mechanically restrictive deviation

A mechanically restrictive deviation is an incomitant strabismus caused by elements within the orbit that interfere with muscle contraction or prevent free movement of the globe. The restriction may be direct, as with a tight or shortened muscle or tendon, or indirect, as with a large retinal explant.¹ This mechanism separates a restrictive deviation from a neurogenic palsy, where the nerve supply fails, and from a myogenic palsy such as myasthenia gravis, where the muscle itself is diseased.¹

Orbital causes include thyroid eye disease, orbital myositis, and tethering of muscles or tissue after an orbital wall fracture.² Loss of elasticity in the conjunctiva and Tenon's capsule also restricts motility, either after an eye has been held deviated in one position for a long time or after scars form from earlier operations.³

The forced duction test separates a restrictive deviation from a paresis. Under topical anesthesia the examiner grasps the muscle insertion and rotates the globe toward the limited field, and checked movement indicates restriction, while a neurological lesion gives no resistance.² Saccades into the restricted field stay close to normal velocity, and the underaction appears abruptly as the eye moves into the field of the affected muscle.¹