The multiple eye movements that happen simultaneously during near work can slightly increase axial length over time and contribute to myopia development, researchers recently found. However, the reasons behind this axial elongation from certain eye movements have not been clearly determined. Optic nerve (ON) traction force could help explain this phenomenon, as it’s exerted on the eye globe during eye movements, potentially also leading to deformation of the optic nerve head. Now, researchers are also proposing that a greater disc tilt may relieve the traction that worsens myopia.
This recent study used MRI imaging to assess the differences in optic nerve tortuosity and displacements between healthy and highly myopic eyes. Data was observed from 20 such patients and 18 healthy emmetropic controls. The study subjects were all diagnosed with staphyloma and had at least one eye with an axial length of at least 27mm. They underwent a dilated fundus examination, B-scan ultrasonography and swept-source OCT. During the MRI, all participants were asked to stare at three distinct targets, one at a time; one accounting for primary gaze and one for each abduction and adduction (both at 15 degrees from baseline).
Using the 3D MRI images, researchers calculated and compared eye globe length, globe displacements and ON tortuosity between the two groups. They came up with the following conclusions:
• The high myope group had a greater axial length (28.62 vs. 22.84).
• In primary gaze, adduction and abduction, estimated ON tortuosities from before myopia onset were lower in highly myopic eyes than in controls (0.9063 vs. 1.0152).
• ON displacements in adduction were significantly different from those in abduction in all eyes, but only in the naso-temporal direction (not supero-inferior).
• High myopes had smaller ON displacements in the posterior segments in both gaze directions but larger displacements in the anterior-most ON segment in adduction.
Researchers concluded that the adjusted tortuosity of the ON was significantly lower in highly myopic eyes, suggesting they may be at a higher risk for myopia if they exhibit higher ON traction forces during eye movements prior to developing the disease. ON disc tilt severity, measured by the ON insertion angle, could be contributing to the damage caused by ON traction force. Results showed that in adduction, the ON insertion angle was positively correlated with ON tortuosity, axial length and temporal translation of the globe. It was also negatively correlated with ON displacements in plane 2, 3 and 4 in abduction.
“These observations showed that the greater the disc tilt (and greater the insertion angle), the less taut the ON and the less the ON would move, suggesting that ON traction might be relieved by the increase in insertion angle,” the researchers explained in their paper. “It is possible that disc tilt is a compensatory mechanism of the eye to mitigate the effects of a large ON traction force during eye movements.” However, they noted, it’s also possible that the stretching of peripapillary tissues by repeated eye movements could be causing or impacting the disc tilt.
The small sample size of the study limits its generalizability, as well as the many conflicting variables to which researchers could attribute increased axial length and ON traction force. Even with its limitations, this data represents a promising potential link between various eye movements and axial elongation, which may help define more risk factors for myopia in the near future.
Wang X, Chang S, Grinband J, et al. Optic nerve tortuosity and displacements during horizontal eye movements in healthy and highly myopic subjects. Br J Ophthalmol. 2021;0:1–7.