Optometry Flashcards

A patient with keratoconus presents for contact lens fitting. Corneal topography shows a central K reading of 52.00 D steeply inferiorly displaced. The technician notes significant irregular astigmatism. Which contact lens modality is MOST likely to provide the best visual acuity for this patient?

Answer: Scleral lenses vaulting over the entire cornea

Scleral lenses vault entirely over the irregular corneal surface and rest on the sclera, creating a smooth optical surface filled with saline. This completely masks the irregular astigmatism of keratoconus. Standard RGPs can work but may be uncomfortable and unstable on severely ectatic corneas. Soft toric lenses conform to the irregular surface and provide poor vision. Piggyback systems are used when RGP comfort is inadequate but don't offer the same stable vault as sclerals.

During automated perimetry using the SITA Standard algorithm on a 24-2 test pattern, a patient demonstrates a mean deviation (MD) of -8.5 dB with a pattern standard deviation (PSD) of 9.2 dB. The glaucoma hemifield test (GHT) reads 'Outside Normal Limits.' Which interpretation is MOST consistent with these indices?

Answer: Localized arcuate scotoma pattern consistent with glaucomatous damage

A high PSD (9.2 dB) in the context of moderate MD (-8.5 dB) indicates that the loss is NOT uniform — there is significant variability across test points, suggesting localized defects superimposed on some generalized loss. An arcuate scotoma pattern (localized nerve fiber bundle loss) produces exactly this pattern. Diffuse media opacity causes a high MD depression but LOW PSD (uniformly reduced). An unreliable field would be flagged by catch trial indices, not these threshold values. The GHT being 'Outside Normal Limits' confirms asymmetric superior/inferior hemifield differences consistent with glaucoma.

A technician performs corneal pachymetry on a glaucoma suspect and obtains a central corneal thickness (CCT) of 510 microns OD and 515 microns OS. The Goldmann applanation tonometry reads 18 mmHg OU. Applying the Ehlers correction factor, what is the MOST accurate interpretation?

Answer: IOP is likely underestimated; true IOP is higher than 18 mmHg

The average CCT is approximately 545-555 microns. A CCT of 510-515 microns is THINNER than average. Goldmann applanation tonometry (GAT) was calibrated for a CCT of approximately 520-540 microns. With a thinner-than-average cornea, GAT systematically UNDERESTIMATES true IOP because less force is required to applanate a thinner, less resistant cornea. The Ehlers correction suggests approximately +1 mmHg correction for every ~35 microns below the normative mean, meaning the true IOP may be 1-2 mmHg higher than measured.

When performing A-scan biometry for IOL power calculation in a patient who has previously undergone LASIK for myopia correction, which formula and adjustment is MOST appropriate to minimize refractive surprise?

Answer: Barrett True-K or Haigis-L formula using historical refraction data or no-history methods

Post-LASIK eyes present two major challenges for IOL calculation: (1) standard keratometers measure the anterior corneal surface and assume a fixed anterior/posterior corneal radius ratio, which is violated by LASIK; and (2) the effective lens position (ELP) is mispredicted when using post-LASIK K readings. Formulas like Barrett True-K (no history) or Haigis-L are specifically designed for post-refractive corneas and either use adjusted ELP predictions or corneal power adjustments. SRK/T and Holladay 1 using raw post-LASIK Ks consistently produce hyperopic surprises. SRK II is an outdated regression formula not validated for these cases.

A 45-year-old patient presents with sudden-onset diplopia. Cover testing reveals a right hypertropia that increases in left gaze and right head tilt (positive Bielschowsky head tilt test). Which cranial nerve palsy does this pattern MOST specifically indicate?

Answer: Right CN IV (trochlear) palsy

The Parks-Bielschowsky three-step test is the diagnostic algorithm for CN IV palsy. Step 1: right hypertropia identifies right eye as hypotropic muscle or left eye as hypertropic muscle dysfunction. Step 2: increases in left gaze — the right superior oblique (SO) is a depressor/intorter, so its underaction is unmasked in adduction (looking left for the right eye). Step 3: positive right head tilt (Bielschowsky) — when head tilts right, the right eye must intort; without functioning right SO, the right SR over-elevates producing more hypertropia. This triad is pathognomonic for right CN IV (trochlear nerve) palsy, as the trochlear nerve exclusively innervates the superior oblique muscle.

During fluorescein angiography (FA), a lesion appears hyperfluorescent in early frames and the fluorescence INCREASES in size and intensity in late frames. A second lesion appears bright in early frames but fades to isofluorescence by the late phase without enlarging. What do these two patterns MOST likely represent, respectively?

Answer: Leakage from choroidal neovascularization; staining of a scar or drusen

In FA interpretation, three types of hyperfluorescence must be distinguished by their temporal behavior: (1) LEAKAGE — starts hyperfluorescent and progressively increases in both intensity AND size beyond the lesion borders into surrounding tissue, caused by breakdown of the blood-retinal barrier (e.g., choroidal neovascularization, cystoid macular edema); (2) STAINING — hyperfluorescent early and becomes brighter in late phase but does NOT increase in size, as fluorescein molecules bind to structural proteins (drusen, fibrovascular tissue, chorioretinal scars); (3) POOLING — fills a space (subretinal or sub-RPE) progressively but within defined anatomical boundaries. The first lesion (enlarging) = leakage from CNV; the second (brightens without spreading) = staining.