Why single-instrument screening for keratoconus remains a dream
The promise of simple, objective, single instrument-based screening for highly asymmetric keratoconus in patients considering LASIK is still more of an idea than reality. Multiple metrics can identify early keratoconus in a minority of affected eyes, but none of the metrics consistently identify early corneal abnormalities and none of the metrics identify the same eyes as being at elevated risk for ectatic disease.
“We know that the risk of ectasia is highest in patients with some predisposition to keratoconus,” said J. Bradley Randleman, MD, professor of ophthalmology, University of Southern California Keck School of Medicine and director of cornea and refractive surgery at the USC Roski Eye Institute, Los Angeles, CA. “In patients with highly asymmetric keratoconus, the less affected eye should, in theory, be the one to be able to test imaging modalities and develop simple screening criteria. The problem is that none of the metrics we have and none of the instruments we have, do a particularly good job in isolation of distinguishing between normal eyes and the less affected eye with great precision.”
Dr. Randleman, who is also Editor-in-Chief of The Journal of Refractive Surgery, presented an analysis of metrics derived solely from Scheimpflug imaging (Pentacam HR, Oculus) for the detection in highly asymmetric eyes with keratoconus.
LASIK is one of the most successful optical surgeries, but success is dependent on good patient selection. Keratoconus can dramatically increase the risk of ectatic disease following refractive surgery. A minority of patients with keratoconus present with what is traditionally called unilateral keratoconus, or pronounced disease in one eye only.
But “unilateral” is misleading, Dr. Randleman explained, as patients rarely have keratoconus in only one eye. Like many ophthalmologists, he prefers the term highly asymmetric keratoconus as a more accurate description. Keratoconus is usually easy enough to determine when patients have obvious signs of keratoconus and avoid LASIK, but the subtler, initial findings are the ones still in discussion and the ones that make screening challenging.
Clinicians traditionally assess these questionable eyes using a subjective combination of corneal measurements and clinical experience. A global consensus published in 2015 (Gomes et al. Cornea. 2015;34(4):359-369) reported that tomography was the best way to screen for early or subclinical keratoconus and that posterior corneal elevation abnormalities must be present. Many authorities take exception to these conclusions, citing a lack of evidence.