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Miosis

Miosis is abnormal reduction in pupillary size, which may be unilateral or bilateral. Causes include:

Oculosympathetic paresis of whatever cause, e.g., Horner’s syndrome (unilateral), pontine hemorrhage (bilateral), early stages of central cephalic herniation (bilateral)
Drug-induced: e.g., opiates (bilateral)
Pupils tend to be small and reactive in metabolic-toxic encephalopathies (bilateral)
"Senile miosis" (bilateral): age-related

If only one pupil appears small (anisocoria), it is important to distinguish miosis from contralateral mydriasis, when a different differential will apply.

References

Kawasaki A. Approach to the patient with abnormal pupils. In: Biller J (ed.). Practical neurology (2nd edition). Philadelphia: Lippincott Williams & Wilkins, 2002: 135-146