One of the leading hypotheses on glaucomatous damage in myopic eyes is related to the mechanical strain on optic nerve axons. Optic disc tilt is a typical characteristic found in myopic eyes. This tilt feature may cause a mechanical distortion of the lamina cribosa leading to axonal damage.1
Lee and colleagues hypothesized that different patterns of disc tilt may differentially affect the pattern of damage. Their study compared patterns of 'glaucomatous-appearing' damage between vertical disc tilt (VDT) and horizontal disc tilt (HDT) in young myopic eyes. They enrolled 52 eyes for each group with age, degree of refractive error, and severity of neural rim loss matching.
HDT is an independent risk factor for having more advanced glaucomatous-appearing visual field defects
The study found that HDT had significantly greater chance for nasal step and Bjerrum visual field defect subtypes compared to VDT which was more likely to have paracentral and temporal wedge defects. The frequency of glaucomatous-appearing visual field defects was significantly higher in HDT compared to VDT. The authors concluded that HDT is an independent risk factor for having more advanced glaucomatous-appearing visual field defects (MD < -6).
Although it was not described in the study that the RNFL defect location from optic disc photography correlated with the location of the visual field defects, the study had the strength of using objective criteria in determining the tilt characteristic and masking all disc and visual field grading.
Of note, the mean peak outpatient IOP was approximately 17 mmHg, indicating that a majority of the subjects were normal-tension type. This observation may suggest the presence of mechanical susceptibility of myopic eyes to glaucoma at any given IOP level, at least in a Korean population.2 It also should be emphasized here, as the authors acknowledge, that the subjects are those who had a mild degree of visual field defect (MD < -10) or no visual field defect at all. Thus, the patterns of glaucomatous-appearing damage in this study may be present in glaucoma patients as well as non-glaucomatous myopic patients. Therefore, it is still inconclusive, given the cross-sectional nature of the study, whether these visual field defects are the result of glaucomatous damage or represent pre-existing, non-progressive findings in myopic eyes.
In conclusion, the information and analysis from this paper provides another aspect for clinicians to consider in myopic patients - that certain optic nerve tilt directions may contribute to higher chance of myopia-related visual field defects which could also be pattern-specific. Furthermore, there is a possibility that these defects show greater susceptibility for subsequent glaucomatous damage.