In some of these retinal diseases, minimal changes in the pigment epithelium or other layers of the retina may not be readily detected by ophthalmoscopy. A test to expose such subtle retinal changes is to estimate the time required for recovery of visual acuity following light stimulation (macular photostress test). The test is conducted by shining a strong light through the pupil of an affected eye for 10 s and measuring the time necessary for the acuity to return to the pretest level (normally 50 s or less). With macular lesions, recovery time is prolonged, but with lesions of the optic nerve, it is not affected. As described earlier, this phenomenon may also be observed in the eye on the side of a carotid occlusion, which in essence causes an ischemic retinopathy. Retinal diseases reduce or abolish the electrical activity generated by the outer layers of the retina, and this can be measured by the electroretinogram (ERG). Fluorescein retinography and various new imaging tests are now essential for proper diagnosis of retinal disease. OCT uses reflected light to construct a high-resolution two-dimensional image of the retinal layers; it is able to demonstrate with remarkable resolution retinal edema, tears, macular holes, and the thinning of the retinal nerve-fiber layer that follows optic neuropathy.
Age-related macular degeneration This is a frequent cause of visual loss in the elderly. As ARMD begins to disturb vision, the straight lines on the Amsler grid are observed by the patient to be distorted. Over time, central vision gradually diminishes, impairing reading, but these patients can navigate because of retained peripheral vision. Examination discloses a central scotoma with pigmentary changes in the region around the macula. The two most common types of macular degeneration are an atrophic “dry” type, which is a true pigmentary degeneration associated with retinal drusen, of unknown cause but with a genetic component, and an exudative “wet” type, which is the result of choroidal neovascularization that results in secondary macular damage. The wet form is amenable to laser treatment and injection into the orbit of ranibizumab or similar antiangiogenic monoclonal antibodies against vascular endothelial growth factor. Progression of the dry form may be slightly reduced by the use of antioxidants and zinc. The pathophysiology and treatment of ARMD have been reviewed by DeJong.
Diabetic retinopathy Although not strictly speaking a problem taken up by neurologists, this is such an important cause of reduced vision and blindness that the basic facts should be known to all physicians. The earliest changes are of microaneurysms, and tiny intraretinal hemorrhages; these are present in almost all diabetics who have had type 1 disease for more than 20 years. Cotton-wool spots and small hemorrhages appear as the retina becomes ischemic. Subsequently, there is a more threatening proliferative retinopathy that consists of the formation of new blood vessels, and consequent leakage of proteins and blood. The proliferative feature occurs in half of type 1 diabetics, and 10 percent of those who have had type 2 disease for 15 to 20 years. The new vessels can grow into the vitreous, and hemorrhages from them may cause traction on the retina, which results in detachment. Visual loss may also be the result of macular edema. Reabsorption of the edema leads to the deposition of lipid “hard exudates.” The maintenance of glucose control reduces the frequency and severity of retinopathy but does not prevent it. Locally elevated levels of vascular endothelial growth factor have been shown to be involved in the pathophysiology of diabetic retinal neovascularization, and recent studies show that intravitreal injections of the antivascular endothelial growth factor (anti-VEGF) antibody, bevacizumab, can improve neovascular leakage, at least in the short term. The review of the subject by Antonetti and colleagues is recommended.