More than 40% of North African population has corneal thickness below limit for refractive surgery

PRK with or without corneal cross-linking, corneal rings and phakic implants are potential alternatives for these patients.

Central corneal thickness in the North African population in Algeria, Tunisia and Morocco is lower than the average calculated thickness in populations in Europe and central Africa, according to a study.

“Conventionally, what we consider as a ‘normal’ cornea ranges between 537 µm and 550 µm. Five hundred microns has been accepted as a cutoff value for safe refractive surgery. Thinner corneas are at a risk for ectasia, haze and less predictable refractive outcomes. If we take this limit into consideration, more than 40% of Maghrebians should be excluded from refractive surgery,” Sihem Lazreg, MD, said at the meeting of the French Society of Ophthalmology in Paris.

This conclusion was based on a study involving 1,615 patients from three countries in the Maghreb region of North Africa. Control groups included 200 Caucasian European patients from Bordeaux, France, and 170 West African patients from Abidjan, Ivory Coast. Exclusion criteria were younger than 18 years, presence of corneal pathology and keratometry greater than 48 D. Age distribution was homogeneous in the three groups, and there was a slightly higher percentage (56%) of female subjects.

“We studied one eye per patient, the right eye in all the cases. Pachymetry was performed using the Pentacam (Oculus) in Algeria and Tunisia and the Orbscan (Bausch + Lomb) in Morocco. Keratometry and refraction were also measured,” Dr. Lazreg said.

Mean central corneal thickness in the Maghrebian population was 519 µm, with no significant difference among the three countries. However, more than one-third of the population had corneal thickness less than 500 µm.

“This was not a negligible percentage,” Dr. Lazreg said.

Mean keratometry in the same population was 44.23 D. More than 50% of the patients had myopia of more than 3 D.

“Compared with both control groups, and particularly with the European group, Maghrebian patients showed significantly lower corneal thickness values. In more details, 11% of Maghrebians had pachymetry less than 450 µm, 32% between 450 µm and 500 µm. In the European group, mean corneal thickness was 553 µm, with more than 90% of the corneas over 500 µm. No correlation was found between central pachymetry and the level of myopia,” Dr. Lazreg said.

Other studies

Dr. Lazreg said that there is little published literature on this topic.

A study conducted in Israel by Lifshitz and colleagues compared the corneal thickness of populations of different origins, reaching the conclusion that North African patients had significantly thinner corneas than other patients, averaging 518 µm compared with 545 µm.

A study by Aghaian and colleagues compared Asian, Hispanic, Caucasian and African-American populations. African-Americans were found to have a significantly thinner cornea, averaging 521 µm compared with 542 µm in the other populations.

The potential causes of this difference in corneal thickness are not evident, Dr. Lazreg said. Environmental factors, such as more exposure to sunlight, genetic factors or other factors could be involved.

“The consequence of what we found in our study is that more than one-third of our candidates in Morocco, Algeria and Tunisia cannot be accepted for refractive surgery,” she said.

Abstention from surgery and continuing surveillance might be the only option in some cases, but many patients could be offered alternatives.

“PRK with or without corneal cross-linking is an option, and so are corneal rings. Femto-LASIK and thin-flap LASIK should be considered with caution, because there is no proof that they provide sufficient protection against postoperative ectasia. Phakic implants could be implanted in some of these eyes,” Dr. Lazreg said.

She also noted that, beyond all other considerations, the corneas in this population should be considered thin, but not pathological. – by Michela Cimberle

References:

• Aghaian E, Choe JE, Lin S, Stamper RL. Central corneal thickness of Caucasians, Chinese, Hispanics, Filipinos, African Americans, and Japanese in a glaucoma clinic. Ophthalmol. 2004;111(12):2211-2219.
• Lifshitz T, Levy J, Rosen S, Belfair N, Levinger S. Central corneal thickness and its relationship to the patient’s origin. Eye (Lond). 2006;20(4):460-465.

• Sihem Lazreg, MD, can be reached at 21325417455 or 13770417369; e-mail: slazbkt@yahoo.fr.

These research findings further support the fact that there is not a direct relationship between corneal thickness and corneal rigidity in eyes with normal topography. Otherwise, there would be an extremely high rate of keratoconus in patients from the North African population in Algeria, Tunisia and Morocco.

In the medical literature, there are multiple published articles of LASIK in eyes with thin corneas (below 500 µm) with normal topography, and there were no cases of ectasia after LASIK reported.

We know that with corneal collagen cross-linking, the cornea becomes thinner as it becomes stiffer and stronger.

We know that in the same individual, as they get older, the cornea becomes stiffer and stronger, yet there is no change in corneal thickness. This is another finding that supports that the thickness of the cornea is not directly related to corneal rigidity. Otherwise, we would expect either the cornea to become thicker as we age, or we would expect that there would be no change in corneal stiffness with age.

I believe this research supports the fact that patients with thin corneas and normal topographies can potentially be considered appropriate candidates for LASIK, although I would recommend intraoperative pachymetry in these cases to ensure the flap is not too thick.

Further research in this area is important. An evaluation of eyes with preoperative thin corneas that have developed ectasia after LASIK should be analyzed, with special attention to cases in which the intraoperative bed was measured to ensure that a deeper than expected flap did not occur.

– William B. Trattler, MD
OSN SuperSite Board Member