Diagnosing AMD

Brief Description
Epidemiology
Pathological changes
Patient Symptoms
Imaging and other testing
Summary

Brief Description

Age Related Macular Degeneration (AMD) is a disease characterized by loss of central vision with relative preservation of peripheral sight. It can occur in one eye or in both, either contemporaneously or asynchronously. It is a debilitating illness resulting in significant quality of life erosion and dependency.

Epidemiology

AMD is the commonest cause of acquired blindness in the developed world affecting over 10% of individuals attaining 80 years or greater and many more with less advanced disease. Classical AMD is very rare before the age of 50. The prevalence is modest until age 70 and rises steeply thereafter.

Environmental exposures can increase the risk for AMD. Smoking cigarettes increase the odds ratio for disease over 3 fold. Excessive ultra violet light, and previous cataract extraction have been linked to this disease possibly by contributing to retinal oxidative damage.

AMD is primarily a genetic disease, as content elsewhere on this site details. As much as 70% of the risk is inherited. Genes for the complement family of inflammatory molecules, cholesterol biosynthetic enzymes, and oxidate stress-associated proteins are the strongest predictors of risk. Recent tools to measure disease-associated genes have become widely available. 

Pathological Changes

The hallmark of early AMD is the accumulation of cholesterol-containing extracellular inflammatory protein deposits in Bruch’s membrane between the retinal pigment epithelium (RPE) and its vascular supply, the choriocapillaris. These deposits, “drusen”, can become large and confluent, but usually only affect vision when retinal atrophy or bleeding neovascularization are induced. Neovascularization develops as vessels grow toward the photoreceptors. Bleeding, leaking, and scarring eventually causes irreversible damage and rapid vision loss if left untreated.

Patient Symptoms

Eyesight is usually preserved during the early stages of AMD when drusen are the only manifestation of the disease. As the disease advances, the loss of central vision severely affects visual acuity resulting in the perception of a blurred or dark central scotoma. Reading and driving become impossible. There is also loss of contrast sensitivity; making contours, shadows, and colors less vivid.

Imaging and other testing

Ophthalmoscopy permits direct visualization of the retina using a light source and magnification. The diagnosis and monitoring of retinal diseases using an opthalmoscope requires dilation of the pupils by instillation of mydriatic eye drops. Pupilary dilation may result in sensitivity to light, requiring protective eye-wear until normal light response returns.

Digital Fundus Photography

Digital fundus photography records the appearance of the retina and can be useful for documenting temporal change. It is conceptually similar to ophthalmoscopy electronic image. One variation is retinal artery angiography, which permits a detailed study of vessel architecture after the systemic administration of a fluorescent agent followed by the recording of stimulated retinal light emission.

Optical Coherence Tomography (OCT)

OCT is an imaging method that uses ultrasound technology to provide detailed cross-section images of the retina and its underlying layers. OCT is useful for checking retinal thickness since it is capable of clearly displaying well-defined tissue boundaries in high resolution. Images are electronically pseudocolored to highlight specific anatomical detail. This non-invasive and sensitive technology has become a practice standard and is capable of detecting choroidal neovascular disease while still presymptomatic.

Microperimetry

Microperimetry assesses the visual function of a specific area of the retina and fovea, which may be correlated to structural changes that can be identified with an OCT. It provides a quantifiable way to measure the regression or progression of retinal visual function in the examined eye.

New microperimetry devices perform automatic perimetry independent of fixation characteristics. Furthermore, automated eye tracking ensures that followup examinations will quantify retinal thresholds over exactly the same retinal points that are tested during baseline examinations. Differential maps can be created that can be superimposed on high-quality color retinal photographs.

PHP (Preferential Hyperacuity Perimetry)

PHP detects small defects in the visual field. Rather than measuring peripheral visual fields using the ability to resolve objects, PHP relies on the concept of hyperacuity. Hyperacuity is the ability to discern a subtle misalignment of an object. Hyperacuity’s threshold is approximately 10 fold lower than that required for optimal resolution of an object.

The brain’s ability to discern such small misalignments is exploited by PHP: hyperacuity stimuli are projected onto the macula, and if the patient’s photoreceptors are perfectly in position from an absence of edema or choroidal neovascularization, then no misalignment is perceived. If the contour of the patient’s photoreceptors are slightly misaligned due to choroidal neovascularization or from elevation of the retinal pigment epithelium due to drusen, then this is perceived as a misalignment of the image.

 
An advantage of hyperacuity for measuring macular function is its high resistance to retinal image degradation allowing diagnostics in patients with cataracts and vitreal opacities. Hyperacuity thresholds of individuals do not tend to show variation with increasing age.

Summary

The combination of modern genetics and imaging technologies aid the diagnosis and prognostication of AMD. Once diagnosed with AMD - repeated, regular evaluation by an eye care professional can preserve vision.