Introduction
The oculomotor nerve (CN III) is a motor nerve supplying all the extraocular muscles except the superior oblique and lateral rectus muscles.
It also supplies levator palpebrae superioris (elevator of the upper eyelid) and the parasympathetic supply to the ciliary muscle (pupillary constriction).
Palsy of CN III causes a depressed and abducted eye (down and out pupil), ptosis and pupillary dilatation.
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| Fig 1: Abducted eye | Fig 2: Ptosis (drooping eyelid) and pupillary dilatation |
Anatomy
The nerve arises from the anterior midbrain as two nuclei:
- Oculomotor nucleus (motor)
- Edinger-Westphal nucleus (parasympathetic)
The oculomotor nerve passes between the superior cerebellar artery and posterior cerebral arteries, then passes between the free and attached borders of the tentorium cerebelli. Following this, it runs along the lateral wall of the cavernous sinus. It enters the orbit through the superior orbital fissure.
Fibres from the Edinger-Westphal nucleus travel to the ciliary ganglion on the outside of the oculomotor nerve.
CN III, IV and VI are tested together.
Examine for ptosis.
Test eye movements. Ask the patient if there is any diplopia during testing.
Imaging of the brain is important to delineate the aetiology. The emergent requirement for imaging and the modality is influenced by the clinical presentation.
Occulomotor palsies can be divided into surgical (compressive) vs medical palsies, or complete vs incomplete. This is in reference as to whether there is pupillary involvement or not. The parasympathetic fibres which contribute to pupillary constriction run on the outside of the nerve and thus are affected by compressive pathology. These would include:
Trauma, Intracranial haemorrhage, cavernous sinus disease, aneurysm of the posterior communicating artery or basilar artery, tumours (sphenoid wing meningioma, carcinomatous lesions of the skull base, brainstem metastasis) etc.
Pupil sparing pathologies (incomplete or medical oculomotor palsy) include stroke, ischaemic neuropathy secondary to diabetes or hypertension and multiple sclerosis. The most common is ischaemic vasculopathy caused by diabetes or hypertension. This is often painful and resolves over 3 to 6 months.
Given these aetiologies imaging may include plain CT, CT angiogram and/or MRI.
Compressive causes of CN III palsy cause pupillary dilatation because the parasympathetic fibres run adjacent to the circumference and are easily compressed.
Compression is usually due to either aneurysm (posterior communicating artery or basilar artery) or tumour (sphenoid wing meningioma, carcinomatous lesions of the skull base, brainstem metastases).
A palsy due to an aneurysm is usually painful.
The most common cause of a pupil-sparing lesion is ischaemic vasculopathy, secondary to diabetes or hypertension. This is often painful. It usually resolves over 3 to 6 months. In such circumstances, the peripheral parasympathetic fibres are usually spared.