Biometry and A-Scans Flashcards

A vitrectomized eye is filled with 1000-centistoke silicone oil. If the A-scan biometer is inadvertently left at the standard phakic velocity of 1548 m/s instead of the correct silicone oil velocity of 980 m/s, what error will result in the axial length measurement?

Answer: The measured axial length will be artifactually longer than the true axial length

Ultrasound travels more slowly through silicone oil (~980 m/s) than through normal vitreous (~1532 m/s). When the machine uses the faster velocity setting, it calculates distance as: distance = velocity × time. Because sound actually takes longer to traverse the silicone oil, the machine — assuming the higher velocity — calculates a longer distance than truly exists. This produces an artifactually elongated axial length and would lead to an underpowered IOL (hyperopic outcome).

A patient who had LASIK for −6.00 D myopia 12 years ago now requires cataract surgery. Standard automated keratometry reads 40.25 D OU. Using the 'clinical history method' to determine true corneal power, which data combination is required?

Answer: Pre-LASIK keratometry readings and the change in manifest spherical equivalent (pre-op minus post-op SEQ)

The clinical history method calculates true corneal power as: True K = Pre-op K − (Pre-op SEQ − Post-op SEQ). The refractive change is subtracted from the pre-operative keratometry because standard keratometers apply a fixed index of refraction (1.3375) that assumes a normal anterior-to-posterior corneal radius ratio — a ratio that is permanently altered by ablative surgery. Without the pre-op refraction delta, the corneal power will be overestimated, leading to a hyperopic refractive surprise post-cataract surgery.

In the Haigis IOL power calculation formula expressed as d = a0 + (a1 × ACD) + (a2 × AL), which constant is specifically weighted by the preoperatively measured anterior chamber depth, and what does this allow the formula to predict more accurately than single-constant formulas?

Answer: a1; it allows individualized prediction of the effective lens position (ELP) based on the patient's own anterior segment anatomy

In the Haigis formula, a1 is the coefficient multiplied by the measured ACD. This allows the predicted effective lens position to vary with actual anterior chamber anatomy — a shallower ACD shifts ELP prediction anteriorly, a deeper ACD posteriorly. Single-constant formulas (like SRK/T with only an A-constant) cannot make this individualized adjustment, which is why Haigis performs comparatively better across a wider range of ACD values, particularly in short or unusual anterior segments.

During contact A-scan biometry on a highly myopic eye with suspected posterior staphyloma, a technician records the following axial length measurements: 28.4 mm, 30.2 mm, 30.0 mm, 30.1 mm, 29.1 mm, 30.3 mm. Which value should be selected for IOL power calculation, and why?

Answer: 30.2 mm — the longest reproducible cluster (30.0–30.3) most likely represents the foveal measurement at the staphyloma apex

In posterior staphyloma, the fovea is located at the apex — the deepest outpouching — of the ectatic posterior pole. A probe correctly aligned along the visual axis toward the fovea will record the longest axial length. Off-axis measurements (hitting the non-ectatic retina adjacent to the staphyloma) produce shorter, misleadingly 'normal' values. The cluster of reproducible long measurements (30.0–30.3 mm) represents consistent alignment with the true foveal path. Using the shorter outlier would result in selection of an IOL that is too weak for the actual visual axis length.

A patient with a dense posterior subcapsular cataract (PSC) undergoes contact A-scan biometry. The printout shows a high-amplitude spike located approximately 12 mm posterior to the corneal peak, well anterior to the expected retinal spike. The technician accepts this as the retinal echo and records an axial length of 15.8 mm. What error has occurred and what is the most appropriate next step?

Answer: The PSC has produced a false posterior lens capsule echo; the technician should switch to immersion A-scan or optical biometry and discard this measurement

Dense posterior subcapsular cataracts can generate a strong acoustic echo that the machine — and an inattentive technician — may misidentify as the retinal spike, yielding a grossly short (and fictitious) axial length. An adult eye with an axial length of 15.8 mm would be profoundly nanophthalmic, which is clinically inconsistent with a patient presenting for routine cataract surgery. The correct response is to recognize the artifact, abandon the contact technique, and switch to immersion A-scan (which separates probe from cornea and reduces artifactual anterior spikes) or to optical biometry (IOLMaster/Lenstar), which is unaffected by this acoustic artifact.

Optical coherence biometry (e.g., IOLMaster 700) fails to obtain a reliable axial length measurement in a patient. Which clinical finding most directly explains this failure, and which fallback method provides the most accurate measurement in this scenario?

Answer: A mature, brunescent cataract with <10% light transmission; immersion A-scan ultrasound biometry is the appropriate fallback

Optical coherence biometry relies on a coherent infrared light beam passing through the optical media to the retinal pigment epithelium. A dense, brunescent, or mature cataract that transmits less than approximately 10% of light will scatter and attenuate the signal to the point where no reliable measurement is possible. In this situation, immersion A-scan ultrasound — which uses sound rather than light and is therefore unaffected by media opacity — is the gold-standard fallback and provides greater accuracy than contact A-scan by eliminating corneal indentation artifact. High myopia, post-vitrectomy status (without silicone oil), and even moderate keratoconus do not inherently prevent optical biometry measurement.