The reflex is due to:
(1) inhibition of the iris sphincter (which involves the Edinger-Westphal nucleus in the midbrain)
(2) sympathetic stimulation of the iris dilator
Conditions associated with poor dilatation of one or both pupils in the dark:
(1) ocular inflammation affecting the iris or sphincter
(2) surgery or other trauma
(3) Adies’s syndrome tonic pupil
(4) third nerve aberrant reinnervation
(5) drug-induced miosis (cholinergic agent)
(6) unilateral episodic spasm of miosis
(7) congenital bilateral miosis
(8) fatigue or drowsiness
(9) inflammation or malignant lymphoma involving the peri-aqueductal gray matter
(10) central-acting narcotics or general anesthetics
(11) old age
(12) Horner’s syndrome with oculosympathetic defect
