Association between myopia and glaucoma concerning, but may not be real

Although evidence from a number of studies shows an association between myopia and glaucomatous-appearing optic disc damage and visual field loss, whether myopia is really a risk factor for glaucoma remains an unanswered question.

“The issue of myopia as a risk factor for glaucoma is important because there is an explosion in the prevalence of myopia in certain ethnic groups,” reported Shan Lin, MD, “Therefore, the question arises of whether there will be an impending glaucoma epidemic.

Dr. Lin, professor of ophthalmology and co-director, glaucoma service at the University of California San Francisco School of Medicine, outlined his points in a presentation entitled, ”Myopia: The Next Glaucoma Epidemic?”

“However, while population-based studies show that the prevalence of glaucoma is higher among myopes than non-myopes and increases with increasing levels of myopia, perhaps the changes in the myopes are really explained by myopic optic neuropathy,” Dr. Lin said.

Data shows increase

Data about the increasing prevalence of myopia, particularly within Asian populations, is available from a number of studies. Dr. Lin cited a cross-sectional study of persons living in the Taipei region of Taiwan that found 84% of high school seniors had myopia and 16% were highly myopic, defined as ≥ -6 D.

Population-based studies outside of Asia also have found myopia increases the risk for glaucoma. In the Blue Mountains Eye Study, which enrolled an Australian Caucasian population of about 3,200 persons, the risk of glaucoma, as assessed by optic disc and visual field changes, was increased 2.3 fold among low myopes (-1.0 D to -2.9 D) relative to non-myopes, while moderate-to-high myopes had a 3-fold increased risk.

“The Blue Mountains Eye Study found the relationship between myopia and glaucoma remained after adjusting for known glaucoma risk factors and was independent of IOP,” Dr. Lin said. “In addition, when looking at the effect of age on glaucoma risk, there as an asymptotic increase in glaucoma prevalence with increasing age among myopes, just as there is between age and glaucoma in general.”

In the Beijing Eye Study, which is a more recent population-based study including about 4,300 participants, a diagnosis of glaucoma was made based on optic-disc criteria only or defined as perimetric glaucoma requiring optic disc and visual field defects. Relative to emmetropes, persons categorized as having marked (< -6 D to -8 D) or high myopia (≥ -8 D) had a 5.5-fold increased risk of having optic disc glaucoma and a 6.3-fold increased risk for perimetric glaucoma.

A recently published cross-sectional study conducted by Dr. Lin and colleagues [Invest Ophthalmol Vis Sci 2013;54:830-5] looked at the association of myopia and glaucoma in the U.S. population. It included data collected from 2005-2008 from about 5,300 CDC participants aged 40 years and older.

Myopia was categorized based on autorefraction data. Associations between refractive status and glaucoma were analyzed using multivariate models to control for confounders and with the diagnosis of glaucoma based on self-report (patients were asked if they had been told by an eye doctor that they had glaucoma or elevated IOP), vertical cup:disc ratio graded from standard photographs by masked readers at a central reading center, and visual field measured by frequency doubling technology (FDT).

The results showed no association between myopia and glaucoma using the self-reported information or cup:disc ratio. However, there was a strong association between myopia and glaucoma based on visual field data, such that compared with non-myopes, the risk of glaucoma was increased 2.2 fold among mild myopes (-1 D to -2.99 D), 3 fold among moderate myopes (-3 D to -5.99 D), and 14.4 fold among severe myopes (> -6 D).

The study also considered the severity of the visual field defect, based on a validated rating system, and found that as the level of myopia increased, so too did the risk for moderate and severe visual field loss.

“The lack of association between myopia and self-reported glaucoma in the study and the discrepancy with the findings for glaucoma diagnosed by visual field loss raises an important question of whether we are under-diagnosing or under-detecting glaucoma among severe myopes,” Dr. Lin said.

“However, it is also important to note that the visual field defects were based on FDT because that may explain in part why our study, in contrast to some other research, found an almost exponential growth in glaucoma with increasing myopia. FDT may be more sensitive than standard automated perimetry for detecting glaucoma earlier, but it may also generate more false positive results.”

Dr. Lin added that the study methodology aimed to minimize false positives through its criteria for defining perimetric glaucoma.

“However, the finding of more advanced visual field defects among the higher myopes is concerning,” he said.

Differential diagnosis

Weighing in on the issue of whether or not the visual defects found in the myopes are all due to glaucoma, Dr. Lin offered his opinion that many are not, but rather they are fixed visual field defects associated with myopia.

The distribution of glaucoma-related and myopic visual field defects is unknown, but this issue was brought to the forefront in a 2007 retrospective study by Doshi et al. that reviewed data from 16 young to middle-aged Chinese males who were initially diagnosed as glaucoma suspects or having glaucoma. All patients received treatment as if they had glaucoma, and during an average follow-up of 7 years, none showed progression of optic nerve cupping or visual field changes.

To explain the findings, the investigators hypothesized that elongation of the eye in myopes places strain on susceptible axons within the prelaminar optic nerve head tissue, resulting in optic disc damage. However, once the myope reaches adulthood and the eye stops growing, the optic disc damage is fixed and any related visual field defect also stays static.

“This concept colors our idea of whether these myopes really have glaucoma,” Dr. Lin said.

FYI:

Shan Lin, MD, has no direct financial interest in the subject matter he discussed.

e. lins@vision.ucsf.edu

TAKE HOME MESSAGE

A number of studies suggest myopia is a risk factor for glaucoma, but there may be an alternate explanation for the reported association.

Caption:

Optic nerves of a high myopia patient, showing tilting, ovalization, and peripapillary atrophy.

Callout

 ‘The issue of myopia as a risk factor for glaucoma is important because there is an explosion in the prevalence of myopia in certain ethnic groups.’

Shan Lin, MD