Ocular Motility and Strabismus
Core concepts
Understand extraocular muscle actions, innervation, and the laws of ocular motility (Sherrington, Hering). Know the cover test, alternate cover test, prism bar, and the difference between phoria and tropia. Classify strabismus: esotropia, exotropia, hypertropia, and their comitant versus incomitant patterns. Recognise amblyopia types (strabismic, refractive, deprivation) and critical periods. Understand the Parks three-step test for vertical deviations and the significance of the near point of convergence. Know when to refer for surgery versus glasses or patching.
Traps
Examiners test muscle action confusion: the superior oblique intorts and depresses; the inferior oblique extorts and elevates. In the three-step test, remember to identify the hypertropic eye, then the incomitance in side gaze, then the head tilt. Distinguish comitant (equal in all gazes) from incomitant (varies) deviations. Do not confuse dissociated vertical deviation with true hypertropia.
Worked example
A patient has a right hypertropia worse on left gaze and worse on right head tilt. Step one: right hypertropia. Step two: worse on left gaze implicates the right superior oblique. Step three: worse on right head tilt confirms right superior oblique palsy. Rule of thumb: the three-step test narrows a vertical deviation to one muscle.