Hyper-Reactivity to Atropine in Down's Syndrome

Abstract

Patients with Down's syndrome respond with abnormally great mydriasis to conjunctivally instilled atropine. This finding has been attributed to a structural anomaly of the Iris. To define the systemic reactivity to atropine in mongolism, the effects of intravenous atropine on heart rate were determined in three age-matched groups: 12 patients with Down's syndrome; 10 retardates without the syndrome; and 10 normal subjects. The patients with Down's syndrome, although reacting normally to atropine's bradycrotic or vagotonic action, had a markedly increased (twice normal or greater) sensitivity to the cardioacceleratory effects of atropine. This increased sensitivity, a pharmacogenetic abnormality, may result from the genetic imbalance imposed by an extra chromosome 21 in Down's syndrome.