51 Third cranial nerve palsy
Salient features
History
• Diplopia in all directions except on lateral gaze to the side of the third nerve lesion (because the lateral rectus muscle supplied by the sixth cranial nerve is intact)
• Painful onset (berry aneurysm or aneurysmal dilatation of the intracavernous part of the carotid artery causing third nerve palsy)
Examination
• Unilateral ptosis (from paralysis of the levator palpebrae superioris)
• Dilated pupil reacting slowly or incompletely to light (paralysis of the constrictor of the pupil)
• Paralysis of accommodation (from involvement of ciliary muscle)
• Squint and diplopia resulting from weakness of muscles supplied by the third cranial nerve (superior, inferior, medial recti and inferior oblique). The eye will be in the position of abduction (i.e. down and out) if the fourth and sixth nerves are intact
• Diplopia may not be obvious until the affected eyelid is elevated manually.
• Exclude associated fourth cranial nerve lesion (supplies the superior oblique) by tilting the head of the patient to the same side – the affected eye will intort if the fourth cranial nerve is intact. Remember superior oblique intorts the eye (SIN). Inferior oblique externally rotates the eye.
Notes
Diagnosis
This patient has a R/L third nerve palsy (lesion) caused by diabetes mellitus (aetiology).
Questions
What are the common causes of a third nerve palsy?
• Hypertension and diabetes are the most common causes of pupil-sparing third nerve palsy. (The presence of pain is not a good discriminating feature between diabetes and aneurysm, as pain is present in both.) Diabetic third nerve palsy usually recovers within 3 months
• Aneurysms of posterior communicating artery (painful ophthalmoplegia)
• Tumours, collagen, vascular disorder, syphilis
• Meningioma at the wing of sphenoid
• Carcinoma at the base of the skull
• Rhinocerebral mucormycosis (in diabetic ketoacidosis, but about half the patients with diabetes who have this infection do not have ketoacidosis).
How would you investigate such a patient?
Advanced-level questions
What do you know about the muscles of extraocular movement?
• Medial and lateral recti (first pair of muscles): adduct and abduct the eye, respectively
• Superior and inferior recti: elevate and depress the abducted eye
• Superior and inferior obliques: depress and elevate the adducted eye.
Note: Superior and inferior recti act in the abducted position (mnemonic RAB).
What do you know about the anatomy of the oculomotor nerve?
Midbrain. The third cranial nerve originates in the midbrain and courses through the cavernous sinus and superior orbital fissure into the orbit to innervate four muscles and provide parasympathetic fibres for pupillary constriction.
Cavernous sinus. In the cavernous sinus or at the superior orbital fissure, the third cranial nerve may lie very close to the optic nerve. The third cranial nerve divides into superior and inferior rami within either the anterior portion of the cavernous sinus or the posterior optic canal. The superior rami supply fibres to the levator palpebrae and superior rectus muscles, and the inferior rami supply the extraocular muscles innervated by this nerve and also carry the pupillomotor fibres, which are in the superomedial portion of the nerve.
Do you know of any eponymous syndromes in which the third cranial nerve is involved?
• Weber syndrome: ipsilateral third nerve palsy with contralateral hemiplegia. The lesion is in the midbrain.
• Benedikt syndrome: ipsilateral third nerve palsy with contralateral involuntary movements such as tremor, chorea and athetosis. It is caused by a lesion of the red nucleus in the midbrain.
• Claude syndrome: ipsilateral oculomotor paresis with contralateral ataxia and tremor. It is caused by a lesion of the third nerve and red nucleus.
• Nothnagel syndrome: unilateral oculomotor paralysis combined with ipsilateral cerebellar ataxia.
