Hirschsprung Disease (Congenital Aganglionic Megacolon)

Epidemiology

• Incidence: 1 in 5,000 live births
• Male:female ratio ≈ 4:1 (though females tend to have more severe/longer-segment disease)
• ~10% of cases are associated with Down syndrome (trisomy 21)
• Siblings of affected children are at increased risk
• Also associated with: MEN 2A/2B, cardiac septal defects, cleft palate, anorectal atresias, other neurocristopathies

Pathogenesis

• Auerbach myenteric plexus
• Meissner submucosal plexus

Key Genetics

• RET proto-oncogene (chromosome 10q11.2) — loss-of-function mutations account for the majority of familial cases and ~15% of sporadic cases. RET is a receptor tyrosine kinase; its ligands (GDNF, neurturin) are expressed in gut mesenchyme during development
• Other genes involved in neural crest cell development also implicated
• Inheritance is typically non-Mendelian (variable penetrance)

Anatomic Distribution

Key anatomic paradox: The aganglionic distal bowel appears narrow/normal caliber externally; the normal proximal bowel becomes progressively dilated (megacolon) due to obstruction.

Clinical Presentation

Neonatal (>90% of cases)

• Failure to pass meconium within the first 24 hours of life (classic hallmark)
• Abdominal distension
• Bilious vomiting
• Symptoms may temporarily resolve after digital rectal exam or rectal thermometer insertion (decompression), but recur without treatment

Older children / missed diagnosis

• Chronic constipation from birth
• Progressive abdominal distension
• Failure to thrive
• Longer-segment disease can paradoxically present later (milder initial symptoms)

Hirschsprung-Associated Enterocolitis (HAEC) — Most feared complication

• Occurs in 18–50% of patients, both pre- and postoperatively
• Leading cause of death in Hirschsprung disease
• Presents with: severe bloody diarrhea, fever, abdominal distension, sepsis, shock, peritonitis
• Higher frequency in: long-segment disease, delayed diagnosis (after 1st week of life)

Diagnosis

Imaging

• Shows: narrow aganglionic distal rectum → transition zone → dilated proximal colon
• Failure to evacuate contrast after 24 hours is highly suggestive
• Also excludes: meconium plug, small left colon syndrome, intestinal atresia

• Low bowel obstruction pattern; may show a paucity of rectal gas

Hirschsprung disease: (A) preoperative barium enema showing constricted rectum (bottom) and dilated sigmoid colon; (B) intraoperative appearance of the massively dilated sigmoid colon. — Robbins & Kumar Basic Pathology

(a) Barium enema showing the classic transition zone; (b) H&E histology showing absence of ganglion cells in the bowel wall

• Failure of internal sphincter relaxation on rectal balloon distension — can be diagnostic

Histopathology — Gold Standard

• Newborns: suction rectal biopsy at bedside (2+ specimens, ≥5–10 mm above dentate line, 1 cm apart)
• Older children: full-thickness biopsy under general anesthesia (thicker mucosa not amenable to suction technique)

1. Absent ganglion cells in submucosal/myenteric plexuses
2. Hypertrophied nerve trunks
3. Robust acetylcholinesterase immunostaining (increased AChE-positive fibers)
4. Calretinin immunostaining — now a standard adjunct (absent staining in aganglionic segment)

Management

Preoperative / Bridging

• Daily rectal irrigations with warm normal saline via soft red rubber catheter (1–2×/day, until clear effluent returns) — decompresses the bowel
• Leveling colostomy with intraoperative biopsies — used when irrigations are not feasible (comorbidities, family limitations, long-segment disease), to identify the level of normally ganglionated bowel

Definitive Surgery — Pull-Through Procedures

Postoperative Complications

• Anastomotic leak, fistula, abscess, stenosis
• Persistent/recurrent Hirschsprung-associated enterocolitis
• Constipation or soiling (bowel function may take years to normalize)
• Up to 10% of patients may eventually require a permanent colostomy
• Total intestinal Hirschsprung disease — notoriously difficult; requires parenteral nutrition support

Differential Diagnosis of Delayed Meconium Passage

Summary Table

Ribbon-Like (Pencil-Thin) Stool — Causes

Primary / Classic Cause

1. Carcinoma of the Left Colon / Rectum (most important)

• The annular (apple-core/napkin ring) carcinoma of the sigmoid colon or rectum narrows the lumen circumferentially
• Ribbon-like stool is a classic symptom of left-sided colorectal carcinoma — one of the earliest alarming symptoms
• Accompanied by: change in bowel habits, rectal bleeding, tenesmus, weight loss

Other Important Causes

2. Anal Stenosis

• A fibrous ring at the anal verge causes constipation and gives stool a ribbon-like appearance
• Responds well to dilation or surgical disruption
• — Sleisenger & Fordtran's GI and Liver Disease

3. Anal Canal Spasm / Levator Spasm (Proctalgia fugax)

• Functional narrowing of the anal outlet

4. Stricture of the Rectum / Anal Canal

• Post-inflammatory (Crohn's disease, radiation proctitis, lymphogranuloma venereum)
• Post-surgical anastomotic stricture

5. External Compression of the Rectum

• Pelvic masses (ovarian cyst, fibroid, presacral tumor)
• Pelvic floor dysfunction

6. Spinal Cord / Neurological Causes

• Ribbon-like stools are listed among symptoms prompting urological/neurological evaluation (alongside urinary/fecal incontinence and lower extremity neurological signs) — suggests dysfunctional defecation from spinal dysraphism or tethered cord syndrome
• — Campbell-Walsh-Wein Urology

Summary Table

Exam pearl: In most clinical/exam contexts, ribbon-like or pencil-thin stool = carcinoma of the left/sigmoid colon or rectum until proven otherwise. It should be investigated urgently with colonoscopy.