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Research defines subclinical keratoconus before LASIK
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---Elevation patterns of ectasia in keratoconus.
SEATTLE — Elevation corneal topography provides a sensitive and a specific way to detect subclinical keratoconus and screen out poor candidates for refractive surgery, surgeons reported at the annual meeting of the American Society of Cataract and Refractive Surgery.
According to John A. Vukich, MD, “We now have the ability to image the posterior surface of the cornea. Up until this point, we’ve really had no practical means of doing that. We have certainly not known what is normal and what is not.”
Recognizing subclinical keratoconus also keeps surgeons from turning away good candidates for refractive surgery because of false keratoconus, said Anita Nevyas-Wallace, MD.
“We need to recognize characteristic patterns on the elevation maps and also to apply quantitative criteria,” she said. Her research identified two shapes of ectasia, mound and peninsula, and four different positions on the cornea where the ectasia could be located (Figure 1).
Ectasia measuring at least 12 µm in elevation above the best fit sphere is shaped either as a mound or a peninsula and its location is either central, temporal, inferotemporal or inferior. Most patients had inferotemporal ectasia.
Subclinical keratoconus is identified by the presence of either a mound or a peninsula measuring at least 12 µm in height above the best fit sphere. The maximum curvature is between 42 D and 47 D. In clinical keratoconus, the mound or peninsula measures more than 20 µm in height and the maximum curvature is more than 47 D (Figure 2).
Unilateral signals bilateral
---Example of false negative: normal clinical appearance and curvature map with significant ectasia on elevation mapping.
Dr. Nevyas-Wallace evaluated 103 eyes of 53 patients known or suspected to have either clinical or subclinical keratoconus. The remaining three eyes had had corneal grafts for keratoconus.
She identified 11 patients who had unilateral clinical keratoconus. In each of these 11 patients, the fellow eye was normal at slit lamp exam.
“We suspected that patients with unilateral slit lamp signs of keratoconus would have bilateral evidence topographically of keratoconus. We had hypothesized that if these 11 eyes showed abnormalities on elevation topography, then these abnormalities would define subclinical keratoconus,” she said.
She evaluated the elevation maps of 67 eyes that had slit lamp evidence of keratoconus, and the 11 clinically normal fellow eyes of the unilateral keratoconus patients. This established criteria for the diagnosis of keratoconus on elevation corneal topography. Those criteria were then applied to 17 maps of eyes suspected of having subclinical keratoconus.
Dr. Nevyas-Wallace found two false positives and two false negatives. The false positives occurred in two patients whose curvature maps erroneously suggested subclinical keratoconus. The false negatives occurred in two patients whose curvature maps failed to detect subclinical keratoconus but whose elevation maps revealed subclinical keratoconus with measurable ectasia.
Posterior corneal feature
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Dr. Vukich said he undertook a study to learn how common abnormalities occur in laser in situ keratomileusis (LASIK) patients cleared to undergo the procedure. He undertook a study of 200 eyes of 100 consecutive patients referred to him for surgery who had gone through a screening process and were ready to have the procedure.
He used the Orbscan (Bausch & Lomb Surgical; Claremont, Calif.) unit to generate corneal maps using the normal band scale method. He created an anterior surface elevation map, a posterior surface elevation, mean corneal power and a pachymetry map for each patient.
Using a normal band scale developed by Donald R. Sanders, MD, PhD, the anterior and the posterior elevations were set at ±25 µm to filter out anything above or below that range from the normal reference sphere. Total corneal power was set between 40 D and 48 D and pachymetry was set between 500 µm and 600 µm.
Anterior and posterior elevation maps produce a lot of information that is not always useful, Dr. Vukich said. “The normal band scale is an attempt to give us some filtration of the data so that we really see what’s important,” he said.
If abnormalities were detected when those parameters were used for filtration, then a full band scale and a full unfiltered quad map was generated to further analyze those patients.
Results showed that patients who had an anterior elevation always had a co-existing posterior elevation. The converse was not true, however. There were isolated incidents of posterior elevation abnormalities without any other anterior features.
Of 200 consecutive eyes, 11% showed a posterior corneal elevation combined with an associated anterior elevation, a localized corneal steepening or localized corneal thinning. Another 9% showed posterior corneal elevation alone. Also, 2% of cases had corneas thinner than 500 µm and 4% had corneas thicker than 600 µm.
False positives, negatives
In Dr. Nevyas-Wallace’s study, a false negative is a patient whose elevation map reveals subclinical keratoconus that was not detected by curvature mapping. For example, in Figure 3 the curvature map is unremarkable, with a maximum steepening of 44 D. The patient had no slit lamp sign of keratoconus. However, the elevation map reveals an inferotemporal mound of 15 µm in elevation, indicative of subclinical keratoconus.
In contrast, a false positive is a patient whose curvature map suggests keratoconus but whose elevation map reveals no ectasia. For example, the patient in Figure 4 has a Placido-derived curvature map showing inferior steepening of more than 2 D greater than superior, erroneously suggesting subclinical keratoconus. However, the elevation map reveals this inferior steep area to be depressed rather than elevated. Because there is no mound of height, the patient does not have subclinical keratoconus and does not need to be denied refractive surgery, Dr. Nevyas-Wallace pointed out.
For Your Information:
- Anita Nevyas-Wallace, MD, practices at Two Bala Plaza — 333 City Line, Bala Cynwyd, PA 19004-1501; (610) 668-2935; fax: (610) 668-1509. Dr. Nevyas-Wallace has no direct financial interest in any of the products mentioned in this article, nor is she a paid consultant for any companies mentioned.
- John A. Vukich, MD, practices at 1025 Regent St., Madison, WI 53715; (608) 282-2002; fax: (608) 282-2048. Dr. Vukich has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.