The patient had abdominal distensions. Give me differentials.
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I now have a solid evidence base from multiple textbooks. Here is a thorough differential diagnosis organized by the classic "6 Fs" framework and beyond.
Differentials for Abdominal Distension
The classic teaching organizes causes by the "6 Fs" — Fat, Fluid, Flatus, Feces, Fetus, and Fatal/Focal mass. Each is expanded below with clinically relevant causes.
1. Fat (Adiposity)
- Obesity / morbid obesity - the most common cause of a "large" abdomen in the outpatient setting; no true intraperitoneal pathology
- Lipomatosis (rare, diffuse intra-abdominal fat deposition)
2. Fluid (Ascites)
The most clinically significant category. Subdivided by the serum-ascites albumin gradient (SAAG):
High SAAG (>1.1 g/dL) - portal hypertension related:
- Cirrhosis (most common cause of ascites worldwide)
- Congestive heart failure / right heart failure
- Constrictive pericarditis
- Budd-Chiari syndrome (hepatic vein thrombosis)
- Sinusoidal obstruction syndrome (veno-occlusive disease)
- Massive hepatic metastases
Low SAAG (<1.1 g/dL) - peritoneal disease:
- Peritoneal carcinomatosis (e.g., ovarian, gastric, colorectal cancer)
- Tuberculous peritonitis
- Spontaneous bacterial peritonitis
- Nephrotic syndrome
- Pancreatitis / pancreatic ascites
- Mesothelioma
Other fluid collections:
- Urinary ascites (urinoma - obstruction or extravasation, especially relevant in urology/neonates)
- Chylous ascites (lymphatic disruption - trauma, lymphoma, post-surgical)
- Hemoperitoneum (trauma, ruptured ectopic pregnancy, ruptured AAA, ruptured solid organ)
3. Flatus (Gas)
- Adynamic ileus - diminished/absent bowel sounds, seen post-operatively, with electrolyte disturbances (hypokalemia), opioid use, sepsis, or peritonitis
- Mechanical small bowel obstruction (SBO) - adhesions (most common), hernias (inguinal, femoral, internal), Crohn's disease stricture, volvulus, intussusception, radiation-induced stricture, gallstone ileus, malignancy/carcinomatosis
- Large bowel obstruction (LBO) - colorectal carcinoma (most common cause), sigmoid volvulus (most common volvulus type, 2/3 of all), cecal volvulus, diverticular stricture, fecal impaction
- Aerophagia - excess air swallowing; functional, from anxiety or poorly fitting dentures
- Celiac disease / malabsorption syndromes - fermentation of unabsorbed carbohydrates
- Small intestinal bacterial overgrowth (SIBO) - excess bacterial fermentation
- Lactose intolerance - most common carbohydrate malabsorption; worldwide 75% of adults show some decrease in lactase activity (Yamada's Gastroenterology)
- Irritable bowel syndrome (IBS) - visceral hypersensitivity, altered gas transit
4. Feces (Constipation/Fecal Loading)
- Chronic constipation - idiopathic, dietary
- Hirschsprung disease - aganglionic segment; presents in neonates/children with failure to pass meconium and progressive distension
- Fecal impaction - common in elderly, institutionalized, or opioid-using patients
- Dyssynergic defecation (anismus)
- Hypothyroidism - systemic cause of severe constipation with distension
5. Fetus (Pregnancy & Related)
- Intrauterine pregnancy - always consider in women of reproductive age
- Ovarian hyperstimulation syndrome (OHSS) - after fertility treatment
- Uterine fibroids (leiomyomata) - can cause massive enlargement mimicking pregnancy
6. Fatal/Focal Mass (Organomegaly or Tumor)
- Hepatomegaly - malignancy (primary HCC or metastases), cirrhosis, congestion (right heart failure), storage disorders (Gaucher's, amyloid)
- Splenomegaly - portal hypertension, haematological malignancy (CML, lymphoma), infections (EBV, malaria)
- Ovarian cyst / ovarian cancer - including mucinous cystadenoma (can grow massive)
- Retroperitoneal sarcoma
- Abdominal aortic aneurysm (AAA)
- Pancreatic pseudocyst or malignancy
- Lymphoma / lymphadenopathy
- Enteric duplication cysts (especially in children - can present with distension, vomiting, or as lead point for intussusception)
- Mesenteric cysts (intra-abdominal lymphangioma) - may cause distension, pain, or bowel obstruction
Special Populations / Additional Causes
Neonates: Mechanical or functional bowel obstruction, abdominal mass, ascites, pneumoperitoneum (perforation), meconium ileus (associated with cystic fibrosis), malrotation, atresias, Hirschsprung disease (Grainger & Allison's Diagnostic Radiology)
Post-operative: Ileus (most common), hematoma, anastomotic leak with peritonitis, internal hernia
Peritoneal/Inflammatory:
- Peritonitis (primary or secondary) - guarding, rigidity, fever
- Neutropenic enterocolitis (typhlitis) - in immunocompromised patients; presents with fever, distension, diminished bowel sounds
Quick Diagnostic Framework
| Feature | Likely Cause |
|---|---|
| Shifting dullness, fluid thrill | Ascites |
| Tympanic all over, high-pitched bowel sounds | SBO/LBO |
| Tympanic, absent bowel sounds | Ileus |
| Dull, firm, irregular | Solid mass/organomegaly |
| Suprapubic dullness | Pregnancy, fibroids, bladder retention |
| Neonatal + failure to pass meconium | Hirschsprung, meconium ileus |
Sources: Grainger & Allison's Diagnostic Radiology; Yamada's Textbook of Gastroenterology 7th ed.; Schwartz's Principles of Surgery 11th ed.; Campbell-Walsh-Wein Urology; Harrison's Principles of Internal Medicine 22nd ed.; Goldman-Cecil Medicine
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