Globe Retraction During Adduction Tethering of the Optic Nerve (ON) Occurs in Primary Open Angle Glaucoma (POAG) With and Without Elevated Intraocular Pressure (IOP)

Abstract

Purpose : ON head deformation in adduction (Suh et al. IOVS58:5015-5021, 2017) due to ON traction has been proposed as a risk factor for optic neuropathy in POAG (Demer, et al., IOVS, 58: 4114-4125, 2017). We used MRI to determine if similar globe retraction by ON sheath traction occurs in patients who have POAG regardless of IOP elevation.

Methods : 47 normal control subjects and 36 patients with POAG underwent,surface coil, axial & quasi-coronalorbital MRI in target-controlled central gaze & mean 30° ab- & adduction. All patients with POAG were under medical and/or surgical treatment to reduce IOP to clinical target pressures. Maximum IOP with or without treatment did not exceed 20 mmHg in patients in 26 patients the lower IOP group (POAG-LP, average mean deviation -8.2±1.2 dB, SEM), but did exceed 21 mmHg pre-treatment in 10 patients in the higher IOP group (POAG-HP, average mean deviation -6.6±1.3 dB). ON area centroids in 2 mm thick planes permitted computation of minimum paths as a straightness index. Globe dimensions were determined from cross sections, and translations by centroid displacement. Generalized estimating equation analysis corrected for possible interocular correlations among individuals.

Results : For all 3 subject groups, the ON was significantly straighter in adduction than central gaze & abduction (P<0.0005), but in central gaze the ON was more redundant than normal in both POAG groups (P<0.05). For adduction, the ON had 102.0%±0.1% of minimum path length versus 104.0%±0.3% in central gaze for POAG-LP, and 102.0%±0.2% of minimum path length versus 103.8%±0.4% in central gaze for POAG-HP, compared with 101.7%±0.1% versus 102.9%±0.2% for controls. During adduction, globes retracted similarly at 0.68±0.08 mm in POAG-LP and 0.74±0.08 mm in POAG-HP, both significantly more than 0.08±0.06 mm in controls (P<10^-6). Generalized linear regression showed the effect of POAG on globe retraction in adduction to be highly significant in models controlling for age, globe size, & gender.

Conclusions : Although globe tethering of the ON & sheath are normal in adduction, adduction is associated withsignificantlygreater globe retraction in POAG irrespective of IOP. Traction in adduction may cause neuropathic mechanical overloading of the ON head & peripapillary sclera, and contribute to glaucomatous damage at all levels of IOP.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.