Knowledge of corneal topography essential for LASIK screening

NAPLES, Italy — Surgeons should have a good working knowledge of corneal topography and topographers to help them effectively screen patients for LASIK contraindications such as forme fruste keratoconus, according to a surgeon here.

Corneal topography is a mature diagnostic tool that can predict within a narrow margin of error the risk for ectasia after LASIK, Stephen D. Klyce, PhD, said at the joint meeting of Ocular Surgery News and the Italian Society of Ophthalmology.

Educated use and the choice of the most suitable equipment are essential to meet the standards of care in screening patients and to avoid LASIK disasters, Dr. Klyce said.

As corneal topographers have evolved over the years, some have developed complex displays with a lot of data. While an expert might understand how to integrate all of this information, most of it is unnecessary in routine clinical use.

"A topographer that adopts a fixed-interval and contrasting-color scale with a few added familiar measures like pupil diameter and simulated keratometry should be sufficient," Dr. Klyce said.

It is useful to have a routine benchmark with which to occasionally test a corneal topographer to ensure calibration is being maintained, he said.

In reality, corneal topography actually measures the shape of the tear film, he noted. If the patient has dry eyes or has been given drops before examination, irregularities in the topography will be recorded. They are temporary artifacts and do not represent the true shape of the cornea. To avoid misreadings, the patient should be asked to blink several times before capturing the images, Dr. Klyce recommended. If the mires remain irregular, the presence of a corneal pathology should be considered.

A normal cornea has smooth contours and centrally uniform power and is flatter in the periphery, particularly toward the nasal side. Simulated keratometry readings should be about 42.75 D. A good rule of thumb would be to consider keratometry readings less than 38 D or greater than 47.5 D abnormal. Normal corneas may also present the characteristic bow-tie pattern of corneal cylinder.

"Any deviation from this pattern, like a truncated bow tie or a 'lazy eight' bow tie, should be red flags during the screening process," Dr. Klyce said.

Contact lens warping can masquerade as keratoconus by causing inferior or superior steepening in corneas that have with-the-rule astigmatism. Discontinuation of contact lens wear and re-examination with corneal topography 2 to 3 weeks later should allow the clinician to test for refractive stability and differentiate between true keratoconus and pseudo-keratoconus due to contact lens warping.

"With true keratoconus, the area of steepening will generally increase, while with contact lens warpage, a symmetrical bow-tie pattern will often re-emerge," Dr. Klyce explained.

Currently, several classification schemes are available on corneal topographers to assist in differentiating keratoconus suspect from normal variations in corneal topography. These include the Tomey Smolek/Klyce keratoconus program, the Humphrey Pathfinder and the NIDEK Magellan Navigator. The latter program can differentiate between keratoconus, keratoconus suspect, pellucid marginal degeneration and several other conditions, and it has the capability of device-independence. Alternatively, a modified Rabinowitz test can be used. If the diopter value along a 3-mm superior arc less the diopter value along a 3-mm inferior arc is greater than 1.4 D and less or equal to 1.9 D, then it qualifies as a keratoconus cornea.

"While this test is not very specific, it can provide guidance to distinguish between normal and abnormal topography and has the advantage of being applicable to any corneal topographer using the computer cursor to display dioptric values from the color-coded map," Dr. Klyce said.

Corneal pachymetry should be performed with either ultrasound or one of the slit-based pachymeters.

"Be suspicious of thin cornea, of a pair of corneas with significantly different central thickness and peripheral values not substantially thicker than the central values," he said.