AMD Diagnostic Imaging

Fundus Fluorescein Angiography

Fundus fluorescein angiography (FFA) in AMD exists mainly to localize treatable leaking vessels: it maps the extent of choroidal neovascularization (CNV) precisely enough to guide treatment planning. Reading an AMD angiogram well comes down to telling apart two patterns that look alike on the frame but mean opposite things.

Window Defect vs. Blocked Fluorescence

A window defect occurs when atrophic RPE lets underlying choroidal fluorescence shine through unobstructed, the pattern seen in geographic atrophy or an old laser scar, where the RPE's normal blocking pigment is simply gone. Blocked fluorescence is the opposite mechanism: an overlying opacity, such as subretinal hemorrhage, masks background fluorescence that would otherwise be visible. Confusing the two gives the wrong read on the RPE itself. A window defect implies atrophy; blocked fluorescence implies something sitting on top of an intact, possibly still-leaking, layer.

Optical Coherence Tomography

Optical coherence tomography (OCT) is the gold standard for detecting fluid in wet AMD. In active neovascular disease it shows subretinal or intraretinal fluid directly, a cross-sectional view of what fluorescein angiography can only infer from a leakage pattern. OCT is fast, non-invasive, and quantitative, which is why it has become the primary tool both for diagnosing conversion to wet AMD and for tracking the response to anti-VEGF therapy over time.

OCT and fundus autofluorescence are also how reticular pseudodrusen get identified. These subretinal drusenoid deposits sit above the RPE instead of beneath it, unlike conventional drusen, so routine color fundus photography often misses them. On autofluorescence they appear as a network of fine hypoautofluorescent lines, best seen with near-infrared imaging. Their presence adds its own risk of progression to late AMD, including geographic atrophy, on top of whatever risk the conventional drusen already carry.

Amsler Grid Home Monitoring

The Amsler grid is a simple home test for monitoring central vision in patients at risk for macular disease, typically those with known drusen or intermediate AMD in one eye. Patients cover one eye at a time and look at the grid's central dot. New wavy lines, missing squares, or distortion (metamorphopsia) signal a change worth same-week evaluation, since it is a sensitive early sign of subretinal fluid or new CNV. Its value is that it catches change between scheduled office visits, when a patient with intermediate AMD would otherwise feel fine.

Low Vision & Blindness Certification

Diagnostic staging in AMD eventually feeds into formal visual disability classification. In the UK, a Snellen visual acuity of 3/60 or less qualifies a patient for registration as "Severely Sight Impaired" (blind), the same threshold used for a severely constricted visual field. This downstream certification is a practical consequence of accurate imaging and staging: it determines a patient's access to disability services. Getting the exact threshold right on an exam question, 3/60 rather than 6/60 or 6/18, is a common testable detail.

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