Ensuring accuracy of A-scan ultrasound

Correlate A-scan & biometry measurements to improve accuracy in IOL calculations. This case shows how disparate measurements can lead to an incorrect IOL power calculation.

While there are multiple methods and formulas for calculating the IOL power for cataract surgery, none of them will give an accurate result if the biometry is imprecise. As the saying goes, “Garbage in, garbage out.” One of the most common situations in which we have inaccurate measurements is when an opaque cataract prevents our optical coherence biometry from measuring the axial length.

A mistake of 1 D in the keratometry reading will give about 1 D of error in the IOL power calculation. However, a 1 mm mistake in axial length measurement can give a 3 D error in the IOL power calculation. When we use optical coherence biometers to measure the axial length, the process is fairly automated, and even a novice ophthalmic technician can do an accurate job. But when we need to revert back to using the A-scan ultrasound to measure the axial length, operator experience is far more critical in order to get a precise measurement.

In the clinical case presented here, the patient had a posterior subcapsular cataract that was much worse in the right eye, leaving her with just 20/400 vision and terrible glare from light sources (see pic above). The patient had a history of being close to emmetropia in both eyes, and she passed her driving test, without glasses, decades ago with both eyes seeing the required 20/40 or better. There was no anisometropia in her glasses from 5 years ago, with each eye having a refraction of –0.50 + 1.00 × 90 with a +2.50 progressive near add.

The technician repeated axial length testing with the optical coherence biometer 20 times in the right eye but still was not able to get a measurement (see pic above). The left eye was measured eight times, and a consistent measurement was achieved. Importantly, the composite axial length data in the right eye had a signal-to-noise ratio of 1.2 to 1. This means that it was nearly equal parts signal and noise. This is too noisy to get any meaningful estimation of the axial length. The left eye was much better, and the composite axial length was accurate, determined with data that had a signal-to-noise ratio of 42.9 to 1. This is a good signal strength, and this axial length is accurate and repeatable.

The technician knew that the next step would be to measure the axial length using the A-scan ultrasound probe. This allows an axial length measurement even if the cataract is completely opaque or if the posterior subcapsular opacities are confluent and obstructive. When placing the A-scan ultrasound on the ocular surface, care must be taken to avoid corneal compression, which would result in an erroneously short axial length. A more accurate way is to use immersion A-scan, which involves placing a saline-filled shell on the ocular surface so that the probe tip is immersed in fluid and not directly touching the cornea.

The ophthalmic technician performed a contact A-scan, not an immersion A-scan, and determined the right eye axial length to be 22.89 mm. The technician did not measure the left eye using contact A-scan because he figured that it was already measured using the optical coherence biometer. This was not the best clinical decision.

The A-scan value of 22.89 mm was manually entered into the biometer in order to perform IOL calculations (see pic above). The machine alerts the surgeon to this manually entered value by placing an asterisk after it. This surgeon was using the Holladay formula for calculations, but the choice of formula was not the critical step in this case. The IOL calculation for the right eye showed a power of +23.0 for a goal of close to plano; however, the left eye showed a power of +20.5 for the same refractive target. This anisometropia of +2.5 D at the IOL plane would be about +1.6 D of anisometropia at the spectacle plane. But this patient had a history of an equal refractive error in both eyes.

The A-scan ultrasound should have been performed in both eyes. Even though the left eye had an accurate axial length with optical coherence biometry, performing the A-scan would allow the technician to determine if the technique was good or if corneal compression was occurring. Likely, the technician would have measured the left eye as significantly short with contact A-scan as compared with the optical coherence-determined value. This would then be a red flag that would be brought to the attention of the ophthalmologist. Fortunately, this surgeon realized the issue before the surgery was performed, and the A-scan was repeated using immersion for both eyes. This time the right eye was measured to be 23.69 mm and the left eye 23.70 mm. Using this new data, a +20.5 IOL was placed in the right eye, and the patient achieved the desired plano spherical equivalent post-op refraction.

click below for the video that explains this important biometry lesson:


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