Supplemental Testing 8 Flashcards

During dark adaptation testing, a patient's curve shows a normal cone branch but the rod branch fails to reach the expected final threshold, remaining elevated throughout the rod-mediated phase. This pattern is MOST consistent with which underlying deficiency?

Answer: Vitamin A deficiency impairing rhodopsin regeneration

Vitamin A (retinol) is the essential precursor for rhodopsin, the photopigment in rod photoreceptors. Without adequate vitamin A, rhodopsin cannot regenerate fully during dark adaptation. The cone branch appears normal because cone opsins rely on different regeneration kinetics and thresholds, but the rod branch — which should ultimately reach a much lower (more sensitive) final threshold — fails to do so, leaving an elevated rod-mediated dark adapted threshold. This is the biochemical basis of night blindness in vitamin A deficiency. Retinitis pigmentosa would affect both branches more globally, and cone dystrophy would spare rods.

A specular microscopy report returns an endothelial cell density (ECD) of 1,700 cells/mm², a coefficient of variation (CV) of 51%, and hexagonality of 39%. A colleague argues that the ECD is the most clinically urgent value. Which response is MOST accurate?

Answer: The CV of 51% is actually the most sensitive early indicator of endothelial stress and warrants greater concern

While ECD, CV, and hexagonality are all reported together, the coefficient of variation (CV) — a measure of polymegethism (cell size variability) — is the most sensitive early indicator of endothelial stress. Normal CV is ≤30–33%. A CV of 51% indicates marked cell size variability, typically seen early in conditions like Fuchs' endothelial dystrophy, often before ECD drops significantly. An ECD of 1,700 cells/mm² is low-normal and does not become critical until approximately 500–700 cells/mm². Hexagonality (pleomorphism) below 60% is abnormal, but CV elevation typically precedes significant hexagonality loss. The colleague's focus on ECD alone misses the more sensitive early biomarker.

When using the Potential Acuity Meter (PAM) pre-operatively to predict post-cataract visual acuity, which patient would be MOST likely to receive a falsely optimistic (overestimated) prediction?

Answer: A patient with a visually significant nuclear sclerotic cataract and a childhood history of anisometropic amblyopia

The PAM projects an optotype through a tiny beam (0.15 mm), bypassing media opacities to directly stimulate the macula and test its functional potential. Because it circumvents optical defects, it can yield excellent acuity readings in patients whose true post-surgical outcome will be limited by factors beyond the media. In amblyopia, the cortical suppression and abnormal binocular visual development limit functional acuity regardless of optical clarity — the macula responds to the PAM stimulus normally, giving a falsely optimistic prediction. Posterior subcapsular cataracts can paradoxically cause PAM to UNDERESTIMATE potential (the dense central opacity blocks the beam). POAG and corneal guttata would not typically cause PAM overestimation in the same way.

On fluorescein angiography, a hyperfluorescent lesion first appears in the early venous phase and by the late recirculation phase has increased in BOTH intensity AND area, with borders that have become progressively less distinct. This behavior BEST describes which angiographic phenomenon?

Answer: Active leakage of dye into surrounding tissue

The temporal behavior of hyperfluorescence is the critical differentiator in FA interpretation. Active leakage is uniquely characterized by hyperfluorescence that increases in both intensity AND geographic size over time, with progressively blurred, indistinct borders as dye diffuses freely into surrounding tissue (as seen with choroidal neovascularization or disc/retinal neovascularization). Tissue staining also increases in intensity over time but maintains sharp, defined borders without expanding in size. Pooling (in a sub-RPE bleb or subretinal space) increases in intensity and may enlarge slightly within the anatomic boundary of the space, but expansion is constrained. A window defect is bright early, matching the choroidal phase, then fades as background fluorescence washes out — it does not expand.

An EOG is performed with a measured dark trough of 360 µV and a light peak of 414 µV, yielding an Arden ratio of approximately 1.15. The simultaneous full-field ERG is entirely normal. Which conclusion is MOST appropriate?

Answer: The abnormal EOG with normal ERG is the classic dissociation pattern of Best disease (vitelliform macular dystrophy)

The Arden ratio = Light Peak ÷ Dark Trough = 414 ÷ 360 ≈ 1.15 (115%). A normal Arden ratio is ≥1.85 (185%), so this result is markedly abnormal. The EOG measures RPE function through the standing electrical potential driven by ion channels (bestrophin chloride channels) in the RPE. The hallmark of Best disease (caused by BEST1/VMD2 mutations in bestrophin) is a severely reduced Arden ratio with a characteristically NORMAL full-field ERG — this EOG–ERG dissociation is pathognomonic. Option C is explicitly incorrect because Best disease is diagnosed precisely by this dissociation. Diffuse RPE atrophy from AMD would typically also show ERG changes and a different clinical context.

A Humphrey 24-2 visual field shows a mean deviation (MD) of −16 dB and a pattern standard deviation (PSD) of 1.9 dB, with a reliable test (false positives 2%, false negatives 8%, fixation losses 1/20). Before attributing this to structural retinal or neuro-ophthalmic disease, which etiology should be investigated FIRST?

Answer: Diffuse media opacity or significant uncorrected refractive error

MD reflects overall average sensitivity loss versus age-matched normal; PSD reflects the variability or 'unevenness' of that loss across the field. A HIGH MD loss (−16 dB = severe) with a LOW PSD (1.9 dB, near normal) indicates that sensitivity is depressed diffusely and uniformly — every point is roughly equally affected, rather than some areas being selectively worse than others. This uniform, diffuse pattern is the classic signature of optical causes: media opacities (cataract, corneal edema, vitreous haze), significant pupillary miosis, or uncorrected refractive error. Glaucoma and most structural diseases produce localized defects (arcuate scotomas, altitudinal loss) that would drive PSD upward. Bilateral optic nerve disease could cause this pattern but media opacity is the first step to rule out given its prevalence and reversibility.