Inflection in inactive lateral rectus muscle: evidence suggesting focal mechanical effects of connective tissues.

PURPOSE

Paths of inactive lateral rectus (LR) muscles were studied to investigate putative roles of orbital fat and intrinsic muscle stiffness suggested to be alternatives to connective tissue pulleys as determinants of pulling direction.

METHODS

Surface coil orbital magnetic resonance imaging was performed in axial planes in adult humans: seven with chronic unilateral LR paralysis, three with nonparalytic concomitant esotropia of similar angle, and 15 healthy controls. Fixation was controlled using targets placed at a broad range of horizontal positions.

RESULTS

Paralyzed LRs exhibited marked atrophy compared with functional contralateral LRs and LRs of orthotropic and esotropic subjects without LR paralysis. The normal LR exhibited a gradual 18.8 degrees +/- 4.5 degrees (mean +/- SD) lateral inflection 14.4 +/- 2.6 mm posterior to the globe center, bowing the LR away from the orbital center. The paralyzed LR exhibited a significantly (P < 0.002) larger and typically more discrete 29.2 degrees +/- 8.8 degrees lateral inflection, similar to that observed in concomitant esotropia in maximal adduction. Average position of this inflection was 11 to 14 mm posterior to the globe center in all three subject groups, but in LR palsy only the inflection of the paralyzed LR-0.17 mm further posterior per degree of abduction (linear fit, R = 0.85)-depended on horizontal gaze. The behavior of the paralyzed LR inflection was consistent with LR pulley anatomy.

CONCLUSIONS

Sharper lateral inflection in the flaccid rather than the tense LR seems inconsistent with intrinsic muscle stiffness or diffuse orbital fat pressure but suggests the influence of discrete connective tissue.