Ординатура / Офтальмология / Английские материалы / Primary Care Ophthalmology_Palay, Krachmer_2005
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150 CHAPTER 10 • Retina
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FIGURE 10–1 Normal fundus. A normal, pink optic nerve, cup-to-disc ratio of 0.1, normal arterial-to-venous diameter ratio of 7:10, and a cilioretinal artery (1) extending from the temporal border of the optic nerve and inferior to the fovea can be seen. Note that the branching points of the blood vessels “point” (2) toward the optic nerve. The foveolar reflex (3), fovea (4), and macula (an area approximately 5.5 mm in diameter, centered on the fovea) (5) also can be seen.
diameter. The arteries appear thinner and more orange-red, whereas the veins are larger and more crimson.
The normal ratio of arterial to venous diameter (A : V ratio) is approximately 7:10 or 8:10 (i.e., the retinal artery diameter is 70% to 80% of the apparent diameter of the adjacent vein). The A : V ratio is best judged by comparing vessel calibers after the initial branch point. As a rule, veins do not cross veins, and arteries do not cross arteries. Veins and arteries travel together in pairs and frequently cross over each other (arteriole-venule crossings).
Expertise with a direct ophthalmoscope is an important skill for a primary care physician to master. When the examining physician asks the patient to look directly into the light of the direct ophthalmoscope, the area with a slightly darker orange pigmentation and an absence of retinal vasculature is the fovea. Typically, this anatomic region is located mostly temporal and slightly inferior to the optic nerve. The small yellow reflex in the center of this area is the foveolar light reflex; it usually is present on examination in the normal eye. In children and young adults, a circular reflex of light identifies the foveal reflex, of about the same size as that of the optic
Related Anatomy |
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nerve. The foveal reflex is formed by light reflecting from the internal limiting membrane of the retina at a circular area of peak thickness formed by the high density of ganglion cells that supply the fovea. The macula is a circular area 5.5 mm in diameter centered on the fovea (generally, the area within the temporal blood vessel arcades adjacent to the optic nerve) and is primarily responsible for central, critical visual function. Peripheral retinal structures and pathologic conditions located anterior to the equator of the globe require an indirect ophthalmoscope, a condensing lens (+20 diopter), and additional expertise.
The normal retina and blood vessel walls normally are transparent. The visible retinal vessel is actually an arterial or a venous “blood column.” The normal orangered color of the red reflex is produced by the vasculature of the choroid, retinal pigment epithelium, and choroidal melanocytes. The retina has two separate blood supplies. The central retinal artery supplies the inner retina, or the retinal layers toward the center of the eye. The outer retina, or the retinal layers toward the outer wall of the eye, is supplied by the highly vascular choroid. The high oxygen requirement of the photoreceptors, located in the outer retina, is provided by the choroid.
The fundus of lightly pigmented patients has a “blond” appearance, with readily visible choroidal vessels (Fig. 10–2). The fundus of darkly pigmented patients has a “brunette” appearance, with less apparent choroidal vasculature. Choroidal blood vessels are readily differentiated from retinal vessels. The retinal vasculature follows a typical branching pattern centered on the optic nerve; the choroidal vasculature demonstrates an irregular branching pattern of larger-caliber, poorly defined vessels. Retinal vessels pass anterior to choroidal vessels. A blood vessel that supplies the retinal circulation frequently arises from the choroidal circulation at the optic nerve; this is called a cilioretinal vessel (see Fig. 10–1). This vessel arises from the short posterior ciliary vasculature, rather than the central retinal artery, and offers an “accessory” supply to the distribution of the vessel.
A basic understanding of the cross-sectional anatomy of the retina helps in the identification of important pathologic states (Fig. 10–3). The inner retina contains the
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FIGURE 10–2 Fundus of eye in an albino with the absence of ocular pigmentation. The choroidal blood vessels (1) are readily visible. These are differentiated from the normal
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retinal vessels (2) by their size and branching pattern. Normal |
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retinal vessels are seen emanating from the optic nerve and |
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crossing over the choroidal vessels. |
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152 CHAPTER 10 • Retina
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Vitreous |
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Nerve fiber layer |
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with retinal vessels |
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Inner nuclear |
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FIGURE 10–3 Cross-sectional |
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schematic anatomy of |
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the retina and choroid. |
Photoreceptors |
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Retinal pigment |
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epithelial cells |
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Bruch's membrane |
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Choroid and vessels |
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Sclera |
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FIGURE 10–4 Multiple cotton-wool spots (1) in the perifoveal region in a young man with disseminated microemboli after bone marrow transplantation. Ischemia of the perifoveal capillaries is responsible for the cotton-wool spots. Multiple flame-shaped hemorrhages (2) in the nerve fiber layer also are present. The arcuate, whitish, radiating striae extending from the optic nerve next to the retinal vasculature (3) rep-
resent the light reflex from the normal nerve fiber layer.
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nerve fiber layer and ganglion cell nuclei that extend their terminal axons through the optic nerve and chiasm to synapse primarily in the lateral geniculate nucleus of the brain, with some fibers extending to the midbrain to subserve the pupillary light reflex. The inner retinal layer also contains the retinal vasculature, originating from the central retinal vessels. Conditions that result in ischemia from the retinal vasculature manifest primarily in the nerve fiber layer and are seen clinically as opacification of the inner retina. Areas of axoplasmic stasis of the nerve fiber layer are seen clinically as fluffy white patches, also known as cotton-wool spots (Fig. 10–4). These whitish lesions follow the distribution of the nerve fiber layer. In the past, these lesions have been described as “soft exudates”—a misnomer, because a cotton-wool spot represents an infarction rather than an exudation.
Flame-shaped hemorrhages also occur in the nerve fiber layer. If a retinal vessel bleeds into the inner retinal layer, the blood intercalates with the nerve fibers in the manner of red paint spilled on the fibers of split wood, giving it a flame-shaped appearance. Pathologic conditions located in the deeper layers of the retina are more
Proliferative Diabetic Retinopathy |
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localized circular hemorrhages with fuzzy or blurry margins. These “dot and blot” hemorrhages commonly are seen in patients with diabetes mellitus or hypertensive retinopathy. Localized, yellow lipid exudates, or “hard exudates,” form in the outer layers of the retina and remain more circular in contour and well localized.
Frequently, fluorescein angiography and, less commonly, indocyanine green angiography are used in the diagnosis and treatment of retinal disorders. Both dyes are extremely safe, water-soluble dyes that appear white or hyperfluorescent in photographs taken during dye circulation through the retinal and choroidal vasculature. Although fluorescein best images the retinal vasculature, indocyanine green is able to better image the choroidal vasculature. The differences between these dyes relate primarily to their light absorption and emission properties. A detailed examination at the capillary level is possible using these angiographic techniques. A disruption of the “blood-retina barrier” (analogous to the blood-brain barrier with intercellular tight junctions) results in leakage of the dye into the retina, which then appears irregular in the angiogram. In addition, areas of nonperfusion or ischemia appear dark or hypofluorescent on the angiogram.
Diabetic Retinopathy
Diabetic retinopathy is the leading cause of blindness in the Western world in people youger than 50 years of age. The current standard of care is for all patients with diabetes to be evaluated by an ophthalmologist annually. Patients with newly diagnosed type 2 diabetes mellitus should be scheduled for a baseline evaluation within weeks of diagnosis and should establish a long-term relationship with an ophthalmologist. The duration of type 2 diabetes mellitus is frequently indeterminate, and many patients will demonstrate some level of diabetic retinopathy at the time of diagnosis. Ophthalmologic referral is therefore more urgent.
Patients with type 1 diabetes mellitus are unlikely to have retinopathy at the time of diagnosis, and an ophthalmologic examination should be scheduled within 5 years of diagnosis and then annually. Although some studies have argued that cost effectiveness is optimized with screening every 2 years, annual examination is still recommended (“preferred practice patterns” of the American Academy of Ophthalmology).
Progression of diabetic retinopathy may accelerate during periods of strong hormonal influence such as pregnancy and puberty, so clinicians are advised to monitor patients closely during such periods. Most commonly, patients with diabetes have irreversible vision loss as a result of secondary complications that arise from injury to the retinal vasculature. Cataracts also may occur in the setting of diabetes and may be more challenging to manage than in eyes without diabetic retinopathy. The basic forms of diabetic retinopathy are proliferative diabetic retinopathy, diabetic macular edema, and macular ischemia.
Proliferative Diabetic Retinopathy
Proliferative diabetic retinopathy results from retinal ischemia. As perfusion to the retina is compromised, ischemic retinal tissue releases an angiogenic factor (vascular endothelial growth factor [VEGF]) that in turn stimulates abnormal new vessel growth,
154 CHAPTER 10 • Retina
FIGURE 10–5 Fronds of neovascularization (1) on the disc are |
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present in this right eye. Inferior and temporal, a cotton-wool |
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spot has an adjacent hemorrhage. Native retinal arteries are |
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narrowed and show evidence of sclerosis (2). |
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FIGURE 10–6 Dilated pupil of a patient with poorly controlled diabetes displaying advanced rubeosis with neovascularization of the iris. The vessels on the surface of the iris may occlude the normal drainage of aqueous fluid from the eye, leading to a severe form of neovascular glaucoma.
or neovascularization. VEGF also increases the permeability of retinal vessels and results in leakage and macular edema. Panretinal ischemia results in neovascularization emanating from the optic nerve or disc and is termed neovascularization of the disc (NVD), a high-risk condition (Fig. 10–5). Neovascularization may occur at any location elsewhere in the retina (NVE) but typically occurs along the vascular arcades. Neovascularization also may occur on the surface of the iris (NVI), imparting a redbrown color. The resultant condition is known as rubeosis iridis; these changes may lead to neovascular glaucoma, a severe form of glaucoma (Fig. 10–6). Almost any form of retinal neovascularization may be dangerous because the vessels grow into the vitreous gel, instead of providing needed vascularization of the retina. Movement or traction of the vitreous gel may cause shearing of these fragile vessels, leading to vitreous hemorrhage. Recurrent vitreous hemorrhage will lead to the formation of contractile,
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FIGURE 10–7 A large white sheet of fibrovascular tissue is present on the surface of the retina in this eye of a patient with diabetes. Contraction of the tissue distorts the retinal vessels and tractionally detaches the retina. The yellow substance deep to the fibrovascular tissue (1) is lipid, which results from chronic exudation of incompetent vessels. Surgical intervention (vitrectomy) is needed to remove the fibrovascular tissue and reattach the retina.
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FIGURE 10–8 Proliferative diabetic retinopathy with high-risk |
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characteristics. Neovascularization is present at the optic |
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nerve (1) and along the vascular arcades (2). Retinal veins are |
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engorged (3), and a preretinal hemorrhage (4) is present infe- |
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rior to the fovea. This boat-shaped hemorrhage blocks the |
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view of the retinal vessels. A more diffuse hemorrhage (5) is |
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present in an arcuate pattern just inferior to the preretinal |
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hemorrhage that represents a mild vitreous hemorrhage. A |
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few small hard exudates are visible in the fovea (6). |
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fibrous scar tissue that may pull the retina toward the center of the eye, producing fractional retinal detachments (Fig. 10–7). Neovascularization can be differentiated from normal retinal vasculature by the characteristic growth pattern. New vessels typically are smaller and have more frequent branching points that grow in an irregular, haphazard manner (Fig. 10–8; see also Fig. 10–5).
Symptoms
•Some patients with severe proliferative retinopathy may have 20/20 visual acuity and be unaware of any visual symptoms.
•Vision may become blurry slowly or suddenly.
•Distortion of vision may occur (i.e., things may appear crooked or wavy).
•Floaters, possibly indicating vitreous hemorrhage, may be reported, usually in a pattern described as a “shower.”
•Blind spots in the vision, or scotomata, may be noted by the patient.
156 CHAPTER 10 • Retina
Signs
•Neovascularization, or fine lacy blood vessels, are seen on the optic nerve, retina (see Figs. 10–5 and 10–8), or iris surface (see Fig. 10–6).
•Preretinal hemorrhages are boat-shaped hemorrhages that may be located anterior to the retinal vessels and block the view of these underlying vessels (see Fig. 10–8).
•Cotton-wool spots often are present (see Fig. 10–5).
•Venous beading, dilation, or engorgement is present.
•Dot and blot intraretinal hemorrhages are a common finding.
•Loss of the red reflex and resulting inability to view the fundus are possible with a vitreous hemorrhage.
•Areas of traction retinal detachment may be observed (see Fig. 10–7).
•Whitish fibrovascular tissue on the retinal surface may be seen in a distribution along the vascular arcades and above the optic nerve.
Differential Diagnosis
Considerations in the differential diagnosis include the following:
•Diabetes
•Vascular occlusions: central or branch retinal vein or artery occlusions
•Radiation retinopathy may appear identical to diabetic retinopathy
•Blood dyscrasias: sickle cell retinopathy (especially in patients with SC hemoglobin), anemias, leukemias, thalassemias, and hyperviscosity syndromes
•Retinal emboli
•Carotid disease, aortic arch syndrome, and carotid artery–cavernous sinus fistula
•Uveitis (e.g., sarcoidosis)
Workup
•To confirm the diagnosis of diabetes mellitus, fasting blood glucose level, oral
glucose tolerance test, and hemoglobin A1C are appropriate tests. If the diagnosis is confirmed, assessment of urine microalbumin by 24-hour urine protein assessment or random urine for albumin-to-creatinine ratio should be performed to evaluate for diabetic nephropathy.
•To address other possibilities in the differential diagnosis consider: complete blood count (CBC) with differential count, serum protein electrophoresis, angiotensinconverting enzyme level, chest x-ray, or vascular imaging studies (carotid ultrasound and echocardiography).
Treatment
•Note: The Diabetes Control and Complications Trial (DCCT) has shown that “tight” glycemic control decreases the progression of diabetic retinopathy, nephropathy, and neuropathy. The primary care physician’s role is therefore critical in decreasing the incidence of vision loss from diabetic retinopathy in cases of both proliferative retinopathy and diabetic macular edema.
•Diabetic patients with neovascularization should be promptly referred to an ophthalmologist.
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•Retinal laser photocoagulation destroys the peripheral retina and decreases the release of vasoproliferative growth factors from ischemic retinal tissue. Photocoagulation also creates multiple choroid-to-retina adhesions that limit the progression of a tractional retinal detachment.
Follow-up
•To minimize the risk of anticoagulation, the following considerations are important:
The guidelines for anticoagulation regimens in patients at risk for coronary artery disease, stroke, or other conditions are not altered by the presence of proliferative diabetic retinopathy. The risk of vitreous hemorrhage is no greater during anticoagulation therapy. However, a hemorrhage that occurs during anticoagulation may be more severe. Although many hemorrhages clear with time, others will require vitrectomy surgery. Anticoagulation recommendations generally should not change because of retinopathy, especially in life-threatening conditions requiring anticoagulation medication.
Ophthalmologic consultation is indicated to monitor the diabetic retinopathy when the patient is medically stable. For example, a diabetic patient taking anticoagulants for unstable angina is best seen by an ophthalmic consultant after appropriate cardiac care, rather than being seen first by the ophthalmologist to rule out proliferative diabetic retinopathy while anticoagulation therapy is delayed. Even if proliferative diabetic retinopathy is present, ophthalmologic treatment is deferred until the patient is medically stable.
Diabetic Macular Edema
The earliest detectable clinical alteration in the vasculature of a patient with diabetes is the formation of small 50-mm red dots on the retinal vasculature or microaneurysms (Fig. 10–9). Generally, the retinal vascular endothelium has “tight junctions” that form the inner blood-retina barrier, analogous to the blood-brain barrier. The microaneurysms represent proliferations of endothelial cells with increased permeability and an incompetent blood-retina barrier. Microaneurysms leak intravascular serum into the retinal tissue. The fluid may accumulate in the foveal area, leading to edema formation with decreased visual acuity. The proteinaceous and lipid portion of the serum may accumulate, leading to the formation of exudates (see Fig. 10–9). Alternatively, decreased visual acuity may result from ischemia or lack of perfusion to the fovea, usually due to loss of the capillaries in the foveal region.
Symptoms
•No symptoms may be noted. Diabetic patients may have normal vision and edema requiring laser treatment; therefore, all of these patients need ophthalmologic screening.
•Vision may be blurry unilaterally or bilaterally.
•Distorted or wavy vision may be described.
158 CHAPTER 10 • Retina
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FIGURE 10–9 Clinically significant macular edema in this eye of an insulin-dependent diabetic patient. Multiple small red dots (microaneurysms) (1) are present throughout the macular area. Linear streaks of yellow deposits or hard exudates
(2) are accumulations of lipid material from chronic leakage of the microaneurysms. Loss of the normal foveal light reflex with a more prominent yellow spot centrally suggests the presence of macular edema.
Signs
•Graying or a slight opacification of the retina results from edema in the macula.
•Microaneurysms usually are adjacent to retinal veins but may be found at any location in the retina.
•Dot and blot intraretinal hemorrhages occur.
•Cystoid changes or yellowing in the fovea may be observed.
•Hard exudates are dense yellowish lesions with discrete borders that may be isolated, linear, or stellate or form a “circinate ring” around leakage sites.
Differential Diagnosis
Considerations in the differential diagnosis include the following:
•Diabetes
•Vascular occlusions: central or branch retinal vein occlusions
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•Hypertensive retinopathy
•Radiation retinopathy
•Choroidal neovascularization (subretinal neovascularization, usually associated with age-related macular degeneration [AMD])
•Macroaneurysms (larger retinal aneurysms associated with systemic hypertension)
•Epiretinal membrane
Treatment
•Note: As mentioned previously, the DCCT has shown that tight glycemic control decreases the progression of diabetic retinopathy. Therefore, the actions of the primary care physician are critical in decreasing the incidence of vision loss from diabetic macular edema.
•Patients should be referred to an ophthalmologist.
•Laser photocoagulation (based on Early Treatment of Diabetic Retinopathy Study [ETDRS] findings) is performed to decrease the further decline in visual acuity by reducing the amount of macular edema.
Hypertensive Retinopathy
Systemic arteriolar hypertension may affect the retinal vasculature in several ways. The traditional classification systems of hypertensive retinopathy were designed in the 1930s to 1950s and predate the current antihypertensive regimens now used to manage patients with hypertension. For this reason, the retinal vascular changes described in those classification systems are important to understand. Such classification systems rarely are used today, however, and the best measure of systemic arteriolar hypertension is the blood pressure cuff reading. Nevertheless, the appearance of hypertensive retinal vascular changes is a good indicator of “end-organ” damage and may give the clinician valuable information about long-term compliance with medication regimens or the effects of preexisting untreated damage from hypertension. Autoregulatory mechanisms are present in the retinal vasculature that limit the ability of a clinician to detect short-term or acute elevations in systemic arteriolar blood pressure.
The hallmark of hypertensive retinopathy is diffuse arteriolar narrowing. As stated earlier, the blood vessel walls are virtually transparent in the normal retina. The examiner sees the blood column, rather than the vessel wall. Chronic hypertension results in thickening of the vascular wall, with a concomitant narrowing of the vessel lumen and an apparent attenuation of the vessel. The normal A:V ratio of 0.8 (artery caliber 80% of the vein caliber) may change to 0.6 or less in patients with chronic hypertension. “Copper-wire vessel” is a dated term used to describe the yellowing of the linear light reflex seen on the surface of a narrowed arteriolar vessel using the illumination source of the older-style ophthalmoscopes. With the halogen light sources in current use, apparent color changes are minimized. “Silver-wire vessel” is a term used to describe a white retinal vessel with a minimal blood column or without a visible blood column. This term reflects the chronic changes that occur in the vessel wall that makes them less transparent. The ophthalmoscopic entities are more appropriately referred to as arteriolar narrowing (copper wire) and sclerosis of the vessels (silver wire).