PUPIL DILATATION - CAUSES III nerve lesion
^-----^—Examination of the light reflex (page 11) distinguishes
^^T^ lesions of the optic (II) and oculomotor (III) nerves.
Failure of the pupil to constrict when light is shone into either the affected or the contralateral eye indicates a lesion of the parasympathetic component of the III nerve.
Look for - ptosis - 70% of levator palpebrae muscle is supplied by the oculomotor nerve - impaired eye movements.
Causes of a III nerve lesion are described on page 149.
In comatose patients, pupil dilatation and failure to react to light is the simplest way of detecting a III nerve lesion; after head injury or in patients with raised intracranial pressure this is an important sign of transtentorial herniation.
The tonic pupil - Adie's pupil
This is a benign condition usually affecting young women. Onset is usually acute and unilateral in 80%.
The pupil dilates and the patient complains of mistiness in the affected eye.
Pupil constriction to both direct and ^ consensual light is often absent but very
✓ sl°w pupillary constriction occurs with / accommodation.
>s relaxed, slow dilatation occurs.
Occasionally the pupil appears completely unreactive to both light and accommodation. When the pupil is associated with reduced or absent limb reflexes this is termed the Holmes-Adie syndrome. More widespread autonomic dysfunction-arthostatic hypotension segmental disturbance of sweating and diarrhoea can co-exist.
Diagnosis: confirmed by pupillary response to pilocarpine (0.1% or 0.05%) - the tonic pupil will constrict (denervation hypersensitivity); the normal eye is not affected. The cause is unknown; the lesion probably lies in the midbrain or ciliary ganglion.
Migraine: Mydriasis persisting for some hours can accompany headache.
Drugs: Mydriasis ocurs with anticholinergic drugs (atropine), tricyclic antidepressants, non-steroidal anti-inflammatories, antihistamines and oral contraceptives. It can precipitate an attack of acute angle-closure glaucoma.
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