Brief Overview

Eye Floaters No More

Cure Eye Floaters Naturally

Get Instant Access

Macular degenerations are a phenotypically and geno-typically heterogeneous group of blinding disorders characterized by central vision loss associated with RPE

atrophy with or without choroidal neovascularization. Of these, age-related macular degeneration (AMD) is a degenerative disorder of the cone-rich macular and perimacular regions of the retina with resulting loss of central visual acuity. Although AMD principally affects the supporting and metabolic structures of the retina including the retinal pigment epithelial (RPE) cells, the choriocapillaris, and Bruch's membrane, vision loss comes from the resulting retinal atrophy and its associated photoreceptor dysfunction (see Figure 68.3). Visual dysfunction is made worse by neovascularization, the ingrowth of choroidal vessels through defects in Bruch's membrane, with secondary hemorrhage, and retinal detachment that characterize the ''wet'' form of AMD. This is contrasted to the ''dry'' or nonneovascular form, which comprises 80% of the disease but results in only roughly 20% of its associated blindness. Drusen, small yellow-white deposits below the retina, are increased in individuals with AMD. Although they do not cause visual loss by themselves, drusen represent a risk factor for development of both the geographical atrophy (dry) and neovascularization (wet) types of AMD, especially when they are soft or indistinct. Recent results from the Age-Related Eye Disease Study suggest that the incidence of AMD could be lowered significantly by diet supplementation with high-dose antioxidant vitamins and zinc.

The clinical terms dry and wet typically are used to refer to different forms of AMD, with the dry form sometimes progressing to the wet form. Early stages of the dry form are characterized by focal pigmentation and accumulation of drusen between the RPE and Bruch's membrane. In later stages, the wet form is characterized by choroidal neovascularization, detachment of the RPE, and geographic atrophy of the RPE in the macular region. Drusen are classified as hard and soft, based on their shape, diameter, and color. Hard drusen are yellowish,

Figure 68.3 Histological section of the retina showing macular degeneration. Although the ganglion cell layer (GCL), inner nuclear layer (INL) and choroid are well preserved, the outer nuclear layer, which should appear similar to the INL, has been in large part replaced by fibrovascular choroidal neovascularization (CNV). Courtesy of Dr. Chi Chao Chan, National Eye Institute, National Institutes of Health.

Figure 68.3 Histological section of the retina showing macular degeneration. Although the ganglion cell layer (GCL), inner nuclear layer (INL) and choroid are well preserved, the outer nuclear layer, which should appear similar to the INL, has been in large part replaced by fibrovascular choroidal neovascularization (CNV). Courtesy of Dr. Chi Chao Chan, National Eye Institute, National Institutes of Health.

smaller (with diameters of less than 50 ^m), and less likely to progress to later stages of the disease. Soft drusen are larger, dark yellowish in color, and more likely to be associated with more advanced stages of the disease. In later stages of AMD, choroidal neovascularization and leakage of serous fluid into the subretinal (occult CNV) or intraretinal (classical CNV) regions leads to cell death and detachment of the RPE. Visual acuity is significantly affected when geographic atrophy of the RPE takes place in the fovea.

Epidemiology of macular degeneration AMD has a multifactorial (or complex) etiology with contributions from a combination of environmental and genetic factors and a strong age effect. The prevalence of AMD increases dramatically with age, although the prevalence cited in various reports is highly dependent on the definition used for AMD. Overall, AMD increases from less than 1 to 2% at 50 years of age to as high as 15% at 90 years old. It has been suggested that increased skin pigmentation tends to protect from AMD, and this correlates to lower prevalence of AMD in African derived populations than Caucasians in some, but not all, studies. Various other risk factors may predispose to AMD including systemic hypertension and atherosclerosis, as well as cigarette smoking. Both photo-oxidation and inflammation have been suggested as possible pathogenic mechanisms for AMD, although the precise mechanism through which these result in disease has not been delineated.

Genetic factors have been implicated in AMD by epidemiological studies including twin studies and formal segregation analyses (Heiba et al., 1994; Hammond et al., 2002; Seddon, Ajani, and Mitchell, 1997). First degree relatives of individuals with AMD appear to have a two-to four-fold increased incidence of AMD over control individuals without a family history of AMD. Twin studies suggest that concordance for AMD in mono-zygotic twins is approximately twice that in dizygotic twins. Formal segregation analysis suggests that there is a major gene effect accounting for approximately 60% of AMD with a single major gene accounting for about 55% of AMD risk. Overall, these data suggest that the etiology of AMD has a significant genetic component.

Was this article helpful?

0 0
Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

Get My Free Ebook


Post a comment