Understanding ectasia

	Preoperative topography of the right eye shows oblique regular astigmatism.
	Right eye topography before enhancement surgery shows asymetric flattening superiorly and nasally and 2 to 3 D steepening inferior and temporal to fixation.
	Ten months after LASIK, right eye topography shows increased paracentral steepening of 7 to 9 D.
	Thirteen months after LASIK, topography shows markedly increased central steepening (left), which progressed during the next 9 months (right).
		External photograph of right eye shows a LASIK flap that is markedly decentered nasally.

Experts believe that ectasias? which usually develop within the first year or 2 after LASIK? occur in cases where the practitioner performed the refractive surgery on an inappropriate cornea? especially one that was too thin.

In a normal cornea? ectasias may result when too large an optical zone has been used; a cut has reached the in-situ stroma? leaving a part of the cornea too thin; or too high a myopic correction has been attempted.

Phillip A. McGeorge, MD, Murdoch Eye Center, Murdoch, Australia, said eyes that are at high risk of corneal ectasia after LASIK may have unstable refractions and variable anterior and posterior surface bowing before the development of frank ectasia.

"This pre-ectasic condition may respond to ocular antihypertensives prior to the permanent stretching of posterior lamellar fibers," he said.

Ocular antihypertensives

In a series, McGeorge used ocular antihypertensives topically to treat late myopic regression following LASIK for high myopia.

In May 1998, a patient with a refractive error of -11 D in each eye was treated with a Hansatome 160-µm head and the 217C Technolas excimer laser, a multizone treatment.

"Preop and intraoperative ultrasonic pachymetry showed relatively thin corneas initially; but fortunately, we had very thin corneal flaps and the residual beds after treatment were at 227 and 232 µm," he said.

Post-LASIK refraction remained stable at 12 months and the patient was uncorrected 20/20 minus (right and left). However, at 18 months postop, she complained of the recent onset of blurred vision. Her uncorrected acuity had dropped to 20/25 and 20/40, with a myopic regression of -0.5 D and -1.5 D.

Orbscan showed a relatively pre-ectasic cornea with a posterior surface forward bowing. McGeorge started the patient on two ocular antihypertensives, Timoptic XE 0.5% (timolol maleate, 5 mL, Merck) drops (once daily in both eyes), and Alphagan (brimonidine tartrate ophthalmic solution 0.15%, Allergan) drops twice daily in both eyes.

She reported marked symptomatic improvement. "Her vision had improved to 20/25 in both eyes with hyperopic shift in refraction of 0.5 D and +1.82 D. She had an improved overall Orbscan appearance as well. There was anterior and posterior surface flattening after treatment with the ocular antihypertensives, and the anterior and posterior elevation map showed central flattening," he reported.

The hyperopic shift appears on the corneal surface, with the difference in optical power of +2 D, he added.

At 36 months postop, the patient's vision has remained stable while maintaining topical treatment. However, within days of stopping the drops, her vision changed. As a result, the patient plans to continue with the drops and no re-treatment is planned.

The point of no return

Harry S. Geggel, MD, director of refractive surgery, Virginia Mason Medical Center, Seattle, reported on a case of a 44-year-old woman with moderate myopia who developed severe ectasia after LASIK. This complication resulted in a corneal transplant.

"The high-volume [LASIK] surgeon in our area did not measure the preop corneal thickness. The patient had relatively normal K readings. Preop topography showed a fairly regular pattern," Geggel said.

In June 1996, the patient had bilateral LASIK, which had a positive initial result. By 3 months postop, the patient had lost best-corrected spectacle vision and complained of ghost images. She underwent enhancement surgery. Pre-enhancement, there was one area of steepening beginning to emerge. One day postenhancement, one area still persisted with a little bit of steepening, he said.

Ten months postop, the patient has increased myopia and cylinder. When Geggel picked up the case at 13 months postop, the patient had increased myopia, loss of best spectacle vision, and a corneal thickness of 449 µm. However, by 22 months postop, her corneal thickness was down to 260 µm and she underwent corneal transplantation. At the time of transplantation, scanning EM photographs and light microscopy were obtained.

"Since the [original] surgeon did not measure corneal thickness, we assumed that if the left eye and the right eye are the same thickness, since the left eye did not develop this complication, postop corneal thickness was 466 µm. That eye also underwent enhancement surgery and, if you add up all the numbers and work backward, her preop corneal thickness may have been 543 µm, which is, as we all know, normal," Geggel said.

Richard L. Lindstrom, MD, clinical professor of ophthalmology at the University of Minnesota, Minneapolis, has seen an unusual case of ectasia in his group practice. One patient with 550-µm corneas had normal corneal topography.

The surgeon performed the cut with a Hansatome 180 with a 150-µm flap and removed about 40 µm of tissue. Residual tissue in the bed was about 360 µm. "At the end of the case, we had perfect results for about 3 weeks and then [it] blew out bilaterally," he said.

"Even with absolutely normal topography, 550 µm, a -4-D myope with a 350-µm residual bed developed frank keratoconus within 6 months and is now wearing a gas-permeable hard contact lens," he said.

Lindstrom said that there are some patients with abnormal corneas that cannot be measured and sometimes, no matter what is done, ectasia will result. "In our series, it would be in one in 35,000 cases, which is the number of LASIKs we've done now," he said.