Cranial Nerves III Oculomotor IV Trochlear and VI Abducens

The principal functions of these nerves include ocular rotation, lid elevation (III), and pupillary constriction. CN III innervates the medial, inferior, and superior rectus muscles; the inferior oblique, which allows inward and upward gaze; and the levator palpebrae superioris muscle, which raises the upper eyelid. It also innervates the pupillary constrictor muscle. CN IV innervates the superior oblique muscle, which causes the eye to look inward and down. CN VI innervates the lateral rectus muscle.

To test ocular rotation, the patient should first be examined by looking straight ahead, to note any imbalances between the eyes. Next, ocular rotation of each eye is tested separately. First, abduction is tested with the patient following the examiner's finger horizontally; this tests the lateral rectus muscle. The patient then looks laterally and upward, to test the superior rectus, and downward, to test the inferior rectus. Adduction of the eye tests the medial rectus muscle. With the eye adducted, the patient looks up, to test the inferior oblique, and downward, to test the superior oblique.

The pupils should be observed for symmetry. Most unequal pupils (anisocoria) are congenital and not clinically significant. The light reflex, direct and consensual, is checked by shining a bright light directly into the pupil. The response to light should be brisk.

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