Differential Diagnoses for Constipation
The differential diagnosis for constipation must systematically distinguish between primary functional disorders (normal transit, slow transit, or defecatory disorders) and secondary causes (medications, metabolic disturbances, neurologic conditions, and structural lesions). 1
Primary Functional Constipation Categories
The functional causes can be classified into three distinct subtypes:
- Normal Transit Constipation (NTC): Normal anorectal function with normal colonic transit time (20-72 hours), often associated with irritable bowel syndrome features 1, 2
- Slow Transit Constipation (STC): Normal anorectal function but delayed colonic transit with reduced propulsive activity and increased uncoordinated distal colonic motor activity 1, 2
- Defecatory Disorders: Impaired rectal evacuation from inadequate propulsive forces and/or increased resistance, including high anal resting pressure, incomplete relaxation or paradoxical contraction of pelvic floor and external anal sphincters 1, 2
Secondary Causes Requiring Systematic Evaluation
Medication-Induced Constipation
The most common pharmacologic causes include:
- Opioid analgesics: Causing opioid-induced constipation (OIC), a distinct entity defined by Rome IV criteria as "constipation triggered or worsened by opioid analgesics" 3, 4
- Anticholinergics, vinca alkaloids, 5-HT3 antagonist antiemetics, iron supplements, antidepressants, and calcium channel blockers 3, 1
Metabolic and Endocrine Disorders
Critical metabolic causes to evaluate:
- Hypercalcemia, hypothyroidism, hypokalaemia, uraemia, and diabetes mellitus 3, 1
- Dehydration (particularly common in elderly and advanced cancer patients) 3
Neurologic Conditions
Neuromuscular dysfunction contributing to constipation:
Structural and Anatomic Causes
Mechanical obstruction and anatomic abnormalities:
- Colorectal cancer or polyps (especially with alarm features: blood in stool, anemia, unintentional weight loss, sudden onset in older adults) 1
- Colonic stricture, abdominal or pelvic mass, radiation fibrosis 3
- Anal fissure, proctitis, rectal prolapse 3, 1
Clinical Approach to Differential Diagnosis
Key Historical Features That Guide Diagnosis
Symptom pattern identification is essential:
- Infrequency alone suggests normal transit or slow transit constipation 1
- Prolonged excessive straining with soft stools or inability to pass enema fluid strongly indicates defecatory disorders 1
- Need for perineal/vaginal pressure or digital evacuation is an even stronger indicator of defecatory disorders 1
- Abdominal pain, bloating, and malaise unrelated to defecation suggests underlying irritable bowel syndrome 1
Alarm Features Requiring Urgent Investigation
These red flags mandate prompt evaluation for serious pathology:
- Blood in stools 1
- Anemia 1
- Unintentional weight loss 1
- Sudden onset of constipation in older patients 1
- Rectal prolapse 5
Physical Examination Findings
Digital rectal examination (DRE) is essential and provides critical diagnostic information:
- Assessment of sphincter tone (lax tone suggests colonic hypotonia, possibly from spinal cord compression) 3
- Presence of impacted feces 3
- Tumor masses, tenderness, obstruction/stenosis 3
- Evaluation of pelvic floor motion during simulated evacuation 1
Abdominal examination should assess for:
- Distension, masses, liver enlargement, tenderness, and bowel sounds 3
Common Pitfalls in Differential Diagnosis
A critical error is assuming constipation equals infrequent bowel movements alone—patients with daily bowel movements may still have constipation if they experience straining, incomplete evacuation, or hard stools 2.
Another pitfall is performing extensive testing without first attempting empiric treatment—discontinue constipating medications if feasible and trial dietary fiber before ordering investigations 1.
In elderly patients, constipation may present atypically with delirium, anorexia, and functional decline rather than classic bowel symptoms 5, requiring heightened clinical suspicion.
Failing to recognize opioid-induced constipation as a distinct entity requiring specific management approaches (not just standard laxatives) leads to inadequate treatment 3.