What does the lateral rectus do?

The lateral rectus pulls straight back along the eye's own axis when the eye looks out, so it has one action and no secondary ones. Ask the patient to follow your finger out to the side: a normal eye abducts until the limbus reaches the outer canthus and no sclera shows lateral to the cornea. Then cover one eye and repeat, because a muscle that is only weak may keep up when both eyes move together and fall short on its own. The muscle is yoked to the medial rectus of the other eye through the medial longitudinal fasciculus, so a right sixth nerve palsy leaves the left eye adducting normally while the right eye lags, and the two images separate further the further the patient looks right.

A sixth nerve palsy leaves the eye unable to abduct: it drifts in, and the patient has horizontal double vision that is worse looking to the affected side and worse at distance. In an older patient with diabetes or hypertension it is usually a microvascular palsy that recovers in three months. In a patient of any age with headache it can be a false localising sign of raised intracranial pressure, because the long nerve is stretched over the petrous ridge. In a child it may be a pontine glioma. A palsy that does not recover, or that comes with other cranial nerve signs, is imaged.

The Duane syndrome mimics it in a child: the lateral rectus is wired to the third nerve instead of the sixth, so the eye cannot abduct and the globe retracts and the lid narrows on adduction, from birth and without double vision.