AMD is the commonest cause of incurable blindness in the elderly in western countries. It is a bilateral disease in which visual loss in the first eye usually occurs at about 65 years of age. The second eye is involved at the rate of approximately 10% per annum and accounts for half of all registered visual impairments in the UK.
There are two main types of AMD: "dry" or atrophic, and "wet" or neovascular. Blindness is usually associated with the wet form of AMD, and among the eyes with severe visual loss, 80-90% of cases are because of wet AMD, while 10-20% are because of the dry form.
Older patients with macular degeneration complain of blurring of their vision and inability to read. Younger or more observant patients notice that straight edges might look kinked. Usually one eye is considerably more affected than the other, although both eyes can be affected simultaneously. Because the degenerative process is limited to the macula,the peripheral field remains unaffected and the patient can walk around quite normally. Difficulty in recognising faces or in seeing bus numbers is also a common complaint. The wet form occurs more commonly in Caucasians and about one-third of the patients give a family history of similar problems. Several preventable factors, including smoking, systemic hypertension, cardiovascular disease and low antioxidant intake, are associated with increased risk of AMD.
In the early stages of dry AMD, inspection of the fundus shows spots of pigment in the macular region. Drusen are also often seen (Figure 19.1). These are small round yellowish spots, often scattered over the posterior pole. Unfortunately, the word "drusen" has been used rather loosely in ophthalmology to refer to two or three types of swelling seen in the fundus. It is used to describe the rare mulberry-like tumours seen around the optic nerve head in tuberose sclerosis and it is also used when referring to the multiple shiny excrescences seen on the optic disc as a congenital abnormality. Drusen seen at the posterior pole of the eye as a senile change are also known as "colloid bodies" and perhaps this term is preferable.
Under the microscope, colloid bodies are seen as a degenerative change in Bruch's membrane. Drusen can have varying degrees of hyperpig-mentation. Most eyes with drusen maintain good vision, but a significant number will develop progressive atrophy of the retinal pigment epithelium (RPE) and choriocapillaris. This is inevitably associated with photoreceptor loss (Figure 19.2). There is usually a moderate loss of vision. This atrophic change in the RPE, choroid and photoreceptors is referred to as "dry" AMD. This is because there is no leakage of fluid or bleeding into the retina or subretinal space.
In the "wet type" of macular degeneration a fan of new vessels arises from the choroid -choroidal neovascularisation (CNV). The growth of these new vessels seems to be important because they invade the breaks in Bruch's membrane. Serous or haemorrhagic exudate tends to occur and this can be either under the RPE or subretinal (Figure 19.3). A sudden loss of central vision might be experienced as the result of such an episode. Subsequently,"healing" of the leaking vascular complex results in scar tissue formation, which further destroys the central vision permanently.
The terms "classic" and "occult" describe the different patterns of CNV leakage on fluorescein angiography.
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