To report a case of isoniazid-induced delirium.

A 56-year-old man affected by alcoholism and hepatic cirrhosis was admitted to hospital for a suspected pneumonia with pleural effusion and thickenings. A Mantoux skin test was positive. A core biopsy of the pleural nodules revealed a tubercular granulomatous pleuritis. An isoniazid, ethambutol and rifampicin therapy was then started. Subsequently the patient developed a state of delirium. A diagnostic work-up was carried out considering different hypothesis: hepatic encephalopathy was ruled out because of lack of response to appropriate treatment; negative encephalic CT scan, liquor examination and EEG excluded a tubercular involvement of the central nervous system or other neurologic etiologies. Haloperidol and delorazepam were administered under the suspicion of Wernicke-Korsakoff syndrome, without any benefit. Finally a toxic etiology was considered. Among isoniazid side effects psychosis is described, though rare. Isoniazid therapy was then stopped and substituted by moxifloxacin, with progressive improvement of the state of consciousness.

Cases of isoniazid-related psychiatric disorders reported include psychosis, obsessive-compulsive disorder and mood alterations. Two mechanisms are generally considered as responsible: pyridoxin deficiency and isoniazid toxicity, a molecule chemically near to iproniazid, a powerful monoamineoxidase inhibitors. This is the first report of a case of isoniazidinduced pure delirium, with all clinical features according to DSM-IV criteria. We also confirmed successful resolutions of symptoms only by discontinuation of therapy.

In the work-up of a state of delirium, when investigating the toxic hypothesis, clinicians should consider neuropsychiatric side effects of isoniazid.