Five steps may help fight progressive myopia
A five-step strategy, based on functional aspects and pathogenetic considerations, could help prevent the onset and reduce the progression of myopia in children and adults.
“Myopia is growing to epidemic proportions. It affects 85% to 90% of adults in Asian cities and as much as 33% in the United States. Developing effective prevention and treatment strategies is crucial and urgent,” OSN Optics Section Editor Jack T. Holladay, MD, MSEE, FACS, said in an interview with Ocular Surgery News.
The pharmacological relaxation of sustained accommodation in childhood has shown mild success, whereas optical strategies such as bifocals, progressive addition lenses, contact lenses, orthokeratology and undercorrection have been less successful. The primary side effect of atropine is mydriasis causing photophobia and the need for photochromic lenses to protect from harmful UV light. In addition, there appears to be an initial increased rate of myopia progression after the cessation of atropine treatment (“rebound”), presumably due to the strong cycloplegic effect of the atropine. Also, the psychological effects on children need to be taken into consideration, Holladay said.
According to Holladay, two factors have been overlooked in the pathogenesis of myopia, and these factors provide the rationale for a new myopia treatment strategy.
The first overlooked factor is sustained accommodative convergence. The convergence is mediated by the medial rectus muscles, which contract simultaneously. The origin of the medial rectus is from both the upper and the lower parts of the common ring tendon (annulus of Zinn) and from the sheath of the optic nerve. The insertion of the medial rectus is about 5.5 mm from the medial limbus.
“The contractile force of the medial rectus is transferred from its insertion as a tensile force to the medial perilimbal sclera, then to the cornea, the temporal perilimbal sclera, the temporal sclera, posterior pole and finally anchored to the lamina cribrosa and sheath of the optic nerve. The stretching or tensile force is equal (in equilibrium) in all these components, like a chain of five rubber bands of unequal elasticity linked end to end. The tensile strength of the cornea is several times that of the perilimbal sclera, which in turn is more than the temporal sclera and posterior pole. The latter two are the weakest links in the chain, and the stretching results in elongation of the axial length with a commensurate increase in myopia,” Holladay said.
As one approaches the posterior pole, the curvature of the globe increases so that the path of the outer scleral collagen fibers is longer than the inner fibers, creating more stretch on the outer fibers in this area. As the outer scleral fibers of the posterior pole fatigue, the tensile force is transferred to the inner sclera and then through the choroid, which provides no support because it is like a sponge. The next structure moving inward is Bruch’s membrane, which is inelastic and more like glass or plastic. As it fatigues from the tensile force, lacquer cracks form and then the pigment epithelium is disrupted, forming Fuchs spots. Finally, there is damage to the photoreceptors and inner retina with loss of central vision, Holladay said.
The second overlooked factor that determines a person’s response to the tensile or stretching force in the temporal sclera and posterior pole is the individual’s elastic properties of the sclera. Instruments such as the Corvis (Oculus) and Ocular Response Analyzer (ORA, Reichert) are available to evaluate the elasticity of collagen in the eye. Just as with keratoconus and open-angle glaucoma, increased elasticity of the collagen makes a person more susceptible to stretching of the sclera and progressive myopia. These elasticity measurements can be taken in young children to identify those at highest risk for myopic progression.
“My goal is to change the way people think about progressive myopia and show a new way of dealing with it by using a five-step approach,” Holladay said.
The first step is to identify children with a family history of myopia.
“Studies on the genetics of myopia are well established. In 10 or 15 years, we may be able to locate the genetic marker and not only change the progressive myopia gene but also the collagen gene. Meanwhile, we can identify children who are at risk by looking at their family history and measuring refraction and scleral elasticity early in their life,” Holladay said.
The second step is to increase the number of hours of outdoor activity. Studies have shown that increasing the hours of outdoor play has a more significant effect on the progression of myopia than any other intervention to date.
“By regularly interrupting near activities, such as playing outside, the sustained stretch of looking at near is reduced to a momentary stretch of maybe 30 to 40 minutes, with pauses in between, in which the medial rectus muscles relax as well as the sustained stretch on the sclera,” Holladay said.
The third step is a new paradigm, measuring collagen elasticity as a risk factor with the Corvis and ORA.
After this, as a fourth step, collagen cross-linking in the temporal sclera and posterior pole will be performed in children and young adults with unusually elastic sclera, like the treatment of keratoconus.
“Finally, it is still beneficial to use bifocals when children are working at near, with a slight undercorrection for distance and including a base-in prism in the add to eliminate convergence at near. Convergence is a major cause of progressive myopia, perhaps even more than accommodation,” Holladay said.
Traditional bifocals that include a base-in prism in the lower segment are already made for low-vision patients. However, the near add is usually +6 D and would have to be reduced to +3 D, with a prism of 10 base-in in each lens. Progressive bifocals will require a progressive base-in prism. In addition, the child should be taught to hold objects no closer than 33 cm and preferably even farther.
“It is a simple five-step strategy and should have a great impact on reducing the myopic epidemic. Expanding cross-linking technology to the sclera will occur in the near future, and the other four steps can be implemented immediately,” Holladay said. – by Michela Cimberle
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- Jack T. Holladay, MD, MSEE, FACS, is a clinical professor of ophthalmology at Baylor College of Medicine. He can be reached at email: email@example.com.
Disclosure: Holladay reports he is a consultant for Oculus.