The number of medical diseases that can present with psychotic symptoms (ie, delusions, hallucinations) is legion. A thorough differential diagnosis of possible medical and toxic causes of psychosis is necessary to avoid the mistaken attribution of psychosis to a psychiatric disorder. One organizing principle separates etiology into primary psychiatric and secondary categories, the latter includes delirium (toxic psychosis), dementia, medical illnesses, and substances (Figure) This terminology avoids the term “organic,” which implies a mind-body dichotomy that is no longer tenable.2Psychosis can be attributable to a combination of factors, and all possible causes must be systematically examined, hence the lack of a hierarchical organization in this nosology.
In this article, I focus on secondary psychosis due to a medical illness or substances and not on the cognitive disorders of delirium and dementia. It is important, however, to be aware that both are commonly associated with psychosis. Psychosis is a frequent ancillary symptom of delirium that can overshadow its cardinal cognitive features.3,4 It is therefore critical to routinely consider the possibility of a delirium in any patient with psychosis. Dementias are also frequently accompanied by neuropsychiatric problems, including psychosis.5 Psychosis is present in about 40% of patients with Alzheimer disease, the most common form of dementia.6 Most patients with Lewy body dementia experience psychosis as well. Hallucinations, usually visual, are the most frequent psychotic symptom; they occur in 78% of patients, followed by misidentifications in 56% and delusions in 25%.7,8
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