In the lens, cataract formation is a common abnormality characterized by clouding that usually develops slowly. The mundane “senile” cataract occurs because lens proteins denature and degrade over time. The “sugar cataract” of diabetes mellitus is the result of sustained high levels of blood glucose, which changes in the lens to sorbitol that causes a high osmotic gradient with swelling and disruption of the lens fibers. Galactosemia is a much rarer cause, but the mechanism of cataract formation is similar, that is, the accumulation of dulcitol in the lens. In hypoparathyroidism, lowering of the concentration of calcium in the aqueous humor is in some way responsible for the opacification of newly forming superficial lens fibers. Prolonged high doses of corticosteroids, as well as radiation therapy, induce lenticular opacities in some patients. Down syndrome and oculocerebrorenal syndrome (see Chap. 37), spinocerebellar ataxia with oligophrenia (see Chap. 38), and certain dermatologic syndromes (atopic dermatitis, congenital ichthyosis, incontinentia pigmenti) are also accompanied by lenticular opacities. Myotonic dystrophy (see Chap. 45) and, rarely, Wilson disease (see Chap. 36) are associated with special types of cataract. Subluxation of the lens, the result of weakening of its zonular ligaments, occurs in syphilis, Marfan syndrome (upward displacement), and homocystinuria (downward displacement).
In the vitreous humor, hemorrhage may occur from rupture of a ciliary or retinal vessel. On ophthalmoscopic examination, the hemorrhage appears as a diffuse haziness of part or all of the vitreous, or takes the form of a sharply defined clot if it lies between the retina and the vitreous (termed preretinal or subhyaloid hemorrhage). The common cause is rupture of newly formed vessels of proliferative retinopathy in patients with diabetes mellitus, but there are many others including orbital or cranial trauma, rupture of an intracranial aneurysm or arteriovenous malformation with high intracranial pressure (Terson syndrome), valsalva maneuvers, retinal vein occlusion, sickle cell disease, age-related macular degeneration (ARMD), and retinal tears, in which the hemorrhage breaks through the internal limiting membrane of the retina. The most common vitreous opacities are benign “floaters” caused by the condensation of vitreous collagen fibers, which appear as darting gray flecks or threads with changes in the position of the eyes; they may be annoying or even alarming until the person stops looking for them.
A sudden burst of flashing lights associated with an increase in floaters may mark the onset of retinal detachment. Patients complaining of bright flashes and spots in vision should be examined with the indirect ophthalmoscope to rule out tears, holes, or detachments of the vitreous or retina. Another common occurrence with advancing age is shrinkage of the vitreous humor and retraction from the retina, causing persistent streaks of light, usually in the periphery of the visual field. These phosphenes, also known as Moore lightning streaks, had been thought to be quite benign, but they may at times indicate incipient retinal or vitreous tears or detachment, and their first appearance requires prompt evaluation by an ophthalmologist. They are most prominent on movement of the globe, on closure of the eyelids, at the moment of accommodation, with saccadic eye movements, and with sudden exposure to dark. The vitreous may also be infiltrated by lymphoma also occurring in the brain; biopsy by planar vitrectomy may be used to establish the diagnosis in those rare instances where the lymphoma is restricted to the eye.