Supplemental Testing 9 Flashcards

A patient with a 3-year history of hydroxychloroquine (Plaquenil) use undergoes multifocal ERG (mfERG). The technician notes a parafoveal ring of significantly reduced response amplitude, while the central foveal response appears relatively preserved. Which interpretation is most accurate?

Answer: This parafoveal ring scotoma is a characteristic early finding of hydroxychloroquine retinal toxicity

Hydroxychloroquine toxicity produces a characteristic parafoveal ring of photoreceptor damage that appears topographically on mfERG as reduced amplitude in a ring pattern surrounding the fovea. The fovea may be relatively spared early in toxicity — this 'bull's-eye' distribution is diagnostically significant and differs from the concentric foveal suppression seen with poor fixation or the diffuse amplitude reduction seen with optic nerve disease.

A patient with a posterior subcapsular cataract (PSC) has a baseline Snellen acuity of 20/40 under standard room illumination. When tested with the Brightness Acuity Tester (BAT) on medium setting, acuity drops to 20/200. A second patient with nuclear sclerotic cataract (NS) of comparable Snellen acuity undergoes the same BAT protocol. What is the most clinically expected difference between these two patients?

Answer: PSC typically causes significantly greater glare disability than NS at equivalent Snellen acuity levels

Posterior subcapsular cataracts are located directly in the central visual axis and cause intense forward light scatter, producing disproportionate glare disability relative to their Snellen impact. Nuclear sclerotic cataracts primarily cause myopic shift and diffuse haze but less dramatic glare sensitivity. A PSC patient with 20/40 baseline dropping to 20/200 on BAT is entirely consistent with PSC behavior — this is precisely why BAT testing is most clinically valuable in this cataract subtype.

A pattern ERG is performed on a glaucoma suspect. The P50 component amplitude is within normal limits for age, but the N95 component is markedly reduced. The calculated N95:P50 amplitude ratio is 1.3 (laboratory reference: >2.1 is normal). Which conclusion is most directly supported by this isolated finding?

Answer: Retinal ganglion cell loss consistent with early optic nerve pathology

In pattern ERG, the P50 component primarily reflects macular photoreceptor and bipolar cell integrity, while the N95 component is generated predominantly by retinal ganglion cells (RGCs). A selectively reduced N95 with a preserved P50 — reflected as a depressed N95:P50 ratio — specifically implicates RGC loss rather than photoreceptor or inner nuclear layer dysfunction. This pattern is a sensitive biomarker for early glaucomatous damage at the RGC level, even before structural OCT changes become apparent.

On a Humphrey SITA-Standard 24-2 visual field, a patient's reliability indices show a false-positive rate of 31%. The mean deviation (MD) is -1.8 dB and the pattern standard deviation (PSD) is 2.2 dB. The technician is counseling the ordering physician on result interpretation. Which statement about the effect of the elevated false-positive rate is most accurate?

Answer: High false-positive responses inflate threshold estimates, making actual visual field defects appear shallower or absent

When a patient responds to catch trials where no stimulus was presented (false positives, 'trigger-happy' behavior), the algorithm interprets those responses as seeing stimuli that were not there. This inflates the estimated threshold sensitivity for affected test points, causing defects to appear shallower or even disappear entirely. The effect is the opposite of false negatives, which depress thresholds. A high false-positive rate should prompt a repeat field, as existing pathology may be masked rather than revealed.

A 38-year-old patient with a history of resolved optic neuritis in the right eye reports a peculiar symptom: when watching a pendulum swing side-to-side, it appears to travel in an elliptical arc rather than a straight line, and depth perception is disturbed. Supplemental testing with which modality would most directly quantify the physiological basis of this symptom?

Answer: Visual evoked potential (VEP) comparing P100 latency between the two eyes

This patient is describing the Pulfrich phenomenon, which occurs when there is an inter-ocular difference in neural transmission speed — most commonly from demyelinating optic neuritis causing a prolonged latency in one eye. The brain interprets the temporal offset as a spatial depth disparity, causing moving objects to appear to travel in an elliptical arc. VEP testing directly measures P100 latency in each eye; a prolonged latency in the previously affected eye confirms the inter-ocular latency asymmetry that generates this phenomenon.

A Potential Acuity Meter (PAM) is performed on a patient with a 2+ nuclear sclerotic cataract and reads 20/25, predicting good post-operative visual recovery. Following uncomplicated phacoemulsification, the patient achieves only 20/80 best-corrected acuity. The cataract was well-removed with no surgical complications. What is the most likely explanation for the discrepancy between the PAM prediction and the post-operative result?

Answer: Concurrent macular or optic nerve pathology was present that the PAM was unable to detect because it bypasses media opacities

The PAM projects a miniaturized Snellen chart through a very narrow beam that largely bypasses lens opacities, making it useful for estimating retinal potential behind a cataract. However, this bypass means it cannot detect coexisting macular degeneration, epiretinal membrane, optic neuropathy, or other retinal pathology hidden behind the cataract. When the PAM gives an optimistic prediction that fails to match post-operative reality without surgical complications, the most likely cause is concurrent posterior segment disease that the PAM inherently cannot assess. This is one of the PAM's critical clinical limitations.