Colonic motility is coordinated by the autonomic nervous system. Constipation is usually due to slow colonic transit of stool rather than outlet obstruction. Often the colonic slowing is idiopathic and associated with poor dietary habits and psychological issues. Occasionally constipation is due to secondary causes.

Systemic illnesses such as diabetes or hypothyroidism can cause constipation. Inflammatory bowel disease can cause constipation though diarrhea is usually more common. Irritable bowel syndrome is usually associated with alternating diarrhea though often, constipation is predominant. Colonic obstruction due to malignancy is also a potential etiology. Constipation is also a side effect of many medications. A notable example among nonpsychotropic medications is opiates. Often, there is no identified etiology.

Constipation is a common side effect of antipsychotics and requires some pharmacological management in at least one-third of patients [2]. The mechanism is mainly via anticholinergic activity. Clozapine is associated with more cases of constipation than other antipsychotics. There are case reports of fatal complications including paralytic ileus , fecal impaction , bowel obstruction , and intestinal perforation [3]. Among other antipsychotics, olanzapine is a frequent cause of constipation . Even haloperidol, a typical antipsychotic, is associated with constipation [3].

Atypical antipsychotics and risk for constipation

Among antidepressants , tricyclic antidepressants (TCAs) cause constipation due to their anticholinergic activity. The effect of serotonin on the intestines is not well understood. Selective serotonergic reuptake inhibitors (SSRIs) are thought to be prokinetic but they have been associated with constipation [1]. Similarly, benzodiazepines may have some effect on colonic motility but the clinical significance is unclear. Antihistamines with anticholinergic activity (e.g., diphenhydramine) also can cause constipation if used regularly. Mood stabilizers are not known to cause constipation.

Constipation resulting from medications resolves quickly if the medication is stopped. Dose reduction can be tried before stopping the medication.

Constipation from medications does not have any distinguishing features. Patients may complain of reduced frequency, straining at bowel movements, or blocked feeling during defecation. If constipation is severe, bloating , abdominal pain , and obstipation (a complete lack of passage of stools or flatus) occur. Rarely, in severe cases, there may be changes in appetite or nausea. Abdominal pain with obstipation should also raise suspicion for bowel obstruction . Any systemic signs including change in vital signs should raise suspicion for bowel perforation .

Generally, no testing is needed for evaluation of constipation. Plain radiography is useful to look for stool retention and complications of constipation. If constipation is accompanied by red flags for colonic malignancy (e.g., blood in stools, weight loss), endoscopic testing is warranted. Patients with refractory constipation may need to be referred for advanced testing such as colonic transit studies or anorectal manometry.