Axial length minimally affects nonprogressive visual field loss in myopic Chinese patients

Additional research is needed to determine which factors do have an effect on visual field loss in these patients.

Issue: January 2011

Daniel Lee Chao

Axial length may not be a risk factor for visual field loss in Chinese patients who present with nonprogressive glaucomatous nerve damage, according to a study.

Daniel Lee Chao, PhD, and colleagues hypothesized that axial length in this subset of patients caused strain on the optic nerve, leading to visual field loss. According to Dr. Chao, epidemiological studies in Chinese patients have found myopia associated with primary open-angle glaucoma, which prompted the question of whether axial length correlates with vision loss.

“One hypothesis that we favored was that a progressive increase in axial length during youth may cause temporary strain on the optic nerve and thus could explain a nonprogressive glaucomatous visual loss that is present in this population of patients,” Dr. Chao said.

Axial length measurements were obtained in 20 Chinese patients who presented with myopia, tilted discs, glaucomatous visual field loss and asymmetric visual field loss. Each fellow eye was also measured. Results of the axial length measurements were compared with results of the baseline ocular examination and visual field test.

“Even in this small sample, there was not even a trend toward an association between axial length and degree of glaucomatous vision loss in these individuals,” Dr. Chao said.

Results and limitations

Thirty of the 40 eyes showed tilted discs, with an average cup-to-disc ratio of 0.74. The range of IOP was 7 mm Hg to 20 mm Hg, and myopia greater than –6 D was found in 30 eyes. Corneal superior arcuate defect was the most common visual field abnormality. Nine subjects reported a family history of glaucoma.

No correlation was found between axial length and visual field loss after eight axial length measurements were obtained and averaged for each eye and then compared with visual field test results.

According to Dr. Chao, the study had several limitations that may have affected the results.

“It was a very small sample size, 20 patients total. Additionally, all patients were sampled from a referral practice, and thus study subjects may represent a skewed population of patients with this condition. As vision loss was determined using automated visual field testing, there is a degree of variation and reliability in the results of these tests. To help minimize confounding factors, a fellow eye analysis was done to control for age, sex and genetic background.”

Next steps

Further study is needed to determine whether axial length has an effect on visual field loss in this group of patients. The limitations of this study warrant future study to rule out axial length as a contributing factor.

“The next step would be to expand this study by performing a large population-based screening with longitudinal follow-up of Chinese individuals to identify individuals with nonprogressive glaucomatous visual loss and then perform a fellow eye analysis to determine whether axial length is associated with the visual loss in this subset of patients,” Dr. Chao said.

Research into other factors that may contribute to nonprogressive visual field loss in Chinese patients is needed, Dr. Chao said. Other potential causes include corneal curvature, central corneal thickness and other genetic factors.

“Identifying risk factors for this condition would help to better elucidate the pathophysiology of this condition and could guide clinical management and diagnosis of this particular subset of Chinese patients who present with nonprogressive glaucomatous visual loss,” he said. – by Bill Menschner

Reference:

Chao DL, Shrivastava A, Kim DH, Lin H, Singh K. Axial length does not correlate with degree of visual field loss in myopic Chinese individuals with glaucomatous appearing optic nerves. J Glaucoma. 2010;19(8):509-513.

Daniel Lee Chao, PhD, can be reached at Department of Ophthalmology, Stanford University, 900 Blake Wilbur Drive #W3002, Palo Alto, Stanford, CA 94304, U.S.A.; e-mail: dlchao@stanford.edu.