Ocular Motility and Binocular Vision
This domain tests extraocular muscle actions, innervation, strabismus, sensory adaptation, and binocular testing. Lesion localization and comitance are high-yield.
Core rules
- Sherrington law: increased innervation to an agonist is accompanied by reciprocal inhibition of its antagonist. Hering law: equal innervation is sent to yoked muscles, so a primary deviation produces a larger secondary deviation.
- Ductions are monocular; versions are conjugate; vergences are disjunctive. The near triad is accommodation, convergence, and miosis.
- Cranial nerves: III medial rectus, superior rectus, inferior rectus, inferior oblique; IV superior oblique; VI lateral rectus.
- Strabismus can be comitant (similar deviation across gaze) or incomitant (varies with gaze, suggesting paresis). Amblyopia, suppression, and anomalous retinal correspondence are sensory adaptations.
- Tests include cover test, alternate cover test, Worth 4-dot, stereoacuity, and Parks-Bielschowsky three-step for vertical deviations.
Worked example
A patient has a left sixth nerve palsy. Reasoning: the left lateral rectus is weak, so the left eye cannot abduct. The patient has an esotropia that is worse in left gaze and may turn the head left to avoid diplopia. Hering law predicts increased innervation to the right medial rectus, so the secondary deviation may exceed the primary deviation.
Traps that cost marks
- Calling a comitant deviation incomitant without comparing gaze positions.
- Forgetting that superior oblique palsy is worse on contralateral head tilt.
- Confusing suppression with amblyopia; suppression is an active cortical inhibition of one image.