, Hansen RI, Szalay J. Ocular circulation. In: Records RE, editor. Physiology of the human eye and visual system. New York: Harper & Row 1979. p. 98-155.

Kolb H. The neural organization of the human retina. In: Heckenlively JR, Arden GB, editors. Principles and practices of clinical electrophysiology of vision. St. Louis: Mosby Year Book Inc. 1991. p. 25-52.

Polyak SL. The retina. Chicago: University of Chicago Press 1941.

Rodieck RW. The vertebrate retina: principles of structure and function. San Francisco: W.H. Freeman and Company 1973.

Snodderly DM, Auran JD, Delori FC. The macular pigment. II. Spatial distribution in primate retina. Invest Ophthal Vis Sci. 198425:674–685. [PubMed]

Snodderly DM, Weinhaus RS, Choi JC. Neural-vascular relationships in central retina of Macaque monkeys (Macaca fascicularis). J Neurosci. 199212:1169–1193.[PubMed]

Van Buren JM. The retinal ganglion cell layer. Springfield (IL): Charles C. Thomas 1963.

Yamada E. Some structural features of the fovea centralis in the human retina. Arch Ophthal. 196982:151–159. [PubMed]

Zhang HR. Scanning electron-microscopic study of corrosion casts on retinal and choroidal angioarchitecture in man and animals. Prog Ret Eye Res. 199413:243–270.


Ophthalmoscopic Examination

The simplest ophthalmoscopes consist of an aperture to look through, a diopter indicator, and a disc for selecting lenses. The ophthalmoscope is primarily used to examine the fundus, or the inner wall of the posterior eye, which consists of the choroid, retina, fovea, macula, optic disc, and retinal vessels (Figure 1). The spherical eyeball collects and focuses light on the neurosensory cells of the retina. Light is refracted as it passes sequentially through the cornea, the lens, and the vitreous body.

The first landmark observed during the funduscopic exam is the optic disc, which is where the optic nerve and retinal vessels enter the back of the eye (Figure 2). The disc usually contains a central whitish physiologic cup where the vessels enter it normally occupies less than half the diameter of the entire disc. Just lateral and slightly inferior is the fovea, a darkened circular area that demarcates the point of central vision. Around this is the macula. A blind spot approximately 15° temporal to the line of gaze results from a lack of photoreceptor cells at the optic disc.


Figure 1. Anatomy of the eye. A diagram showing a sagittal view of the human eye with the structures labeled.


Figure 2: Normal retina. A photograph showing an ophthalmoscopic view on the normal retina.

Procedure

Since mydriatic eye drops are typically not used in general practice, the view of the fundus is limited to only a section of the posterior retina. Be familiar with these features before attempting to examine the patient.

  1. Unless the patient's refractive errors make it difficult to focus on the retina, it is usually best to remove your own eyeglasses for the exam.
  2. After darkening the room, turn on the ophthalmoscope and shine the light on your hand or on the wall.
  3. Turn the lens disc until the largest white circle of light can be seen, and the diopter indicator reads 0, meaning the ophthalmoscope lens is neither converging nor dispersing the light.
  4. Keep your index finger on the lens disc during the exam, so the diopters can be adjusted as necessary to focus in on the retinal structures.
  5. To examine the patient's right eye, hold the ophthalmoscope in your right hand and look through the aperture with your right eye to exam the patient's left eye, hold the ophthalmoscope in your left hand and look through the aperture with your left eye. This avoids bumping noses with the patient.
  6. Position yourself about a foot from the patient at eye level, and ask the patient to stare at a spot on the wall just over your shoulder.
  7. As you peer through the aperture, keep both eyes opened, press the ophthalmoscope firmly against your bony orbit, and hold the handle at a slight angle away from the patient's face.
  8. Position the ophthalmoscope about 15° lateral to the patient's line of vision. Direct the light to the pupil and look for an orange-red glow. This is the red reflex. Note any opacity that seems to interfere.
  9. Stay focused on the red reflex, and move the ophthalmoscope inward along the 15° line until you are almost on top of the patient's eye. While doing this, the optic disc and retinal vessels should come into sharp focus. The disc appears as a yellow, orange, or pinkish oval that largely fills the field of view.
  10. If the disc isn't seen right away, identify a blood vessel and follow it toward the disc. It appears to get wider if you are going in the right direction. The light may need to be dimmed in order to keep the patient comfortable and to avoid spasmodic constriction of the pupil.
  11. If the disc appears out of focus, try adjusting the diopters by rotating the lens disc one or two settings in the positive or negative direction. The retina only appears perfectly sharp if neither you nor the patient have refractive errors.
  12. Carefully examine the disc for outline clarity, color, relative size of the yellowish-white central cup, and symmetry with the contralateral eye. White or darkly pigmented rings and crescents are often seen around the disc and have no pathologic significance.
  13. Follow the retinal vessels as they extend away from the disc in all four directions. Veins appear redder and wider than arteries.
  14. Look for spontaneous venous pulsations, which appear as subtle variations in the width of the veins with each heartbeat. It's possible to discern subtle pulsations of the veins.
  15. Make special note of arteriovenous (AV) crossings. Since the walls of the normal retinal arteries are transparent, only the column of blood is visible within. Veins crossing behind arteries, therefore, are normally seen right up to the column on either side.
  16. Look for any lesions elsewhere in the retina, noting their size.
  17. Examine the fovea and surrounding macula by asking the patient to look directly into the light. The macula often appears to shimmer.
  18. Finally, look for opacities in the lens by adjusting the diopters between 10+ and 12+.
  19. If the image is lost while searching the retina, it means the light fell out of the pupil as the ophthalmoscope was moved. It takes some practice to keep it inside.

The ophthalmoscopic examination is one of the most important parts of the physical exam. If conducted properly, it can be used as a tool to not only assess the patients' eyes but also their overall health. The simplest ophthalmoscope consists of a light source with a dimmer for adjusting the brightness, an aperture to look through, a disc for selecting lenses of different diopters and a diopter indicator that displays the power of the lens to focus light.

A diopter of zero means that the ophthalmoscope lens is neither converging nor diverging the light passing through it. Turning the dial counterclockwise toward negative, or red, diopter settings is useful in myopic or nearsighted patients whose retina lies closer than normal to the ophthalmoscope. Conversely, turning the dial clockwise toward positive, or green, diopter settings is useful in hyperopic or farsighted patients whose retina lies farther than normal from the ophthalmoscope.

This video will review the important landmarks that a physician should look for during an ophthalmoscopic inspection as well as provide the steps needed to conduct an effective examination.

Let's start with the landmarks. The ophthalmoscope is primarily used to examine the fundus, which is the portion of the posterior wall of the eye where visual processing primarily takes place. Therefore, the exam is also known as the fundoscopic exam.

The fundus consists of the choroid, retina, fovea, macula, optic disc, and retinal vessels. The first anatomical landmark that you should notice when viewing the fundus is the optic disc, which is where the optic nerve and retinal vessels enter the back of the eye. The disc usually contains a central whitish physiologic cup where the vessels enter. The cup normally occupies less than half the diameter of the entire disc. Just lateral and slightly inferior to the optic disc is the fovea, a darkened circular area that demarcates the point of central vision. Around the fovea is the macula, which appears as an oval-shaped pigmented area.

Now that we have an understanding of the landmarks, let's review the procedural steps needed to effectively carry out fundus evaluation. Upon entering the examination room, greet your patient and explain the procedure briefly. As with any examination, wash your hands thoroughly or apply topical disinfectant solution before proceeding. Unless the patient's refractive errors make it difficult to focus on the retina, it is usually best to remove your own eyeglasses for the exam.

Turn on the ophthalmoscope to its brightest setting. Remove any filters by adjusting the filter setting until the largest white disc appears. Turn the diopter indicator to zero. Be sure to keep your index finger on the lens disc during the exam, so the diopters can be adjusted as necessary to focus in on retinal structures. Position yourself about a foot away from the patient, making sure that your eye and the patient's eye are at the same level. Ask the patient to stare at a spot on the wall just over your shoulder&hellip

To examine the patient's right eye, hold the ophthalmoscope in your right hand and look through the aperture with your right eye. As you peer through the aperture, keep both eyes opened. Press the ophthalmoscope firmly against your bony orbit and hold the handle at a slight angle away from the patient's face. Placing your opposite thumb on the patient's eyebrow will prevent you from bumping the ophthalmoscope against the patient's orbit during the exam.

Position the ophthalmoscope about 15° laterally to the patient's line of vision. Direct the ophthalmoscope's light to the patient's pupil and look for an orange-red glow, which is known as the red reflex. Be sure to note any opacities that seem to interfere. As you remain focused on the red reflex, move the ophthalmoscope inward along the 15° line until you are almost on top of the patient's eye. If the image appears out of focus, try adjusting the diopters by rotating the lens disc one or two settings in the positive or negative direction. After adjustment, the optic disc and retinal vessels should come into sharp focus.

The disc appears as a yellow, orange, or pinkish oval that largely fills the field of view. Sometimes the disc isn't visible right away in that case, identify a blood vessel and follow it towards the disc. You will know you are going in the right direction if the blood vessel appears to get wider. Keep in mind, the ophthalmoscope's light may need to be dimmed in order to keep the patient comfortable and to avoid the spasmodic constriction of the pupil.

Carefully examine the disc for color, outline clarity, relative size of central cup, and symmetry with the contralateral eye. White or dark pigmented rings and crescents are often seen around the disc and have no pathologic significance. Then, follow the retinal vessels as they extend away from the disc in all four directions. Veins will appear redder and wider than arteries. As you follow the retinal vessels, look for spontaneous venous pulsations, which appear as subtle variations in the width. Take special note of arteriovenous crossings and look for any lesions in the retina, noting their size, shape and location. If the image is lost while searching the retina, it means the light fell out of the pupil as the ophthalmoscope was moved. It takes some practice to keep the light inside.

Next, ask the patient to look directly into the light of the ophthalmoscope to examine the fovea and the surrounding macula. The macula often appears to shimmer. Finally, look for opacities in the lens by adjusting the diopters to a point between 10 positive and 12 positive. To examine the patient's left eye, perform the same steps while holding the ophthalmoscope in your left hand and looking through the aperture with your left eye.

You have just watched a JoVE video documenting an ophthalmologic examination. You should now know the important landmarks in the fundus of the eye viewed during this exam and understand the systematic sequence of steps that every physician should follow in order to conduct an effective ophthalmologic assessment. As always, thanks for watching!

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Applications and Summary

The ophthalmologic exam is probably the most challenging for students to master. With time, however, it becomes routine. It is also one of the most productive parts of the physical exam, as it not only offers a window into the condition of the eye, but also provides evidence of disease elsewhere in the body. Elevated intracranial pressure from a variety of causes may lead to swelling of the optic nerve, which appears as papilledema on a funduscopic exam. In papilledema, the optic disc is swollen, its margins are blurred, the central cup is lost, and venous pulsations are absent. Papilledema signals a serious, life-threatening condition. Death of optic nerve fibers, which can occur in disorders such as optic neuritis, multiple sclerosis, and temporal arteritis, causes the disc to atrophy and lose its smaller blood vessels. Uncontrolled hypertension leads to "copper wiring" of thickened arterial walls in the retina, causing them to appear less transparent. Veins crossing these arteries seem to stop abruptly before reaching either side, a condition called AV nicking. Other signs to look for in hypertensive retinopathy are hard exudates and cotton-wool patches, which result from infarcted nerve fibers. In patients with diabetes, the retina may reveal microaneurysms, hemorrhages, and neovascularization.

Common eye diseases observable on a funduscopic exam include glaucoma and macular degeneration. In glaucoma, increased intraocular pressure may cause the central cup of optic disc to deepen and widen, so it occupies greater than half of the disc diameter. In age-related macular degeneration (AMD), patches of hyperpigmentation and deposits composed of cellular debris, called drusen, can be seen scattered throughout the retina (particularly in the macula). In more severe stages, choroidal neovascularization is visible in the neovascular ("wet") form of AMD, whereas depigmentation and loss of the choriocapillaris are visible in the advanced atropic ("dry") form of AMD (also known as geographic atrophy). Cataracts can be more thoroughly examined by focusing the ophthalmoscope on opacified lenses.

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Transcript

The ophthalmoscopic examination is one of the most important parts of the physical exam. If conducted properly, it can be used as a tool to not only assess the patients' eyes but also their overall health. The simplest ophthalmoscope consists of a light source with a dimmer for adjusting the brightness, an aperture to look through, a disc for selecting lenses of different diopters and a diopter indicator that displays the power of the lens to focus light.

A diopter of zero means that the ophthalmoscope lens is neither converging nor diverging the light passing through it. Turning the dial counterclockwise toward negative, or red, diopter settings is useful in myopic or nearsighted patients whose retina lies closer than normal to the ophthalmoscope. Conversely, turning the dial clockwise toward positive, or green, diopter settings is useful in hyperopic or farsighted patients whose retina lies farther than normal from the ophthalmoscope.

This video will review the important landmarks that a physician should look for during an ophthalmoscopic inspection as well as provide the steps needed to conduct an effective examination.

Let's start with the landmarks. The ophthalmoscope is primarily used to examine the fundus, which is the portion of the posterior wall of the eye where visual processing primarily takes place. Therefore, the exam is also known as the fundoscopic exam.

The fundus consists of the choroid, retina, fovea, macula, optic disc, and retinal vessels. The first anatomical landmark that you should notice when viewing the fundus is the optic disc, which is where the optic nerve and retinal vessels enter the back of the eye. The disc usually contains a central whitish physiologic cup where the vessels enter. The cup normally occupies less than half the diameter of the entire disc. Just lateral and slightly inferior to the optic disc is the fovea, a darkened circular area that demarcates the point of central vision. Around the fovea is the macula, which appears as an oval-shaped pigmented area.

Now that we have an understanding of the landmarks, let's review the procedural steps needed to effectively carry out fundus evaluation. Upon entering the examination room, greet your patient and explain the procedure briefly. As with any examination, wash your hands thoroughly or apply topical disinfectant solution before proceeding. Unless the patient's refractive errors make it difficult to focus on the retina, it is usually best to remove your own eyeglasses for the exam.

Turn on the ophthalmoscope to its brightest setting. Remove any filters by adjusting the filter setting until the largest white disc appears. Turn the diopter indicator to zero. Be sure to keep your index finger on the lens disc during the exam, so the diopters can be adjusted as necessary to focus in on retinal structures. Position yourself about a foot away from the patient, making sure that your eye and the patient's eye are at the same level. Ask the patient to stare at a spot on the wall just over your shoulder…

To examine the patient's right eye, hold the ophthalmoscope in your right hand and look through the aperture with your right eye. As you peer through the aperture, keep both eyes opened. Press the ophthalmoscope firmly against your bony orbit and hold the handle at a slight angle away from the patient's face. Placing your opposite thumb on the patient's eyebrow will prevent you from bumping the ophthalmoscope against the patient's orbit during the exam.

Position the ophthalmoscope about 15° laterally to the patient's line of vision. Direct the ophthalmoscope's light to the patient's pupil and look for an orange-red glow, which is known as the red reflex. Be sure to note any opacities that seem to interfere. As you remain focused on the red reflex, move the ophthalmoscope inward along the 15° line until you are almost on top of the patient's eye. If the image appears out of focus, try adjusting the diopters by rotating the lens disc one or two settings in the positive or negative direction. After adjustment, the optic disc and retinal vessels should come into sharp focus.

The disc appears as a yellow, orange, or pinkish oval that largely fills the field of view. Sometimes the disc isn't visible right away in that case, identify a blood vessel and follow it towards the disc. You will know you are going in the right direction if the blood vessel appears to get wider. Keep in mind, the ophthalmoscope's light may need to be dimmed in order to keep the patient comfortable and to avoid the spasmodic constriction of the pupil.

Carefully examine the disc for color, outline clarity, relative size of central cup, and symmetry with the contralateral eye. White or dark pigmented rings and crescents are often seen around the disc and have no pathologic significance. Then, follow the retinal vessels as they extend away from the disc in all four directions. Veins will appear redder and wider than arteries. As you follow the retinal vessels, look for spontaneous venous pulsations, which appear as subtle variations in the width. Take special note of arteriovenous crossings and look for any lesions in the retina, noting their size, shape and location. If the image is lost while searching the retina, it means the light fell out of the pupil as the ophthalmoscope was moved. It takes some practice to keep the light inside.

Next, ask the patient to look directly into the light of the ophthalmoscope to examine the fovea and the surrounding macula. The macula often appears to shimmer. Finally, look for opacities in the lens by adjusting the diopters to a point between 10 positive and 12 positive. To examine the patient's left eye, perform the same steps while holding the ophthalmoscope in your left hand and looking through the aperture with your left eye.

You have just watched a JoVE video documenting an ophthalmologic examination. You should now know the important landmarks in the fundus of the eye viewed during this exam and understand the systematic sequence of steps that every physician should follow in order to conduct an effective ophthalmologic assessment. As always, thanks for watching!


Neuroscience For Kids

The retina is the back part of the eye that contains the cells that respond to light. These specialized cells are called photoreceptors. There are 2 types of photoreceptors in the retina: rods and cones.

The rods are most sensitive to light and dark changes, shape and movement and contain only one type of light-sensitive pigment. Rods are not good for color vision. In a dim room, however, we use mainly our rods, but we are "color blind." Rods are more numerous than cones in the periphery of the retina. Next time you want to see a dim star at night, try to look at it with your peripheral vision and use your ROD VISION to see the dim star. There are about 120 million rods in the human retina.

The cones are not as sensitive to light as the rods. However, cones are most sensitive to one of three different colors (green, red or blue). Signals from the cones are sent to the brain which then translates these messages into the perception of color. Cones, however, work only in bright light. That's why you cannot see color very well in dark places. So, the cones are used for color vision and are better suited for detecting fine details. There are about 6 million cones in the human retina. Some people cannot tell some colors from others - these people are "color blind." Someone who is color blind does not have a particular type of cone in the retina or one type of cone may be weak. In the general population, about 8% of all males are color blind and about 0.5% of all females are color blind.

The fovea, shown here on the left, is the central region of the retina that provides for the most clear vision. In the fovea, there are NO rods. only cones. The cones are also packed closer together here in the fovea than in the rest of the retina. Also, blood vessels and nerve fibers go around the fovea so light has a direct path to the photoreceptors.

Here is an easy way to demonstrate the sensitivity of your foveal vision. Stare at the "g" in the word "light" in middle of the following sentence:

"Your vision is best when light falls on the fovea."

The "g" in "light" will be clear, but words and letters on either side of the "g" will not be clear.

One part of the retina does NOT contain any photoreceptors. This is our "blind spot." Therefore any image that falls on this region will NOT be seen. It is in this region that the optic nerves come together and exit the eye on their way to the brain.

To find your blind spot, look at the image below or draw it on a piece of paper:

Hold the image (or place your head from the computer monitor) about 20 inches away. With your right eye, look at the dot. Slowly bring the image (or move your head) closer while looking at the dot. At a certain distance, the + will disappear from sight. this is when the + falls on the blind spot of your retina. Reverse the process. Close your right eye and look at the + with your left eye. Move the image slowly closer to you and the dot should disappear.

Here is another image that will help you find your blind spot.

For this image, close your right eye. With your left eye, look at the red circle. Slowly move your head closer to the image. At a certain distance, the blue line will not look broken!

Did you know? Why can't you see very well when you first go into a darkened room like a movie theater? When you first enter the movie theater, the cones in your retina are working and the rods are not yet activated. Cones need a lot of light to work properly rods need less light to work, but they need about 7-10 minutes to take over for the cones. After 7-10 minutes in the dark, the rods do work, but you cannot see colors very well because the rods do not provide any color information. The cones, which do provide color information, need more light, but do not work well in the dark. After the movie is over and you leave the theater, everything looks very bright and it is hard to see for a minute or two. This is because the rods become "saturated" and stop working in these bright conditions. It takes a few minutes for the cones to begin to function again, and for normal vision to be restored.

A complete lesson plan on the eye and its connections - teacher and student guides available. Also, try some experiments to test your sense of sight and take a short, interactive quiz about the eye and sight.


Watch the video: Wahrnehmung, Der blinde Fleck oder Zeichen wegzaubern - Experiment. Tutorial (August 2022).