incomitant (strabismus)

Incomitant strabismus is a strabismus whose angle of deviation changes with the direction of gaze.¹ The angle increases consistently each time the eyes turn in one particular direction, and it also differs according to which eye is fixating.²

If one or more extraocular muscles underact or are paralyzed, the deviation is largest with the eyes turned into the field of action of the weak muscle.¹ The cause may be innervational, as in paralytic strabismus, or a mechanical restriction.^1,3 A restriction can mimic a paresis; contracture of the medial rectus, or its entrapment in a fracture of the medial orbital wall, limits abduction just as a lateral rectus paresis does.⁴

A comitant deviation, by contrast, stays the same in all directions of gaze for a given fixation distance, within physiologic limits.¹ In a comitant esotropia, the angle varies by no more than $5\Delta$ across the horizontal gaze positions.³ Incomitant deviations may be congenital or acquired at any age.²