The most common reason for unilateral mydri-asis is drugs in the form of locally administered eye drops, either prescribed by an ophthalmic department or obtained from a friend's medicine cabinet. The next most common cause is probably the Adie's pupil, a condition that is more common in young female patients. The affected pupil is usually dilated and contracts slowly in response to direct and indirect stimulation. In bright light, the pupil might constrict slowly on the affected side and take some time to dilate in the dark. The vision might also be blurred, particularly at near fixation because of the effect of the disease process on the ciliary muscle (necessary for accommodation). The pupillary constriction to near fixation is tonic and prolonged and worm-like. When this tonic pupil reaction is combined with absent tendon jerks in the limbs, it is known as Holmes-Adie syndrome. When the vision is blurred and the pupil widely dilated, the symptoms can be partially relieved by the use of a weak miotic. After a delay of months or years, the other eye may become affected. The overall disability is minimal and the condition has not so far been related to any other systemic disease. It is thought to have an underlying viral aetiology.
Acute narrow-angle glaucoma can occasionally present in this manner and confusion can arise if the eye is not red; however, closer examination of the eye should make the diagnosis obvious. Because the nerve fibres, which cause constriction of the pupil, are conveyed in the oculomotor nerve,oculomotor palsy if complete, is associated with mydriasis. For this reason, dilatation of the pupil can be a serious sign of raised intracranial pressure after head injury. One pupil might be wider than the other as a congenital abnormality (congenital anisocoria).
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