Abnormal Optic Nerve (ON) Traction on the Globe During Adduction in Normal Tension Glaucoma (NTG)
Robert A. Clark; Soh Y. Suh; Joseph Caprioli; JoAnn A. Giaconi; Kouros Nouri-Mahdavi; Simon K. Law; Laura Bonelli; Joseph L. Demer; Anne Coleman
Stein Eye Institute
Los Angeles, CA
Introduction: ON straightening by medial rectus muscle counterforce during adduction may mechanically load the globe-ON junction, creating repetitive strain that could produce intraocular pressure (IOP) independent progressive ON degeneration in NTG.
Methods: Sixteen NTG patients and 30 normals underwent high-resolution, surface coil, quasi-coronal orbital magnetic resonance imaging in central gaze, abduction, and adduction to angles measured by displacement of globe-ON junctions. Globe size was estimated from images. ON area centroids were plotted in three-dimensions (3D) to determine ON lengths relative to minimum straight-line paths. Globe translation was determined 3D centroid displacement and globe elongation was calculated from differential translation of the posterior border of the globe.
Results: Average abduction (20.8°±1.0° versus 20.7°±0.8°) and adduction (28.2°±0.9° versus 26.6°±1.1°) angles were similar between groups. Coronal globe diameters were significantly larger in NTG (25.9±0.2 versus 25.2±0.2 mm, p=0.01). The ON significantly straightened only in adduction for both groups: 102.1%±0.2% of minimum path length in adduction versus 104.4%±0.5% in central gaze for NTG (p=10-7), and 101.6%±0.1% versus 102.7%±0.3% for normals (p=10-5). During adduction, the globe shifted medially in both groups, but in NTG the globe center retracted significantly farther posteriorly (0.7±0.1 versus 0.1.0±0.1, p= 10-6) and elongated vertically (0.5±0.1 versus 0.2±0.1, p=10-5).
Discussion: In NTG, ON straightening during adduction abnormally pulls the globe posteriorly and elongates the globe inferiorly, reflecting globe tethering with mechanical loading of the globe-ON junction.
Conclusion: Although ON tethering in adduction is normal, in NTG there is greater globe displacement and deformation that may cause IOP-independent, neuropathic mechanical loading of the ON head and peripapillary sclera.
References: 1. Demer, J. L. Optic nerve sheath as a novel mechanical load on the globe in ocular duction. Invest. Ophthalmol. Vis. Sci. 57:1826-38, 2016.