Endogenous anticholinergic substances may exist during acute illness in elderly medical patients.

BACKGROUND The purpose of this study was to determine if serum anticholinergic activity (SACA) arises from endogenous substances produced during illness. METHODS Elderly medical inpatients (N = 612) were screened for anticholinergic medication use in the week prior to the study by interviews of subjects and proxies and review of emergency room, hospital, and nursing home medication administration records. Of 24 subjects without a recent anticholinergic medication history, 15 were recruited and 10 completed the study. Serum samples were obtained on Day 2 of hospital admission. SACA was measured using a radionuclide displacement assay. Medications taken by subjects were assayed for central muscarinic receptor binding at therapeutic concentrations. Results. Eight of the ten subjects had SACA detectable in the serum. No medication used by these subjects had anticholinergic activity at usual therapeutic concentrations. CONCLUSIONS Endogenous anticholinergic substances may exist during acute illness. Characterization of such substances may increase the depth of our understanding of delirium and lead to useful intervention strategies.