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Module 23 |
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Module 23: |
The Posterior Segment, Part 1 | |||||
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This module relies heavily on graphics and images to enhance your educational experience. The images at the top of the page will load first, but may do so slowly if you have a dial-up internet connection. The material was all placed on one html page so that subsequent images can be loading while you are reading. The material is best view at a screen resolution of at least 1024 by 768. You can easily check or change your screen resolution yourself. For windows, go to the desktop and move your mouse to a blank part of the screen. Right click the mouse and click on "properties". Click on the settings tab. Use the slider to change the screen resolution if needed, and click "OK". |
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| Introduction
The eye can be divided into two major segments: the anterior segment and the posterior segment. |
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| The anterior segment (AC) includes the cornea, anterior chamber, iris, and lens. Sometimes the lens is listed as part of the posterior segment. The posterior segment includes the vitreous, retina, choroid, and posterior sclera. | ||||||
| The Vitreous
The vitreous is a clear, gelatin-like substance that accounts for about 75% of the mass of the eye. There are normally no blood vessels in the vitreous, and the vitreous is normally optically transparent. |
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Vitreous does not regenerate, and it shrinks with age, being replaced by other fluid. At a young age, the vitreous is attached to the retina. As the vitreous shrinks with age, it pulls away from the retina. This normal process is called a vitreous detachment. When the vitreous is detached from the posterior pole of the eye, it is called a posterior vitreous detachment (PVD).
Sometimes the vitreous does not detach gracefully from the retina and it adheres and pulls on the retina at one or more points. The tugging of the vitreous on the retina mechanically stimulates the photoreceptors, causing "light flashes" in the patient's peripheral vision. The vitreous can pull hard enough to tear the retina, causing a cloud of pigment and/or blood cells to be released into the vitreous. The patient sees these as a "shower of black specks" or "cobwebs". The result can be a retinal hole or tear that needs to be treated to prevent fluid from entering the tear and detaching the retina. Floaters can be a normal symptom of the aging eye and the younger myopic eye. Fragments of vitreous collagen are more dense than the bulk of the vitreous matter, and they cast shadows on the retina as they float about. Sometimes the floaters settle out of site, away from the optical axis, only to re-appear as the shadow moves over the macula. Floaters associated with a possible retinal tear are almost always "new" floaters that haven't been noticed before by the patient, and they are often associated with the light flashes. Floaters and flashes are indicative of the vitreous detaching from the retina, but they are not always associated with a retinal tear or hole. Only an ophthalmoscopic examination can rule out a retinal tear or detachment for the patient complaining of floaters and flashes. The vitreous can become infected with pathogens, and abnormal blood vessels can grow from the retina into the vitreous. The vitreous can become clouded with inflammatory cells, blood, and/or pigment cells. Opacities in the vitreous can reduce visual acuity. The most common opacity is a vitreous hemorrhage associated with diabetic retinopathy. Vitreous strands can remain attached to the retina and pull on blood vessels damaged by diabetes, causing them to bleed into the vitreous. Also, new blood vessels (neovascularization) growing from the damaged retina can grow into the vitreous and bleed. If the hemorrhage does not clear over time, the blood can be removed from the vitreous with a procedure called a vitrectomy. Small, pipe-shaped instruments are inserted via small holes into the vitreous cavity. The blood and associated vitreous fibers can be chopped up and sucked out. The anterior vitreous can be examined with a slit lamp. A slit lamp and a 90 diopter lens can be used to examine the vitreous along the optical axis. The indirect ophthalmoscope is routinely used to view the vitreous out into the periphery. The B-scan ultrasound is an excellent instrument for visualizing the vitreous. Although not usually needed to confirm the presence of inflammatory cells or blood in the vitreous, the B-scan is useful for ruling out a retinal detachment that may be hidden from view by dense opacities in the vitreous. The normal vitreous appears acoustically clear on the ultrasound, that is, it appears black. Since most opacities in the vitreous are particulate (small), the gain must usually be turned up to view the opacities.
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| B-scan image of opacities in the vitreous. The sensitivity (gain) of the instrument is turned up to be able to view them. |  |
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The retina is a thin (.5mm) film of photosensitive cells that faces the vitreous and lines the back of the posterior segment of the eye. The retina is analogous to the film in the camera. |
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The surface of the retina is only interrupted by the optic nerve head. This "optic disk" is where the nerve fibers, the retinal arteries, and the retinal veins enter the eye and fan out over the surface of the retina.
Retinal arteries and veins form superior and inferior "arcades" above and below the macula, which is the point along the visual axis at which light is normally focused. The macula is loaded with a great number of color sensitive cells (cones) in order to increase the acuity of the central field of vision. The very center of the macula, the fovea, is avascular (no blood vessels), and no nerve fibers run above the cones in this bulls-eye of maximum acuity that is a mere 1.5 mm across. This absence of tissue above the cones in the fovea insures that nothing will distort or block light on the way to the cones. The macular area appears darker than the surrounding retina on fundus photos. This is because the underlying pigment layer (RPE) is more dense in the macula. Optical Coherence Tomography (OCT) uses a light beam the same way that B-scan ultrasonography uses a sound beam to image the retina in a microscopic slice. The following is an OCT image of a normal macula. The scan is 5mm wide. The small depression in the center is the fovea centralis (F). The nerve fiber layer (NFL) and the retinal pigment epithelial (RPE) layer are relatively highly reflective, as indicated by the orange-red coloration.
If you draw a horizontal line through the fovea, you will notice that the macula is anatomically slightly lower than the center of the optic nerve.
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This feature is useful when identifying unlabeled photos of the posterior pole as to right eye or left eye. Since the optic nerve enters each eye nasally, a photo of the posterior pole will have the optic nerve to the left of the macula in a photo of the left eye and vice-versa. | |||||
| However, we know
that the following photo is not of the right eye, even though the optic
nerve is to the right of the macula. This is because the macula is
higher than the optic nerve in this orientation. The photo is of
the left eye posterior pole, it just needs to be flipped over.
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An interesting anatomical feature of the retina is that the cell structure is laid down backwards from what you might think would be logical. Refer to the diagram that follows.
Think of the RPE layer as the basement of the retina. Notice that light has to travel through the inner neuron layer to get to the light sensitive rods and cones. This of course means that these inner layers of the retina must be clear. (Yes, "ganglion" is spelled wrong in the picture above.) |
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| When you look at a color photo of the retina, it is really the pigment epithelial layer that you are seeing through the clear inner layers of the retina. The retinal arteries and veins branch out from the optic nerve and travel over the top of this inner neurosensory layer. |
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If you look at a stereoscopic fluorescein angiogram, you will notice that the retinal vessels appear to be floating above the RPE below. When the ophthalmologist exams the patient with the intense light of the indirect ophthalmoscope, some patients will comment that they can see the pattern of the retinal blood vessels, like a tree branching in their vision. This perception arises from the shadow of the retinal vessels being "projected" onto the rods and cones below as the intense light creates a washed out, monochromatic visual background. The photoreceptor cells are of two types: rods and cones. These light sensitive cells have photo-chemicals that change in nature when exposed to light in the 400 to 700 nm wavelength range. The chemical change is converted to an electrical impulse that travels along the visual pathway to the cerebral cortex where vision is perceived. Rods far outnumber cones, approximately 20 to 1. This is because the 130 million rods are scattered over the peripheral retina, which makes up the vast majority of the surface area of the retina compared to the small area of central vision in the macula. Rods only perceive in black and white, and they are most sensitive to the blue end of the spectrum. Despite the high number of rods, the concentration of rods per square millimeter of peripheral retina is low compared to the concentration of cones in the macula. This means that the acuity of the peripheral retina is very low, less than 20/400. Try looking at a target straight ahead. Hold some large type reading material about 30 degrees away from your central vision. It is impossible to read. That's why patients with macular degeneration have such a hard time. While holding that object out there, also notice that the peripheral retina can perceive color. This is because there are some cones scattered among the rods. One thing that the retina is good at is perceiving motion. Once the peripheral retina senses motion, the brain reflexively directs the eyes to rotate so that the image of the moving object falls on the macula. The technical term for this is "pursuit". The rods are very sensitive to low light levels, but not instantly. The dark adaptation process, which involves the chemical rhodopsin, is 80 percent complete after 30 minutes in the dark. As light dims and the eye converts from photopic (bright illumination) vision to scotopic (dim illumination) vision, the eye becomes less sensitive to colors at the "warm" end of the visual spectrum (cone sensitivity), and it becomes more sensitive to light at the "cool" end of the spectrum (rod sensitivity). This is known as the Purkinje shift. The shift is so gradual as to be barely noticeable, but when outside, pay attention to the relative intensity of colors as evening falls. Vitamin A is essential for rod metabolism. A normal diet gives most people the needed amount of vitamin A. Retinitis pigmentosa is an inherited disease that slowly atrophies rod function until the patient is only left with only cone function and tunnel vision. Massive doses of vitamin A have slowed the degenerative process in some of these patients. |
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| Pigment degeneration and pigment clumping of a retina suffering from retinitis pigmentosa. |
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Cones are color sensitive and are concentrated in the macula. The high visual resolution of the macular area is aided by the one-to-one connection between cones and bipolar/ganglion cells in this region. In the periphery, each bipolar/ganglion cell transmitter is hooked up to more than one rod cell. There are three different kinds of cone cells: red light sensitive, blue light sensitive, and green light sensitive. All other shades of coloration are made up of a combination of these three colors. Persons with normal color vision are termed trichromats, they have normal function with all three types of cone pigments. A person missing one type of cone pigment would be a dichromat (3% of the population), and a person having only one type of cone pigment would be a monochromat (extremely rare). Most persons with "color blindness" are really color deficient. This would be termed an anomalous trichromat, meaning one of the cone types functions below normal. The most common type of deficiency is a congenital red/green deficiency that occurs in varying degrees in about 9 percent of the male population and in only about .5 percent of the female population. Society has accommodated these individuals by always putting the red traffic light above the yellow and green lights and by making traffic signs different shapes as well as different colors. Color vision deficiencies can also be acquired, meaning some external agent causes a decrease in cone function. Glaucoma, optic neuropathy, macular degeneration, and cataracts are known to cause changes in color vision. Plaquenil (chloroquine) has been shown to be toxic to the photoreceptors at doses above 400mg a day. The drug is used to treat rheumatoid arthritis and lupus. Some patients exhibit symptoms of toxicity at the usual dosage level of 200mg BID. Changes in color vision serve as an early warning of damage to the retina. A decrease in retinal sensitivity and visual acuity can follow within months of changes in color vision if the Plaquenil dosage is not reduced or discontinued. Color vision tests for clinical use come in two types: color plate tests and hue tests. An anomaloscope is used for color vision research. |
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Ishihara, H-R-R, SPP, and other pseudoisochromatic plates are color plate tests that consist of a series of plates with random dots that contain a figure or symbol that is visible to a person with normal color discrimination, but it is invisible or different to a person with a color deficiency. |
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How color
testing plates work (this is an animation, you will need to view it
on a computer for the effect):
A number or symbol is formed by coloring same size dots in the color shade being tested. A background is formed by coloring the remaining dots a different color, with varying degrees of color separation. All of the dots form a pattern with a consistent level of shading. If the color differential is removed, as when viewing the dots in back-and-white, the dots that make up the number blend into the pattern and the number disappears. |
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| Color plate tests are clinically useful for screening purposes only. All will tell you if there is normal color perception or if there is a red-green deficiency. Most will classify according to protan (red-green confused) or deutan (red-green confused with milder blue-yellow defect). Some will also test for blue-yellow deficiency (tritan). A study by Ichikawa and Hukami has shown that color plates cannot reliably estimate the degree of the defect, even though some tests attempt to do so. | ||||||
| Hue Tests | ||||||
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The Farnsworth D-15, Farnsworth-Munsell 100, and the PV-16 are hue tests that consist of colored caps (the number in the name refers to the number of caps in the test). The caps are mixed in random order and the patient is asked to place them in smooth order according to hue. Results are recorded on a chart, and a scoring process reveals the type and the degree of the defect. | |||||
| The D-15 and PV-16 are relatively quick and they effectively screen patients with abnormal color vision. They only give a general indication of the severity of the defect. The 100 hue test is complicated and time consuming, but it provides a more detailed analysis. | ||||||
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Illumination — The plates are intended for natural daylight illumination. There are special bulbs that give the proper color temperature. Special glasses can be used with incandescent illumination, or a daylight type fluorescent light can be used. Distance — The plates should be viewed at about 30 inches. Time — Allow a maximum of 3 seconds viewing time. Eyeglasses — Patients needing glasses for good acuity at 30 inches should wear their glasses. Tinted glasses should not be worn. General procedure — The patient is asked to identify the symbol(s) or figure(s). Small children can be asked to trace the figures with a brush. Wrong answers are recorded on a scoring sheet and the results are computed. Follow the instructions included with the particular test. Do not allow fingers to touch the plates, and protect the plates from prolonged exposure to light. The Retinal Neural Transmission Layer Between the rods and cones and the nerve fiber layer is a group of cells responsible for the transmission of the nerve impulse from the rods and cones to the nerve fibers that lead into the optic nerve head.
This transmission layer includes bipolar, amacrine, horizontal, Mueller, and ganglion cells. The axons of the ganglion cells form the optically clear nerve fiber layer. Other axons in the body have an insulating sheath called myelin that is opaque. If the axons in the retina had myelin, light would not be able to get through to the rods and cones. The axons in the NFL are not normally medullated (myelinated) for this reason. Rarely, a patient will be born with a section of myelinated nerve fibers. This condition may create a small visual defect, but it needs no treatment. |
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| The nerve fibers branch out over the retina in a characteristic pattern that affects the pattern of visual field defects. For more information, see Module 11. |
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The Retinal Pigment Epithelium (RPE) The RPE lies beneath the neurosensory retina. This layer of concentrated pigment cells is what gives the retina coloration. Those people with darker skin color also have a darker color to the retina. A healthy RPE acts as a kind of gatekeeper between the retina and the underlying vascular choroid. Nutrients and waste are allowed to pass through and other substances are not. |
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The central retinal artery and the central retinal vein branch from the ophthalmic artery and come into the eye through the optic nerve head. Each then branches from the optic nerve head to serve four main quadrants of the retina. These vessels provide the blood supply for the inner two thirds of the retina. | |||||
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The outer third of the retina is served by the blood vessels of the
choroid. The fovea has no overlying blood vessels, so that no
light is blocked over this small, highly sensitive area of central
vision.
A fluorescein angiogram showing the capillaries in the macula demonstrates the avascular nature of the fovea. The capillaries in this photo are damaged by diabetes. |
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The retina is commonly viewed with a direct ophthalmoscope, with an indirect ophthalmoscope, or with a fundus camera. The retina can be imaged, and abnormalities can be measured with B-scan ultrasonography. Retinal vasculature and blood flow can be evaluated with fluorescein angiography. Retinal function is commonly tested with visual acuity measurements, visual field testing, and color vision testing. Retinal function can also be tested with electrophysiology testing. Electrophysiology testing includes electro-retinography (ERG), electro-oculography (EOG), and Visual Evoked Response (VER). These tests involve placing electrodes at strategic points on the head and measuring difference in electrical potential as the eyes are stimulated visually and kinetically.
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| This material is continued in Part 2, Module 24, but there is a Post-Test for each Module. | ||||||
| Back to Top Get Post-Test | ||||||
| Go to Module 24 | ||||||
