Controlling Myopia with RGPs

BY KARLA ZADNIK, O.D., Ph.D.
MAY 1996

Myopia research has undergone something of a renaissance over the last two decades. The two main avenues of investigation -- animal models of myopia describing how eye growth is modulated, and human studies seeking etiological information -- are converging, and there are at least seven U.S. patents pending for pharmacological modulators of ocular growth. There is also revived interest in non-pharmacological treatments for myopia, including juvenile myopia control with conventional rigid gas permeable contact lenses.

However, if we were to ask optometrists how often they prescribe RGP lenses for myopia control apart from orthokeratology, the response would probably be "underwhelming." What can the scientific literature tell us? Is a positive result possible? What do we know about the long-term effects of short-term myopia progression retardation? Should more studies be done?

PMMA LENSES SLOWED MYOPIA PROGRESSION

PMMA lenses were long touted for their ability to retard myopia progression in children. In the words of Janet Stone 20 years ago, "corneal lenses definitely slow down the rate of progress of myopia slightly, but . . . this is due partly to flattening of the cornea. As it is not entirely due to the corneal flattening, it is suggested that there is some retarding effect on axial elongation, but the mechanism for this is not known and requires further study."

THE HOUSTON STUDY

During the 1980s, clinical investigators at the University of Houston College of Optometry tackled an ambitious study in an attempt to generate a definite answer. They fitted 100 children between the ages of eight and 13 with RGP lenses and followed 56 of them for three years. The mean myopia progression for this group was -0.48 ± 0.70D, compared to a mean myopia progression of -1.53 ± 0.81D for a retrospective control, spectacle-wearing group from the earlier Houston Bifocal Study. Corneal flattening accounted for less than half the difference in myopia progression between the two groups.

The tantalizing aspect of the Houston results lies in this myopia retardation that exceeds that exhibited in corneal flattening alone. Although the Houston authors believe this represents more corneal flattening in the mid-peripheral cornea than was measured by keratometry, a more intriguing possibility exists. Perhaps wearing rigid contact lenses somehow slows ocular growth and prevents axial elongation of the globe.

RGPs FOR MYOPIA CONTROL:
A PRIME STUDY AREA

Are further studies needed? You bet! The Houston Study has been criticized for its complicated matching scheme of the contact lens and spectacle groups on age, gender and severity of myopia. In addition, 44 percent of children were lost to follow-up during the three-year study, limiting its strict interpretation. Although there are ultrasonographic axial length data for the group fitted with RGPs, no such data were available for the spectacle control group. The question simply hasn't been answered.

In the United States, as many as 15 percent of students entering high school have developed juvenile onset myopia. Many more develop first-time myopia or progress in their myopia during the college years. No investigations of the myopia-retarding properties of rigid gas permeable contact lenses in adult onset myopia have ever been conducted. This is a prime area for a carefully conducted study -- in children and adults -- to answer the question of whether skilled contact lens practitioners hold in their hands an efficacious and safe treatment for myopia. CLS

Dr. Zadnik is a senior optometrist engaged in patient-based optometric research at the University of California-Berkeley School of Optometry. She will join the faculty of The Ohio State University College of Optometry in July 1996.