Optic Nerve Sheath as a Novel Mechanical Load on the Globe in Ocular Duction

PURPOSE. The optic nerve (ON) sheath's role in limiting duction has been previously unappreciated. This study employed magnetic resonance imaging (MRI) to demonstrate this constraint on adduction. METHODS. High-resolution, surface coil axial MRI was obtained in 11 normal adults, 14 subjects with esotropia (ET) having normal axial length (AL) < 25.8 mm, 13 myopic subjects with ET and mean AL 29.3 +/- 3.3 (SD) mm, and 7 subjects with exotropia (XT). Gaze angles and ON lengths were measured for scans employing eccentric lateral fixation in which an ON became completely straightened. RESULTS. In all groups, ON straightening occurred only in the adducting, not abducting, eye. Adduction at ON straightening was 26.0 +/- 8.8 degrees in normal subjects, not significantly different from XT at 22.2 +/- 11.8 degrees. However, there was significant increase in comparable adduction in ET to 36.3 +/- 9.3 degrees, and in myopic ET to 33.6 +/- 10.7 degrees (P < 0.04). Optic nerve length at straightening was 27.6 +/- 2.7 mm in normals, not significantly different from 28.2 +/- 2.8 mm in ET and 27.8 +/- 2.7 mm in XT. In myopic ET, ON length at straightening was significantly reduced to 24.0 +/- 2.9 mm (P < 0.002) and was associated with globe retraction in adduction, suggesting ON tethering. CONCLUSIONS. Large adduction may exhaust length redundancy in the normally sinuous ON and sheath, so that additional adduction must stretch the sheath and retract or deform the globe. These mechanical effects are most significant in ET with axial myopia, but may also exert traction on the posterior sclera absent strabismus or myopia. Tethering by the ON sheath in adduction is an important, novel mechanical load on the globe.