Retinal Disorders

Retinal disease is a major cause of blindness and morbidity worldwide, accounting for at least 15 percent of blindness. Precipitating factors include vascular disease, infection, malnutrition, and genetic mutations. Many retinal diseases can be treated without residual visual defects, yet treatment delay, poor access to eye care, lack of prevention education, and poverty lead to preventable vision loss.

Age-related macular degeneration (AMD) accounts for 8.7 percent of blindness worldwide, making it the leading cause of vision loss from posterior segment disease. AMD, manifesting most often in the sixth decade and progressing with age, results from the photoreceptor dysfunction in the macula. The dry subtype is more common, caused by accumulation of yellow deposits known as drusen that interfere with processing fine visual acuity. The pathophysiology involves vascular disease and cellular defects of the retinal pigment epithelium. Surgical therapies can stabilize disease progression, but no cure currently exists. As the global population ages, morbidity due to AMD will warrant increased attention.

Diabetic retinopathy, a chronic complication of diabetes mellitus (DM), is the second leading cause of blindness of retinal origin, contributing to 4.8 percent of cases globally. In nonproliferative retinopathy, the retinal microvasculature develops aneurysms and obstruction to blood flow. As the retina becomes starved of nutrients, neovascularization occurs, which characterizes proliferative retinopathy. These naive vessels tend to leak and produce macular edema, which can produce irreversible vision loss, even in the absence of symptoms. Yearly screening is recommended to detect early disease.

In contrast to the above chronic diseases, central retinal artery occlusion (CRAO), a rare vascular cause of retinal blindness, often presents as a medical emergency. Risk factors include hypertension, DM, and carotid artery disease. The most common etiology is carotid thrombus embolization. Complete vision loss in the respective eye is inevitable if a patient fails to seek medical attention within one hour. Retinal detachment is another medical emergency that occurs secondary to trauma, infection, anatomical defects, and intraocular fibrosis. Central retinal vein occlusion (CRVO) is less emergent but can become complicated by neovascular glaucoma, ischemia, and blindness.

The most significant cause of preventable blindness due to retinal infection is Onchocerca volvulus, a parasite transmitted by black flies indigenous to sub-Saharan Africa, Yemen, and Latin America. [Page 1488]Commonly known as river blindness, onchocerciasis afflicts populations that inhabit regions near streams or rivers. The microfilariae nonselectively destroy eye structures, including the retina. Ivermectin effectively eradicates onchocerciasis, thereby averting devastating complications. Patients with advanced human immunodeficiency virus (HIV), especially those with a CD4 count of 100 cells/mL, are prone to developing CMV retinitis. Occurring in 25 percent of AIDS cases, CMV retinitis leads to visual acuity defects, loss of peripheral vision, and ultimately blindness. Maintaining adequate CD4 counts can prevent CMV retinitis, and antiviral agents such as Valganciclovir and Foscarnet are proven effective therapies.

Treatment delay, poor access to eye care, lack of prevention education, and poverty can lead to preventable vision loss.

Vitamin A deficiency is the most common dietary source of retinal disease. The prevalence is most alarming in sub-Saharan Africa, India, and South America. By impairing rhodopsin production, low vitamin A yields night blindness clinically. A number of landmark studies have further demonstrated that vitamin A supplementation remarkably reduced child mortality. Hence, the World Health Organization, United Nations Children's Fund, and the Vision 2020 initiative have embarked on a global campaign to distribute vitamin A supplements.

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