Third nerve palsy
 

'Down and out' of the left globe in complete third nerve palsy.

The patient may have complete third nerve palsy with the classical signs or partial palsy with aberrant regeneration. a. Features of complete third nerve palsy.  The patient has a complete ptosis. The eye under the lid is depressed and abducted (down and out). The pupil is dilated and unreactive to light or accommodation. The fourth nerve and sixth nerve function are intact as  evidenced by the normal intortion and abduction respectively.  Note: intortion can be seen by getting the abducted eye to look down. Other examination: tell the examiner you would like to examine the head for craniotomy scar (operation for berry aneurysm) neurological examination of the limbs for contralateral hemiparesis (Weber's syndrome), contralateral coarse involuntary tremor (Benedikt's syndrome) or cerebellar ataxia (Norhnagel's syndrome) b. Features of partial third nerve palsy:  The affected eye has a partial ptosis. Pupil of the affected side may be larger or the same size as the opposite pupil. Elevation, adduction and depression of the affected side are limited. But abduction and intortion are normal. Other examination: Check for aberrant regeneration especially there is a dilated ipsilateral pupil (suggestive of a compressive lesion) This may be one or a combination of the following: adduction on attempted upgaze constriction of pupil on lateral gaze elevation of the upper lid on down gaze .

Questions:

1. Why is the pupil affected in compressive third nerve palsy but not if the cause is infarction?

answer
This is because the parasympathetic nerve that innervates the pupil travels at the surface of the third nerve. In compressive lesion, this is affected early. Whereas in infarction, the centre of the nerve is usually affected more than the surface of the nerve which even if involved can still obtain nutrients and oxygen from the cerebrospinal fluid.

2. How would you manage a patient with a pupil-involving third nerve palsy?

answer
This is an emergency and cerebral aneurysm must be ruled out. Refer the patient urgently to the neurosurgeon for further management such as MRI and cerebral angiography.

3. How would you manage a patient with a pupil-sparing third nerve palsy?

answer
The most likely cause is infarction of the nerve. It is important to take a medical history looking for predisposing factors such as diabetes mellitus and hypertension. Physical examination should include the cardiovascular examination especially the blood pressure. Blood tests should include glucose and ESR (to exclude giant cell arteritis). Other tests such as auto-antibodies for autoimmune disorders may be ordered if there were signs or symptoms in the patient's examination or history.

4. What is the significance of aberrant third nerve degeneration?

answer
This sign suggests that the patient has a slowly compressive lesion such as tumour (especially meningioma) or long-standing aneurysm. Its presence suggests that a medical cause ie. infarction is unlikely.

Return to the main page