Biometry and A-Scans Flashcards

A technician performs applanation A-scan biometry and obtains an axial length of 22.50 mm. The same eye is then measured using immersion A-scan, yielding a measurement of 22.85 mm. What is the most likely reason for this discrepancy?

Answer: Corneal compression from the applanation probe.

Applanation (contact) biometry involves touching the ultrasound probe to the cornea, which can indent or compress it, artificially shortening the axial length measurement. [2, 4] Immersion biometry avoids corneal contact by using a saline-filled scleral shell, providing a more accurate and typically longer measurement. [2, 9] The difference of 0.2 mm to 0.3 mm is classic for this effect. [2]

When evaluating an A-scan waveform for accuracy, which of the following represents the ideal characteristics for the retinal spike?

Answer: A tall, steeply rising spike that is perpendicular to the baseline.

The ideal A-scan measures along the visual axis, hitting the macula perpendicularly. This alignment produces a high-amplitude (tall), sharply rising retinal spike, indicating a strong, direct echo. [26] A sloping or notched spike suggests the sound beam is hitting an off-axis point, like the slope of the optic nerve head, leading to an inaccurate (longer) measurement. [12, 23]

An ophthalmologist is selecting an IOL formula for a patient with a very short axial length of 21.2 mm. Which of the following formulas has traditionally been preferred for this type of eye?

Answer: Hoffer Q

The Hoffer Q formula is a third-generation formula specifically designed and generally considered more accurate for eyes with short axial lengths (typically < 22.0 mm). [3, 6] Formulas like SRK/T are more commonly used for average to long eyes. [6] While newer multi-variable formulas like the Kane or Barrett are now often used, the Hoffer Q is the classic choice for short eyes among the options provided.

During an A-scan measurement, the technician fails to identify a distinct posterior lens capsule spike, and the lens thickness value appears abnormally large. This is most characteristic of:

Answer: A dense posterior subcapsular cataract.

A dense posterior subcapsular cataract can absorb or scatter the ultrasound waves, preventing them from reflecting cleanly off the posterior lens capsule. [12] This results in a weak or absent posterior lens spike. The machine may then incorrectly identify the retinal spike as the posterior lens capsule, leading to an erroneously large lens thickness measurement and an inaccurate axial length. [12]

A technician is preparing to perform an A-scan on a patient whose vitreous cavity is filled with silicone oil. Which of the following is the most critical machine setting to adjust for an accurate axial length measurement?

Answer: The sound velocity setting.

The speed of sound (sound velocity) is significantly slower in silicone oil (around 980-1040 m/s) compared to the natural vitreous humor (around 1532 m/s). [22] Failing to change the biometer's sound velocity setting to the correct value for silicone oil will result in a grossly inaccurate axial length measurement, typically making the eye appear much longer than it is and leading to a major IOL power calculation error. [10, 20]

Which of the following is a primary advantage of optical biometry (e.g., IOLMaster, Lenstar) over ultrasound A-scan biometry?

Answer: It is a non-contact procedure that avoids corneal compression.

Optical biometry is a non-contact technique, which is a key advantage as it completely eliminates the variable of corneal compression that can occur with applanation ultrasound, leading to more accurate and reproducible measurements. [8, 13] Ultrasound is actually necessary for very dense cataracts that light cannot penetrate, and it measures the anatomic axis, whereas optical biometry measures the more visually relevant refractive axis. [13, 18]