Chronic Constipation: What Could Be Causing It

Defining Chronic Constipation: Rome IV Criteria

Chronic constipation is not simply infrequent bowel movements. The Rome IV criteria require at least two of six symptoms present for three months with onset at least six months prior: straining, lumpy or hard stools, sensation of incomplete evacuation, sensation of anorectal blockage, manual maneuvers to support defecation, or fewer than three spontaneous bowel movements per week [1]. These criteria must be met without the routine use of laxatives.

The distinction between functional constipation and irritable bowel syndrome with constipation (IBS-C) matters for treatment selection. IBS-C includes abdominal pain as a dominant symptom, occurring at least one day per week, related to defecation [1]. A 2020 multinational study using the Rome IV criteria across 33 countries found that functional constipation affected 11.7% of participants, while IBS-C affected 4.1%, with significant geographic variation [2]. Some patients shift between diagnoses over time, which complicates management.

The Bristol Stool Form Scale provides an objective stool consistency measure. Types 1 and 2 indicate constipation. Patients who report "constipation" may actually have normal frequency but experience excessive straining or incomplete evacuation, making symptom characterization more important than stool counting alone.

Functional Causes: The Three Subtypes

Three distinct pathophysiologic mechanisms drive functional chronic constipation: normal transit constipation, slow transit constipation, and defecatory disorders. Each requires different management.

Normal transit constipation is the most common subtype, accounting for approximately 59% of patients in referral populations [3]. Stool moves through the colon at a normal rate, yet patients perceive constipation. Visceral hypersensitivity and altered rectal compliance may explain the disconnect. These patients typically respond well to fiber supplementation and osmotic laxatives.

Slow transit constipation affects predominantly young women. Colonic transit studies using radiopaque markers show delayed passage beyond 72 hours [3]. The pathology involves reduced interstitial cells of Cajal (the pacemaker cells of the gut) and diminished enteric nerve density. A study published in Gastroenterology found a 37% reduction in interstitial cells of Cajal in colonic specimens from patients with slow transit constipation compared to controls [4].

Defecatory disorders (dyssynergic defecation, pelvic floor dyssynergia) affect 25-50% of patients referred for refractory constipation [5]. The pelvic floor muscles paradoxically contract rather than relax during attempted defecation. Diagnosis requires anorectal manometry and balloon expulsion testing. This is the most treatable subtype. Biofeedback therapy produces sustained improvement in 70-80% of patients, outperforming laxatives in randomized trials [5].

Medication-Induced Constipation

Drug-induced constipation accounts for a substantial proportion of chronic cases, particularly in older adults taking multiple medications. Identifying the offending agent is the single most impactful diagnostic step in many patients.

Opioid-induced constipation (OIC) affects 40-80% of patients on chronic opioid therapy [6]. Opioids bind mu-receptors in the enteric nervous system, reducing peristalsis, increasing fluid absorption, and raising anal sphincter tone. Unlike other opioid side effects, tolerance rarely develops. The peripherally acting mu-opioid receptor antagonists (PAMORAs) naloxegol (25 mg daily) and methylnaltrexone address OIC without compromising analgesia. In the KODIAC-04 trial (N=652), naloxegol 25 mg produced spontaneous bowel movements within 12 hours in 40.8% of patients versus 29.4% for placebo [7].

Anticholinergic medications represent another major category. Tricyclic antidepressants, first-generation antihistamines, overactive bladder drugs (oxybutynin, tolterodine), and antipsychotics all carry constipating potential. The cumulative anticholinergic burden correlates with constipation severity in polypharmacy patients.

Other common offenders include:

• Calcium channel blockers (verapamil is the worst; amlodipine less so)
• Iron supplements (ferrous sulfate more than ferrous gluconate)
• Calcium supplements (particularly calcium carbonate)
• NSAIDs and aspirin (through prostaglandin inhibition)
• 5-HT3 antagonists (ondansetron, granisetron)
• Clonidine and other centrally acting antihypertensives

A medication reconciliation should precede any constipation workup. Switching verapamil to amlodipine, replacing ferrous sulfate with ferrous bisglycinate, or deprescribing unnecessary anticholinergics may resolve constipation entirely without adding laxatives.

Metabolic and Endocrine Causes

Several systemic conditions produce constipation through direct effects on gut motility, fluid balance, or neural function. A targeted laboratory panel helps exclude these before labeling constipation as "functional."

Hypothyroidism slows colonic transit through reduced smooth muscle contractility and myxedematous infiltration of the bowel wall. Constipation occurs in 15-20% of hypothyroid patients and resolves with adequate levothyroxine replacement targeting TSH within the reference range [8]. A simple TSH measurement is warranted in any new presentation of chronic constipation.

Diabetes mellitus causes constipation in up to 60% of patients with longstanding disease [9]. The mechanism involves autonomic neuropathy affecting the enteric nervous system, hyperglycemia-induced delayed transit, and medication effects (particularly from GLP-1 receptor agonists, which slow gastric and colonic motility). Semaglutide and tirzepatide both list constipation as a common adverse event occurring in 20-25% of patients at higher doses.

Hypercalcemia from any cause (hyperparathyroidism, malignancy, excessive supplementation) reduces smooth muscle contractility and decreases intestinal secretion. Serum calcium should be checked in the initial workup. Correction of hypercalcemia typically resolves constipation within days.

Hypokalemia and hypomagnesemia impair smooth muscle function. Chronic diuretic use, eating disorders, and malabsorption syndromes predispose to electrolyte-driven constipation. Checking a basic metabolic panel covers this.

Less common metabolic causes include Addison disease, pheochromocytoma, uremia, and heavy metal poisoning (lead). These are worth considering when standard workup is unrevealing.

Neurological Etiologies

The enteric nervous system contains approximately 500 million neurons. Any condition disrupting central or peripheral neural pathways to the gut can produce constipation.

Parkinson disease causes constipation in 70-80% of patients, often preceding motor symptoms by years or decades [10]. Alpha-synuclein aggregates accumulate in the enteric nervous system early in disease progression. New-onset constipation in a middle-aged or older patient with subtle motor findings (reduced arm swing, micrographia) warrants neurological evaluation.

Multiple sclerosis produces constipation in 40-70% of patients through demyelination of autonomic pathways. Spinal cord injuries at or above T6 dramatically impair colonic motility.

Hirschsprung disease is typically diagnosed in childhood but rare adult-onset or ultra-short segment variants exist. The absence of ganglion cells in a colonic segment produces functional obstruction. Rectal suction biopsy is diagnostic.

Autonomic neuropathies from diabetes, amyloidosis, or autoimmune conditions (anti-neuronal antibodies in paraneoplastic syndromes) can present with constipation as the primary gastrointestinal complaint. Chagas disease remains relevant in endemic regions due to destruction of myenteric plexus neurons by Trypanosoma cruzi.

Structural and Anatomical Causes

Mechanical obstruction must be excluded, particularly in patients with alarm features. Colonoscopy is indicated for new-onset constipation after age 50, presence of rectal bleeding, iron deficiency anemia, or significant unintentional weight loss.

Colorectal cancer can present as progressive constipation, especially left-sided or rectal tumors. The American Cancer Society recommends screening beginning at age 45, but symptomatic patients with alarm features need diagnostic colonoscopy regardless of screening history [11].

Rectocele (anterior rectal wall prolapse into the vagina) affects up to 80% of parous women to some degree, though only larger rectoceles (>3 cm) typically cause obstructed defecation symptoms. Defecography identifies clinically significant rectoceles.

Intussusception and rectal prolapse produce outlet obstruction. Internal rectal intussusception is common on defecography but its clinical significance is debated; only full-thickness external prolapse clearly requires surgical correction.

Colonic strictures from diverticular disease, ischemia, radiation, or Crohn disease can mimic functional constipation. CT colonography or barium enema identifies strictures in patients unable to undergo complete colonoscopy.

Megarectum and megacolon (idiopathic or acquired) represent end-stage motility failure where the dilated bowel loses effective propulsive function. These are diagnosed radiographically and may require surgical intervention when refractory to medical management.

Diagnostic Workup: A Stepwise Approach

The initial evaluation combines history, physical examination, and limited laboratory testing. The American Gastroenterological Association (AGA) guidelines recommend a pragmatic approach that avoids unnecessary testing in typical presentations [12].

Step 1: History and physical. Characterize symptoms using Rome IV criteria. Assess stool form (Bristol Scale), frequency, straining, evacuation completeness, and duration. Review all medications. Perform abdominal and digital rectal examination. The digital rectal exam alone identifies pelvic floor dyssynergia with 75% sensitivity when performed by an experienced clinician [12].

Step 2: Laboratory testing. Check TSH, serum calcium, fasting glucose or HbA1c, and a complete metabolic panel. Add CBC if alarm features are present.

Step 3: Empiric treatment trial. In the absence of alarm features, a 4-8 week trial of fiber plus osmotic laxative is appropriate before advanced testing. The AGA considers this "test and treat" approach cost-effective.

Step 4: Colonoscopy. Indicated for alarm features, age-appropriate screening, or failure to respond to empiric therapy. Rules out structural pathology.

Step 5: Physiologic testing. For refractory cases: anorectal manometry with balloon expulsion test (evaluates defecatory function), colonic transit study with radiopaque markers or wireless motility capsule (distinguishes slow transit from normal transit), and defecography (identifies structural outlet obstruction).

Dr. Satish Rao, a gastroenterologist at Augusta University who has published extensively on dyssynergic defecation, has stated: "At least one-third of patients referred for refractory constipation have a treatable defecatory disorder that was simply never tested for" [5].

First-Line Treatment: Fiber, Fluid, and Osmotic Laxatives

Evidence supports a graduated approach beginning with lifestyle modification and over-the-counter agents.

Soluble fiber (psyllium) is the best-studied fiber type. A meta-analysis of 7 RCTs found psyllium increased stool frequency by 1.5 bowel movements per week (95% CI 0.6-2.4) compared to placebo [13]. The recommended dose is 5-10 g daily, titrated slowly to minimize bloating. Insoluble fiber (wheat bran) has weaker evidence and may worsen symptoms in slow-transit constipation.

Polyethylene glycol (PEG 3350) at 17 g daily is the most studied osmotic laxative. A Cochrane review of 10 RCTs (N=868) found PEG superior to lactulose for stool frequency, stool consistency, and need for rescue laxatives [14]. PEG is inexpensive, well-tolerated, and does not cause electrolyte disturbances at standard doses.

Lactulose (15-30 mL daily) is an alternative osmotic agent that works through bacterial fermentation in the colon. It produces more bloating and flatulence than PEG.

Stimulant laxatives (bisacodyl, senna) are appropriate for intermittent or rescue use. Contrary to older beliefs, chronic stimulant use does not cause colonic nerve damage or "cathartic colon." A 2010 systematic review in Alimentary Pharmacology & Therapeutics found no evidence supporting laxative dependency at standard doses [15].

Adequate hydration (1.5-2 L daily) and physical activity complement pharmacotherapy but have limited independent evidence for constipation relief.

Prescription Therapies for Refractory Cases

When first-line agents fail after 4-8 weeks, prescription secretagogues and prokinetics offer evidence-based alternatives with NNTs (numbers needed to treat) between 4 and 8.

Linaclotide (Linzess) is a guanylate cyclase-C agonist that increases intestinal chloride and water secretion. At 145 mcg daily (chronic constipation dose) or 290 mcg (IBS-C dose), linaclotide produced at least 3 complete spontaneous bowel movements (CSBMs) per week in 21.2% versus 6.0% for placebo in phase III trials (N=1,272) [16]. Diarrhea is the main side effect (16-20%). Take 30 minutes before breakfast on an empty stomach.

Plecanatide (Trulance, 3 mg daily) shares the guanylate cyclase-C mechanism but has pH-dependent activation, theoretically reducing upper GI side effects. Efficacy is similar to linaclotide.

Prucalopride (Motegrity, 2 mg daily) is a selective 5-HT4 receptor agonist that stimulates high-amplitude propagating contractions in the colon. The pooled analysis of three phase III trials (N=1,555) showed 23.6% of prucalopride patients achieved 3 or more CSBMs per week versus 11.3% for placebo [17]. It is particularly suited to slow-transit constipation.

Lubiprostone (Amitiza, 24 mcg twice daily) activates type-2 chloride channels on the apical membrane of intestinal epithelial cells. Nausea (30%) limits tolerability; taking with food reduces this. It is the only prescription constipation agent with an FDA indication in OIC that is not a PAMORA.

For opioid-induced constipation specifically, naloxegol (Movantik, 25 mg daily) and methylnaltrexone (Relistor, subcutaneous or oral) block peripheral mu-receptors without crossing the blood-brain barrier at standard doses.

Biofeedback for Dyssynergic Defecation

Biofeedback therapy retrains the pelvic floor muscles to relax appropriately during defecation. It is the treatment of choice for dyssynergic defecation, not a last resort.

A landmark randomized trial by Rao et al. (N=77) compared biofeedback to standard therapy (diet, exercise, laxatives) in dyssynergic defecation [5]. Biofeedback produced a treatment response in 79.2% versus 22.2% for standard care. Improvements persisted at 12-month follow-up. The NNT was 1.8, making biofeedback one of the most effective gastroenterology interventions available.

The protocol typically involves 4-6 sessions over 2-3 months. Patients learn diaphragmatic breathing techniques, practice abdominal push coordination, and receive visual or auditory feedback from anorectal manometry or surface EMG sensors. Success requires a motivated patient and an experienced therapist. Access remains limited outside major academic centers.

Dr. Adil Bharucha of the Mayo Clinic has noted: "Dyssynergic defecation is among the most common and most treatable causes of refractory constipation, yet it remains under-diagnosed because clinicians skip the rectal exam and proceed directly to prescribing more laxatives" [12].

When to Worry: Red Flags and Specialist Referral

Most chronic constipation is benign and manageable in primary care. Specific features should prompt urgent evaluation or gastroenterology referral.

Immediate evaluation indicated:

• New-onset constipation after age 50 without prior screening colonoscopy
• Rectal bleeding or heme-positive stool
• Unintentional weight loss (>5% in 6 months)
• Iron deficiency anemia
• Palpable abdominal or rectal mass
• Acute obstruction symptoms (vomiting, distension, absolute constipation)

Gastroenterology referral warranted:

• Failure of adequate empiric therapy trial (fiber + osmotic laxative at full dose for 8 weeks)
• Suspected defecatory disorder on digital rectal exam
• Need for physiologic testing (anorectal manometry, transit study)
• Suspected slow transit constipation in a young patient
• Consideration of prescription secretagogues after OTC failure

Fecal impaction in elderly or immobile patients requires disimpaction before initiating maintenance therapy. Untreated impaction can progress to stercoral ulceration, perforation, and death, though these outcomes are rare.

Patients with refractory slow-transit constipation who have failed all medical therapies may be candidates for subtotal colectomy with ileorectal anastomosis. This surgery has high satisfaction rates (>85%) when patient selection is rigorous, confirmed by pan-colonic inertia on transit studies and exclusion of generalized motility disorders [3].