Cranial Nerve Lesions
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Direct answer
Localising a cranial nerve palsy depends on identifying which nerve is affected, what it supplies and which other structures fail alongside it. Third nerve palsy causes ptosis and a down-and-out eye, with a dilated pupil in compressive lesions and a spared pupil in diabetic microvascular palsy. Sixth nerve palsy gives horizontal diplopia and can be a false localising sign of raised intracranial pressure, while an upper motor neuron facial palsy spares the forehead.
What you must remember
- Third nerve: ptosis, eye down and out, and a dilated pupil; the pupil is involved in posterior communicating artery aneurysm but spared in diabetic palsy.
- Fourth nerve: vertical diplopia worse on reading or going downstairs; the head tilts away from the affected side.
- Sixth nerve: failure of abduction with horizontal diplopia; petrous apex lesions cause Gradenigo syndrome with trigeminal pain and otorrhoea.
- Seventh nerve: lower motor neuron palsy involves the forehead and eye, with loss of taste from the anterior two-thirds of the tongue; upper motor neuron palsy spares the forehead.
- Ninth and tenth nerves: palatal weakness with the uvula deviating away from the lesion, hoarseness and dysphagia.
- Twelfth nerve: a lower motor neuron lesion deviates the tongue towards the side of the lesion on protrusion.
- Brainstem syndromes: Weber syndrome (third nerve with contralateral hemiplegia), Millard-Gubler (sixth and seventh with contralateral hemiplegia) and Wallenberg syndrome (ipsilateral Horner and facial pain loss).
A bedside route from double vision to the nerve
A 62-year-old man with hypertension and diabetes reports sudden horizontal double vision, with the right eye turning inwards.
- Test each gaze: Each direction of gaze is tested and the patient is asked which image is outer. Failure to abduct the right eye, with the diplopia worst on looking to the right, confirms a sixth nerve palsy.
- Examine the fundi: Fundoscopy is needed, because papilloedema suggests raised intracranial pressure, which can stretch the long intracranial course of the abducens nerve.
- Check the pupils: Pupils are normal here, but in a third nerve palsy the pupil decides the urgency. A dilated, poorly reacting pupil needs prompt imaging and vascular study to exclude aneurysm.
- Judge the diabetic palsy: A pupil-sparing palsy in a diabetic patient usually recovers over several months. Isolated sixth nerve palsy in a vasculopathic patient is often microvascular and resolves, but imaging is still required when there are other signs.
- Use the head tilt test: A head tilt test helps in fourth nerve palsy, where the tilt pattern guides the diagnosis of an isolated trochlear lesion.
Where examiners test pupils and the forehead
Four angles recur in these questions.
- Trap: treating every third nerve palsy as an aneurysm Correct: a dilated pupil is an emergency, while a pupil-sparing palsy in a diabetic is not.
- Trap: assuming every facial palsy involves the forehead Correct: upper motor neuron weakness from a cortical stroke spares the frontalis, while Bell palsy does not.
- Trap: misreading the uvula and tongue Correct: the uvula points away from a lower cranial nerve lesion, and the tongue points towards a lower motor neuron lesion.
- Trap: stopping at the nerve and missing the brainstem level Correct: examiners like the Millard-Gubler pattern, which pairs a pontine lesion with crossed signs, and reward naming the level from the cranial nerves and long tract signs together.
Frequently asked questions
Which features distinguish a compressive third nerve palsy from a diabetic one?
A compressive third nerve palsy usually involves the pupil, with a dilated and poorly reacting pupil, and needs urgent imaging for a posterior communicating artery aneurysm. A diabetic third nerve palsy typically spares the pupil, may still be painful, and recovers over several months, though compression must still be excluded.
Why does a sixth nerve palsy prompt a check for raised intracranial pressure?
A sixth nerve palsy can be a false localising sign of raised intracranial pressure, because the long intracranial course of the abducens nerve is stretched as pressure rises. Papilloedema should be sought on fundoscopy, and imaging should precede any lumbar puncture.
How does a lower motor neuron facial palsy differ from an upper motor neuron palsy?
A lower motor neuron facial palsy involves the whole half of the face, including the forehead, because the facial nucleus or nerve itself is damaged. An upper motor neuron palsy, usually from a cortical stroke, spares the forehead because the frontalis receives input from both cerebral hemispheres, and it affects mainly the lower face.
Which signs localise a lower cranial nerve lesion to the brainstem?
Lower cranial nerve lesions are localised by palatal weakness with uvular deviation away from the lesion, hoarseness, tongue deviation towards the lesion and dysphagia. Lateral medullary syndrome adds ipsilateral Horner syndrome, facial pain and temperature loss and ataxia, while medial medullary syndrome gives contralateral limb weakness with tongue deviation.