A Simplified Approach To Teaching Medical Students Ocular Movements And The Rationale In Testing The Oculomotor, Trochlear, And Abducent Nerves.

The actions of the individual extraocular muscles are best explained to medical students in a lecture format by showing the relationship of each muscle to the axes of the globe and the walls of the bony orbit. The lateral and medial rectus muscles cross only the vertical axis, and consequently, cause only abduction and adduction, respectively. These muscles can be tested simply by asking the patient to abduct or adduct. Inability to do so indicates that the muscles and/or their innervation have been compromised. In contrast, the superior and inferior oblique and rectus muscles cross all three axes (vertical, horizontal, and anteroposterior) of the globe. Therefore, concentric contraction of each of these muscles results in three movements; abduction or adduction, elevation or depression, and intorsion or extorsion. These muscles must be tested with a sequence of two movements. Gaze can be either elevated or depressed by both an oblique and a rectus muscle. If abduction or adduction occurs first, one muscle of the pair becomes incapable of causing elevation or depression of the pupil. The rationale for abducting or adducting first is that either the oblique or rectus muscle of the pair will no longer cross the horizontal axis and will then be unable to effect the desired movement. Inability to elevate or depress gaze from the abducted or adducted position shows which muscle, the oblique or rectus, and/or its innervation has been compromised.