Life Science Encyclopedia

Oculomotor nerve palsy is an eye condition resulting from damage to the third cranial nerve or a branch thereof. As the name suggests, the oculomotor nerve supplies the majority of the muscles controlling eye movements. Thus, damage to this nerve will result in the affected individual being unable to move their eye normally. In addition, the nerve also supplies the upper eyelid muscle (the Levator palpebrae superioris) and the muscles responsible for pupil constriction (termed miosis) and dilation (termed mydriasis). The limitations of eye movements resulting from the condition are generally so severe that the affected individual is unable to maintain normal alignment of their eyes when looking straight ahead, leading to strabismus and, as a consequence, double vision (diplopia).

Eye Position

A complete Oculomotor nerve palsy will result in a characteristic down n' out position in the affected eye. The eye will be displaced downward, because the superior oblique (innervated by the fourth cranial or trochlear nerve), is unantagonized by the paralyzed superior rectus and inferior oblique and displaced outward, because the lateral rectus (innervated by the sixth or abducens cranial nerve) is unantagonized by the paralyzed medial rectus. The affected individual will also have a ptosis, or drooping of the eyelid, and pupil dilation.

It should be borne in mind, however, that the branched structure of the oculomotor nerve means that damage sustained at different points along its pathway, or damage caused in different ways (compression versus loss of blood supply, for example), will result in different muscle groups or, indeed, different individual muscles being affected, thus producing different presentation patterns.

Cause

Oculomotor palsy can arise as a result of a number of different conditions.

Congenital Oculomotor Palsy

The origins of the vast majority of congenital oculomotor palsies are unknown, or idiopathic to use the medical term. There is some evidence of a familial tendency to the condition, particularly to a partial palsy involving the superior division of the nerve with an autosomal recessive inheritance. The condition can also result from aplasia or hypoplasia of one or more of the muscles supplied by the oculomotor nerve. It can also occur as a consequence of severe birth trauma.

Acquired Oculomotor Palsy

1. Vascular disorders such as diabetes, heart disease, atherosclerosis and aneurysm, particularly of the posterior communicating artery.
2. Space occupying lesions or tumours, both malignant and non-malignant.
3. Inflammation and Infection
4. Trauma
5. Demyelinating disease (Multiple sclerosis)
6. Auto-immune disorders such as AIDS or Myasthenia gravis
7. Post operatively as a complication of neurosurgery.