Overacting oblique muscles in exotropia: a mechanical explanation

Patients with exotropia often show apparent overaction of the inferior and superior oblique muscles. Are the oblique muscles contracted, are they truly overacting overacting, or does the eye flip up or down in adduction from the leash effect of a contracted lateral rectus muscle? Theoretically, if the mechanical limits of ocular rotations were circular or elliptical, rather than square or rectangular, we would expect a patient with exotropia to develop a vertical deviation in extreme gaze into the oblique quadrants, for the abducting eye would reach the mechanical limit, while the adducting eye would still be free to move up or down, giving the appearance of both inferior and superior oblique overaction. The circular or elliptical limits of ocular rotations were documented with tracings from slow-motion video recordings. Also, reduction of pseudo-overation of the obliques in both eyes following unilateral surgery for exotropia was observed in three patients. The strabismus surgeon should be aware of this possible mechanism for pseudo-overaction of the obliques and should avoid muscle surgery in this clinical setting.