What is the most likely cause of abdominal pain in a patient with constipation and what is the appropriate initial management for an adult without red‑flag symptoms?

Abdominal Pain with Constipation: Diagnosis and Management

Most Likely Cause and Initial Approach

For adults with abdominal pain and constipation without red-flag symptoms, the most likely diagnosis is functional constipation or IBS with constipation (IBS-C), and initial management should begin with osmotic or stimulant laxatives combined with antispasmodics for pain relief. 1

Critical First Step: Rule Out Mechanical Obstruction

Before treating as functional constipation, you must distinguish functional from mechanical causes:

Red Flags Requiring Urgent Imaging

• Intermittent colicky pain that worsens after eating with regular vomiting 2
• Absolute constipation (no passage of stool or gas) 2
• Abdominal distension with visible peristalsis 2
• History of previous abdominal surgeries (adhesions cause 55-75% of small bowel obstructions) 2
• Abdominal rigidity (indicates peritonitis requiring emergency surgery) 3

Immediate Action if Obstruction Suspected

• Obtain CT abdomen/pelvis with IV contrast immediately - this is the gold standard for confirming mechanical obstruction 2
• Do NOT use prokinetics or laxatives if complete obstruction is suspected - they can cause perforation 2
• Obtain immediate surgical consultation if complete obstruction or peritoneal signs are present 2, 3

Management Algorithm for Functional Constipation/IBS-C

Step 1: First-Line Pharmacologic Treatment (4+ weeks trial)

For Constipation:

• Start with osmotic laxatives (polyethylene glycol) or stimulant laxatives (senna, bisacodyl 10-15 mg, 2-3 times daily) 1
• Goal: one non-forced bowel movement every 1-2 days 1
• Evidence for traditional laxatives in IBS-C is limited, but they are reasonable first-line based on efficacy in general constipation, low cost, and wide availability 1

For Abdominal Pain:

• Antispasmodics as first-line: hyoscine butylbromide, dicycloverine, or peppermint oil 1
• These reduce smooth muscle spasm and are the recommended initial approach for pain with constipation 1

Step 2: Dietary Modifications (Concurrent with Step 1)

• Reduce dietary fiber if bloating is prominent - fiber increases bacterial fermentation and gas production 1
• Consider low FODMAP diet for patients with significant bloating, but avoid in malnourished individuals 1
• Increase fluid intake and physical activity when appropriate 1

Step 3: Second-Line Treatment (If inadequate response after 4-8 weeks)

For Persistent Constipation:

• Secretagogues: linaclotide or plecanatide - these activate intestinal chloride secretion, soften stools, and accelerate transit 1
• Linaclotide may also have visceral analgesic activity beyond its laxative effect 1

For Persistent Pain:

• Neuromodulators: low-dose tricyclic antidepressants (amitriptyline) for at least 6 months if response occurs 1
• These address visceral hypersensitivity, which is a key pain mechanism in IBS-C 4

Step 4: Management of Fecal Impaction (If present)

• Glycerol suppositories or phosphate enemas for rectal stimulation 1
• Manual evacuation under anesthesia if oral and rectal treatments fail 1
• Avoid colectomy for impaction - outcomes are poor 1

Understanding the Pain-Constipation Relationship

Pain and constipation are partially independent phenomena in IBS-C:

• In functional constipation, increasing bowel frequency with laxatives reduces abdominal pain severity 5
• In IBS-C, pain is driven by factors beyond constipation alone (visceral hypersensitivity), requiring both laxation AND visceral analgesic modulation 5, 4
• Pain sensitivity and colonic motility are independent factors - treatment must address both 4

Red Flags Requiring Urgent Gastroenterology/Surgery Referral

Refer immediately if any of the following are present: 2

• Age >50 years with new-onset symptoms
• Unintentional weight loss
• Nocturnal symptoms waking patient from sleep
• Rectal bleeding
• Family history of GI malignancy or inflammatory bowel disease
• Failure to respond to first-line therapies within 8-12 weeks

Common Pitfalls to Avoid

1. Never assume functional constipation in patients with prior abdominal surgery - adhesive obstruction is frequently missed; obtain CT during severe pain episodes for best diagnostic yield 2

2. Do not use prokinetics (metoclopramide) if obstruction is suspected - they increase perforation risk 2

3. Do not overlook narcotic bowel syndrome in chronic opioid users - this requires opioid reduction, not escalation, plus neuropathic pain medications 1

4. Avoid bulk laxatives in patients with suspected dysmotility - they can worsen symptoms 1

5. Do not delay imaging when clinical examination suggests peritonitis - imaging should not delay surgical intervention 3

6. Recognize that pain thresholds correlate with IBS symptom severity (rho = -0.36), so addressing pain sensitivity is as important as treating constipation 4