Whilst undertaking a night shift, you are called by an out-of-hours GP for advice. He has seen a 39-year-old patient with headaches and an increasingly droopy lid for the last day or so. Examination reveals a fully dilated pupil in an eye that is down and out.
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1. Can this referral wait until the morning to be seen?
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Correct answer: NO! This is an emergency situation. This patient needs urgent cerebral imaging and may need subsequent referral to the neurosurgeons if a posterior communicating, posterior cerebral or superior cerebellar artery aneurysm is diagnosed.
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2. Can you explain the clinical signs?
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Correct answer: The oculomotor nerve (CN3) supplies all of the extraocular muscles except the superior oblique (SO) and lateral rectus (LR). It also innervates the levator palpebrae superioris (LPS) and the sphincter pupillae. In the event of pathology in the nerve, the following will be seen:
- Unopposed action of dilator pupillae: hence, a dilated pupil
- Complete ptosis of the upper lid
- The eye is ‘down and out’:
- Unopposed abduction from the LR
- Action of the SO, which alone acts to depress, abduct and intort the eye.
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3. What is the relevance of the pupil in this situation?
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Correct answer:
In the proximal part of the nerve, the parasympathetic fibres are superficially placed on the sheath. Hence, if there is a ‘surgical’ cause of nerve pathology (i.e. extrinsic compression), the parasympathetic nerves are affected early and the pupil is dilated.
Conversely, if there is a medical cause (i.e. vascular infarct), this is more internal in the nerve fibre and the autonomic nerve is not involved. Hence, the pupil is spared.
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4. Where along the route of this nerve can pathology arise?
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Correct answer:
Broadly categorise into:
- Midbrain lesion
- Interpeduncular fossa lesion
- Cavernous sinus lesion
- Orbital lesion
