Visual Assessment 7 Flashcards

A patient with a dense central scotoma is being tested for visual acuity using an ETDRS chart. The patient consistently reads letters from the lower-right corner of each line rather than the center. Which explanation BEST accounts for this behavior?

Answer: The patient is using eccentric fixation with a preferred retinal locus (PRL) to bypass the scotoma

Patients with dense central scotomas (commonly from macular degeneration) adaptively develop a preferred retinal locus (PRL), an area of functioning peripheral retina they use as a substitute fixation point. This causes them to position letters eccentrically relative to their optical line of sight. A right hemianopia would suppress the right side, not drive rightward gaze, and astigmatism produces blur rather than spatial repositioning behavior.

During potential acuity meter (PAM) testing on a patient with a 3+ nuclear sclerotic cataract, the technician obtains a PAM reading of 20/200. Preoperative corneal topography shows no significant irregularity. Which interpretation is MOST accurate?

Answer: PAM readings are artificially reduced by dense nuclear sclerosis, making this result likely an underestimate of true potential acuity

Dense nuclear sclerotic cataracts significantly degrade the quality of the laser beam used in PAM testing, causing artifactually low readings that underestimate true macular potential. A grade 3+ nuclear sclerosis can prevent adequate transmission of the PAM stimulus. Therefore, the 20/200 result likely underestimates postoperative potential, and additional testing (e.g., ultrasound B-scan, laser interferometry) should be considered before concluding there is macular pathology.

A patient reports that straight lines on the Amsler grid appear wavy only when the grid is held at 10 cm, but appear normal when held at the standard 30 cm testing distance. What is the MOST likely explanation?

Answer: The abnormal result at near represents a larger retinal area being stimulated, unmasking a paracentral scotoma or metamorphopsia

The standard Amsler grid held at 30 cm subtends approximately 20 degrees of central visual field. When held at 10 cm, the same grid subtends a larger angular area, testing a broader retinal zone and sampling more peripheral macular tissue. A lesion causing paracentral metamorphopsia may fall outside the tested area at 30 cm but be captured at the closer distance. This is not a testing artifact — it is clinically meaningful and can detect pathology missed at standard distance.

When administering the Farnsworth-Munsell 100-Hue test to a patient with a suspected toxic maculopathy from hydroxychloroquine use, which error pattern would be MOST characteristic and diagnostically important?

Answer: A tritan-axis error pattern (errors concentrated along caps 46–70), indicating blue-yellow discrimination loss

Hydroxychloroquine toxicity characteristically produces a tritan (blue-yellow) type of color vision loss in early stages, corresponding to damage to the short-wavelength-sensitive (S-cone) pathways predominantly affected by macular toxicity. Errors clustering along the tritan axis on the FM 100-Hue test are a sensitive early indicator of toxicity before structural changes appear on imaging. Protan and deutan patterns are more typical of hereditary red-green deficiencies.

A technician performs Goldmann visual field testing on a patient with suspected bitemporal hemianopia. The IV4e isopter respects the vertical meridian in both temporal fields but the I2e isopter does NOT respect the meridian in the right temporal field. How should this discrepancy be interpreted?

Answer: The larger isopter (IV4e) captures remaining absolute defect at the meridian, while the I2e result suggests a relative defect that becomes absolute only for smaller stimuli — consistent with an incomplete or evolving chiasmal lesion

In Goldmann perimetry, different isopters probe different levels of retinal sensitivity. A defect that respects the vertical meridian for a large, bright stimulus (IV4e) but not for a smaller, dimmer one (I2e) suggests a relative defect at the temporal meridian — meaning residual but reduced sensitivity remains in that area for high-contrast stimuli. This pattern is consistent with an incomplete chiasmal compression where the nasal fibers crossing at the chiasm are partially but not fully damaged, producing sensitivity loss that is absolute for small targets but not yet absolute for large ones.

A patient with 20/20 best-corrected visual acuity complains of significant difficulty driving at dusk despite normal standard visual field testing and normal color vision. Pelli-Robson contrast sensitivity testing reveals a score of 1.05. Which condition BEST explains this clinical picture?

Answer: Early retinitis pigmentosa, which can reduce contrast sensitivity and mesopic function before standard acuity or fields are significantly affected

A Pelli-Robson score of 1.05 (normal ≥ 1.65) indicates significantly reduced contrast sensitivity despite preserved Snellen acuity. In early retinitis pigmentosa (RP), rod photoreceptor loss begins in the mid-peripheral retina and reduces mesopic/scotopic function and contrast sensitivity well before standard perimetry detects significant field loss or acuity drops. This explains the dusk driving difficulty. Glaucoma typically produces high spatial frequency contrast loss but the distribution of standard field loss usually correlates. A cataract would typically affect acuity measurably and produce glare symptoms rather than purely mesopic difficulty.