Prepare Yourself to Comanage LASIK Patients

BY DAVID I. GEFFEN, O.D.

Are you prepared to comanage LASIK patients? It is estimated that by the end of 1998, LASIK will account for more than half of all refractive procedures. Optometrists must become comfortable with the procedure in order to provide proper follow-up care for these patients.

GAUGING ABLATION DEPTH

The average corneal thickness is 550 ± 100 microns, and the normal flap thickness is approximately 160 microns. Of the remaining 390 microns of corneal tissue, how much corneal tissue can be safely ablated?

Although we don't have a specific answer, 250 microns appears to be a safe bet. A corneal thickness of less than 200 microns may result in corneal ectasia. This leaves l40 microns of tissue that can potentially be removed. Current lasers ablate approximately 12.5 microns per diopter, thereby theoretically allowing a maximum of 11.25D of myopic correction that can be performed safely (give or take three or four diopters, depending on initial corneal thickness). Doctors can approximate correction using the Munnerlyn formula: depth of ablation = diopters of correction x ablation diameter x 2/3.

Surgeons perform LASIK on much higher prescriptions at this time, but are we sure this is safe? Hard data has not yet appeared in the literature to support or refute it. Some preliminary data shows that in using LASIK to correct more than 12.00D of myopia, the reliability of the procedure starts to break down, yielding variable results. It appears that the higher the prescription, the higher the incidence of decreased best-corrected visual acuity.

CONSIDER THE K'S

It's also important to consider the keratometry readings. Preliminary data suggests that visual acuity drops when the cornea is flattened beyond 36.00D, although it's not yet clear why. Some researchers believe it's because optical aberrations increase to a point, which affects visual acuity negatively when flattening the cornea excessively. Others feel that the cornea becomes irregular when flatter than 36.00D.

Therefore, when counseling a patient about the possible outcomes, you must take into account the baseline keratometry reading and the anticipated endpoint. Some practitioners will approximate the endpoint by subtracting 80 percent of the desired correction from the initial keratometry readings. If the calculation shows less than 36.00D, you may need to back off of the desired goal.

POSTOP PROTOCOL

After the surgery, follow-up care for LASIK patients is very straightforward. We see the patients at day one, at which point we remove the occluder, check uncorrected visual acuity and perform a careful slit lamp examination. Expect acuity to be 20/40 or better for most people. Biomicroscopy will show a relatively clear cornea. There may be a slight trace of stain at the cut for the flap, which usually disappears after 18 hours postoperatively. You may also see what appears to be debris under the flap. This is not unusual and is probably the result of traces of particles from the air or the tear film that get caught under the flap.

The next scheduled follow-up is at two weeks. We repeat the tests performed at the one-day visit and tack on a refraction. We should have a good idea at this point where the patient will end up in terms of the final prescription. If there's a residual refractive error, it's too soon to schedule an enhancement. You may want to provide a temporary pair of spectacles for patients who need help in the interim.

The next visit is at six weeks. We repeat all previous tests and may schedule an enhancement procedure for approximately two weeks later. If all is well, we see the patient at three months and then again at six months.

While LASIK is a procedure that requires great skill on the part of the surgeon, it is a fairly straightforward task for the optometrist to follow. There's usually no medication management. The primary responsibility at follow-up visits is optical in nature and is uniquely specific to optometry's talents. CLS

Dr. Geffen is in a joint refractive surgery practice in San Diego.

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