Clozapine, constipation and a distended abdomen
A 41-year-old man with treatment-resistant schizophrenia has been on clozapine for 8 months, now 400 mg/day, with a stable level of 480 ng/mL. He presents to the emergency department with 4 days of abdominal pain, distension, vomiting and no bowel action for 5 days. He is afebrile, pulse 104, BP 112/70, abdomen distended and tympanitic with reduced bowel sounds. He has been on clozapine plus sodium valproate and has been taking regular codeine for 'back pain'. White cell count 11.2, neutrophils 8.1, CRP 60, lactate 2.1. Clozapine level is unchanged. The psychiatric team is asked whether clozapine should be stopped.
1. What is the likely diagnosis and how does clozapine contribute?
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This is likely a severe clozapine-associated gastrointestinal hypomotility progressing to ileus or, worse, megacolon or obstruction - a recognised and potentially fatal adverse effect that is under-recognised relative to agranulocytosis. Clozapine has strong anticholinergic and antiserotonergic effects and reduces enteric motility, and the risk is compounded by valproate (which also slows motility and raises clozapine levels) and by codeine (an opioid, which further reduces motility). Constipation is common on clozapine; it should be actively prevented and treated, not accepted as trivial.
The differential includes mechanical obstruction, ischaemic bowel, perforation, and drug-induced ileus. The distension, vomiting, reduced bowel sounds and rising lactate raise concern for obstruction or ischaemia. This is a surgical emergency until excluded: the patient needs urgent surgical review, erect and supine abdominal X-rays or CT, and bloods including lactate and electrolytes.
2. How would you manage the clozapine and the acute abdomen?
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The acute abdomen takes priority: urgent surgical and gastroenterology review, nil by mouth, IV fluids, analgesia (avoiding further opioids), antiemetics, and imaging. Stop the codeine and valproate immediately, since both worsen motility and valproate raises clozapine levels. Do not simply stop clozapine abruptly without a plan - abrupt cessation risks rapid psychotic relapse and cholinergic rebound, and restarting at a previously tolerated dose after a break can cause severe toxicity. The usual approach in severe hypomotility is to hold clozapine while the acute problem is managed, monitor level and mental state, and liaise with the treating psychiatrist about a structured plan: restart at a reduced dose with titration once the bowel is functioning, or switch to another antipsychotic if the problem recurs. Never restart at the old dose after more than about 48 hours off treatment without re-titrating.
Supportive measures: stool softeners and osmotic laxatives, avoidance of constipating drugs, monitoring of bowel function as a routine observation on clozapine, and patient education. Document the adverse effect, report it, and communicate the plan to the patient and the inpatient team. Consider the underlying cause of the back pain and whether codeine was appropriate.