Visual Assessment 9 Flashcards

During ETDRS visual acuity testing, a patient reads all 5 letters correctly on the 20/50 line but only 3 of 5 letters on the 20/40 line. What is the correct LogMAR visual acuity score?

Answer: 0.34

In ETDRS scoring, each correctly read letter is worth 0.02 LogMAR units. The last fully completed line (20/50) has a LogMAR value of 0.40. Each additional letter read on the next line subtracts 0.02 from that baseline. The patient read 3 additional letters on the 20/40 line: 0.40 − (3 × 0.02) = 0.40 − 0.06 = 0.34. This is a key distinction from Snellen scoring, which records only the last complete line.

A 1.0 log unit neutral density filter is placed alternately before each eye of a patient with 20/200 OD from organic macular degeneration and 20/200 OS from longstanding anisometropic amblyopia. Which response pattern is MOST expected?

Answer: The eye with macular degeneration shows a greater reduction in acuity than the amblyopic eye

Eyes with organic macular pathology retain normal photoreceptor light-sensitivity requirements and show a marked, disproportionate drop in acuity under a neutral density filter. The amblyopic eye, by contrast, has a suppressed but functionally adapted visual pathway that is relatively insensitive to further reductions in luminance — it has chronically 'learned' to process a degraded signal. The amblyopic eye therefore shows a similar or lesser acuity drop compared to the normal fellow eye, not a greater one. This differential response is the basis of the ND filter test for distinguishing functional from organic vision loss.

A patient with advanced primary open-angle glaucoma and a dense posterior subcapsular cataract undergoes pre-operative Potential Acuity Meter (PAM) testing. The PAM projects through a small clear zone and reads 20/20. Post-cataract surgery, the patient achieves only 20/100. What is the MOST likely explanation for this discrepancy?

Answer: The PAM bypassed the cataract but could not account for glaucomatous perifoveal and macular nerve fiber layer damage

The PAM projects a miniaturized Snellen chart through a pinhole-sized beam, successfully bypassing optical media opacities like cataracts. However, it targets only the foveal point and cannot detect or account for glaucomatous damage to the perifoveal macular nerve fiber layer or diffuse neural loss. In advanced glaucoma, the macular threshold can be significantly reduced even when fixation appears intact, leading the PAM to give a falsely optimistic acuity prediction. This is a well-documented limitation of PAM in patients with co-existing optic nerve or retinal ganglion cell disease.

A patient's Farnsworth-Munsell 100 Hue Test reveals an error axis consistent with a protan-type color vision deficiency. Which additional clinical finding would BEST differentiate a protan defect from a deutan defect of similar severity?

Answer: Red stimuli appearing significantly darker or dimmer relative to a green of equal measured luminance

The hallmark distinguishing feature of protan (red-deficient) defects is a shifted photopic luminosity function: protanopes and protanomalous individuals have markedly reduced sensitivity at the red end of the visible spectrum, causing red targets to appear abnormally dark or dim compared to green targets of equal photometric luminance. Deuteranopes share nearly the same red-green confusion axis on hue arrangement tests but do NOT show this luminance shift — red and green appear approximately equally bright to them. This relative 'red dimming' effect is the most reliable clinical differentiator and can be elicited with a simple colored light brightness comparison.

A Humphrey 24-2 SITA-Standard visual field shows a well-defined superior nasal step. Reliability indices are: Fixation Losses 0/14 (0%), False Positives 31%, False Negatives 4%. Which interpretation is MOST accurate?

Answer: The 31% false positive rate may artificially elevate sensitivity in apparently normal areas, meaning the superior nasal step could be more severe than the printout suggests

A false positive rate above 15% means the patient is pressing the response button even when no stimulus is presented, which inflates (artificially improves) sensitivity values in areas that appear normal. This makes normal-looking zones of the visual field appear better than they truly are — it does NOT generate false scotomas. Therefore, a defect (such as a superior nasal step) that is visible despite the background sensitivity being artificially inflated is likely a real finding, and may even be underrepresented in severity. False positive errors mask or minimize defects rather than create them — the opposite of the common misconception.

A 47-year-old commercial pilot has 20/15 BCVA bilaterally, normal dilated fundus exam, normal Humphrey visual fields, and normal brightness acuity tester (BAT) results. He reports significant difficulty reading low-contrast cockpit instruments under reduced ambient lighting. Pelli-Robson contrast sensitivity testing yields 1.05 log units (normal ≥1.65 log units). Which etiology is LEAST consistent with this isolated contrast sensitivity deficit?

Answer: Fully corrected myopia with current spectacle prescription

Fully corrected myopia does not impair contrast sensitivity. Uncorrected myopia degrades retinal image quality and reduces CS, but once the refractive error is properly corrected with spectacles or contact lenses, the optical blur is resolved and CS returns to the neurally normal baseline. In contrast, subclinical optic neuritis (demyelination slows signal conduction and impairs spatial frequency processing), early diabetic maculopathy (photoreceptor and RPE changes precede visible funduscopic signs), and lens scatter (intraocular forward scatter degrades retinal contrast even with 20/20 Snellen acuity) are all established causes of isolated CS loss with preserved Snellen acuity.