In highly myopic eyes, B-scan biometry locates macula

Study shows B-scan biometry should be part of preop assessment for clear lens extraction.

ByJeanne Gonzalez

BIRMINGHAM, England – B-scan biometry helps to locate the macula in highly myopic patients scheduled for clear lens extraction, according to clinicians here. This method can provide a better means of calculating IOL power than A-scan measurement in these patients.

“This study was really conducted to show how we can use the B-scan,” said Peter Good, PhD, of the Birmingham and Midland Eye Center here.

“Because the eyes of these patients are not spherical and have staphyloma, it can be very difficult to locate the macula. What you’re using the B-scan for is to accurately locate the macula,” he said.

Previously, A-scan measurement was used for these eyes with a resulting high failure rate. Because the A-scan ultrasound can be “fooled” by tilted discs or by posterior staphyloma, it cannot always locate the macula, he said. Calculating IOL power from an incorrect A-scan measurement can cause a refractive error postop.

“The problem with the A-scan is that you don’t always know if you’re focusing on the macula. If you have not focused on the macula, and then were to use that measurement to asses which power of IOL to put in the eye, the patient would have a significant refractive error postoperatively,” Dr. Good said.

Dr. Good and colleagues studied the accuracy of B-scan biometry in locating the macula. In the study, they compared A- and B-scan measurements. The B-scan was highly effective in measuring the axial length in long myopic eyes, he said.

“With the B-scan, we are up to a 98% success rate in getting patients within 1 D of their desired postoperative refraction.”

Their success with this technique over the past 2 years has led them to use B-scan routinely in evaluation of patients with long eyes. The device can also be used to assess IOL power in preoperative evaluation for cataracts in myopic eyes, he said.

Ultrasound control

In a study of the technique, Dr. Good and colleagues evaluated 12 patients; 10 measurements were taken for each eye. Refractive errors ranged from –11 D to –24 D; the mean was –18 D. The Quantel Medical Axis II biometry machine and the Quantel Medical Cine Scan A/B (Quantel Medical Inc.) were used to measure the axial lengths.

“The anterior chamber depth was confirmed using the Axis II (longest AC depth and shortest axial length), and these measurements were compared with the B-scan measurement of the anterior chamber depth. The location of the macula was achieved by measuring 3.5 mm temporally to the optic disc,” he said.

The B-scan machine has the ability to lower the sensitivity or intensity of the ultrasound near the front of the eye, he said. This allows clarity in viewing the cornea and the front of the lens and helps ensure accurate measurements, he noted.

Results

Of the patients measured, the range of axial lengths was 25.2 mm to 33.2 mm, with a mean length of 27.4 mm. Posterior staphyloma were found in 61%; 78% of patients had tilted discs. All patients achieved postop refraction within 1 D of predicted refraction and 83% achieved postop refraction within 0.5 D of prediction.

“If A-scan measurements alone had been used, only 69% would have achieved a postoperative refraction within 1 D of that predicted; three eyes would have been 2 D away from the predicted value, and two of them were hypermetropic,” Dr. Good said.

For Your Information:

Peter Good, PhD, can be reached at the Birmingham and Midland Eye Center, City Hospital NHS Trust, Dudley Road, Birmingham B187QU, England; +(44) 121-507-6744; fax: +(44) 121-507-6886; e-mail: peter.good@cityhospbham.wmids.nhs.uk.

Quantel Medical, manufacturer of the CineScan and the Axis II Precision A-Scan, can be reached at 21 rue Newton — Z.I. Le Brézet, 63069 Clermont-Ferrand Cedex 2, France; +(33) 4-73-745-745; fax: +(33) 4-73-745-700.