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Abdominal distension in dogs and cats: differential diagnosis

Abdominal distension is enlargement of the abdomen, and in a dog or cat it comes from gas in the stomach or bowel, free fluid in the peritoneal cavity, an enlarged organ or mass, or a weak abdominal wall letting the belly sag.[1,2,3]

Reviewed by Brian Toncray, DVM · October 22, 2026

Confirming the sign

Palpation and ballottement give the first hint, but a focused abdominal ultrasound scan is the fastest way to find free fluid, and it extends the physical examination in emergency patients.[4] Radiographs show gas distension and the position of the stomach, which separates a twisted stomach from a simple dilatation.[1] Obesity can mimic distension, so compare body condition, and ask the owner how fast the change happened: hours suggests gas or bleeding, weeks suggests fluid accumulation, organ enlargement or an endocrine cause.

Gastrointestinal gas

Gastric dilatation-volvulus (dogs)[1,5]

Supports it:Acute cranial abdominal distension with nonproductive retching. On a lateral radiograph, the gastric fundus lies caudoventrally and the pylorus craniodorsally; a funnel-shaped caudal esophagus was seen in every volvulus dog with marked esophageal gas.

Argues against it:A gastric fundus in its normal craniodorsal position on the lateral view with the pylorus in the right cranial abdomen, which fit simple gastric dilatation in that study.

Next test:A right lateral abdominal radiograph. Blood lactate guides broad prognostic conversations only; it should not decide surgery for an individual dog.

Free abdominal fluid

Hemoabdomen from a ruptured splenic mass[6]

Supports it:An older dog that collapsed, with pale gums and fluid on a focused ultrasound scan, and a splenic mass. Hemangiosarcoma was the most common cause in a prospective study of 345 dogs.

Argues against it:A splenic mass does not prove malignancy: benign lesions made up 35.7% of that cohort.

Next test:Abdominocentesis with a fluid PCV, ultrasound of the spleen and liver, and staging before the surgical conversation.

Ascites from low oncotic pressure, right-sided heart failure or portal hypertension[2,7]

Supports it:In dogs, peritoneal transudates were most often caused by decreased oncotic pressure, and modified transudates by increased hydrostatic pressure, which includes right-sided heart failure and pericardial effusion as well as portal hypertension. Portal hypertension shows up as ascites, acquired shunts or hepatic encephalopathy.

Argues against it:An exudate on fluid analysis, which points toward inflammation, a ruptured viscus or neoplasia.

Next test:Fluid total protein, cell count and cytology, serum albumin and liver function testing, plus a focused cardiac scan when jugular distension, muffled heart sounds or a murmur suggest a cardiac cause.

Septic peritonitis[8]

Supports it:An exudate with intracellular bacteria, a lower effusion glucose than blood glucose, and a painful, unwell patient.

Argues against it:A clean cytology on a fresh sample. Prepare cytology immediately, because samples degrade, and be aware that point-of-care glucometers can confound the glucose comparison.

Next test:Abdominocentesis with immediate cytology and paired blood and effusion glucose, then imaging to find the source.

Uroabdomen[9]

Supports it:Trauma or urethral obstruction followed by abdominal fluid and hyperkalemia; urinary tract rupture is a recognized cause of hyperkalemia in dogs.

Argues against it:An intact urinary tract on contrast imaging.

Next test:Paired effusion and serum creatinine and potassium, then contrast imaging of the urinary tract.

Feline infectious peritonitis (cats)[10,2]

Supports it:A young cat with a high-protein effusion and compatible clinical and laboratory findings.

Argues against it:Effusion cytology and immunostaining negative for feline coronavirus antigen. In cats, effusion cytology overlaps widely across causes, so one test rarely settles it.

Next test:Effusion cytology and immunostaining for coronavirus antigen when an effusion is present.

Organ enlargement and body wall

Pyometra[11]

Supports it:An intact female with a fluid-filled uterus, vaginal discharge, depression and fever, often within four months after estrus.

Argues against it:A spayed patient.

Next test:Abdominal ultrasound of the uterus.

Hyperadrenocorticism (dogs)[3]

Supports it:A slowly enlarging, pot-bellied abdomen in an older dog with PU/PD, panting or hair loss, from hepatomegaly and weak abdominal muscles.

Argues against it:No other consistent clinical signs; endocrine tests should be run only when the picture fits.

Next test:Chemistry, urinalysis and abdominal ultrasound, then an adrenal screening test.

First diagnostics for this sign

In an unstable patient, a focused abdominal ultrasound scan is the first test because it finds free fluid within minutes and guides abdominocentesis.[4] Any fluid gets a PCV, total protein, cell count and cytology, and the transudate, modified transudate or exudate category narrows the cause well in dogs.[2] Suspected sepsis adds paired blood and effusion glucose and fresh cytology.[8] A dog retching with a tympanic cranial abdomen gets a right lateral radiograph as soon as stabilization allows.[1]

Example: working the sign

Example. A ten-year-old dog collapses at home and arrives pale and tachycardic with a moderately distended abdomen. A focused ultrasound scan finds fluid in several abdominal views, and abdominocentesis yields blood that does not clot.[4] Ultrasound shows a cavitated splenic mass. Hemoabdomen from a ruptured splenic mass moves to the top of the list, with hemangiosarcoma the most common cause, but the client hears that roughly a third of such masses were benign in a large prospective study, which changes how the surgery is discussed.[6]

Frequently asked questions

Which test comes first for a distended abdomen?

A focused abdominal ultrasound scan finds free fluid quickly, and a right lateral radiograph comes first when the dog is retching with a tympanic cranial abdomen.[4,1]

Does a ruptured splenic mass always mean hemangiosarcoma?

No. Hemangiosarcoma was the most common cause in a prospective study of 345 dogs, but 35.7% of the ruptured masses were benign.[6]

How does fluid analysis narrow the cause of ascites in dogs?

Transudates were most often from decreased oncotic pressure, modified transudates from increased hydrostatic pressure, and exudates from increased vascular permeability, a ruptured viscus or neoplasia.[2]

Can blood lactate decide whether a dog with GDV goes to surgery?

No. Lactate supports broad prognostic conversations at a population level, but survivor and nonsurvivor values overlap too much to use it for an individual dog.[5]

Sources

  1. Maxwell EA, Rivenburg RE, Mitjans T, et al. Caudal esophageal twist sign may help in differentiating between 360 degree gastric dilatation-volvulus and gastric dilatation in dogs. J Am Vet Med Assoc. 2026;264(4):416-422. PMID 41072467 · doi:10.2460/javma.25.06.0431
  2. Sotillo S, Viall AK, Palerme JS, Ward JL. Retrospective Evaluation of the Causes and Fluid Characteristics of Cavitary Effusions in Dogs and Cats. J Vet Intern Med. 2025;39(5):e70205. PMID 40757981 · doi:10.1111/jvim.70205
  3. Behrend EN, Kooistra HS, Nelson R, et al. Diagnosis of spontaneous canine hyperadrenocorticism: 2012 ACVIM consensus statement (small animal). J Vet Intern Med. 2013;27(6):1292-1304. PMID 24112317 · doi:10.1111/jvim.12192
  4. Lisciandro GR. Abdominal and thoracic focused assessment with sonography for trauma, triage, and monitoring in small animals. J Vet Emerg Crit Care (San Antonio). 2011;21(2):104-122. PMID 21463438 · doi:10.1111/j.1476-4431.2011.00626.x
  5. Mackenzie D. Gastric dilatation volvulus in dogs: utility of lactate as a predictor of survival. Vet Evid. 2022;7(4). PMID 42004681 · doi:10.18849/ve.v7i4.537
  6. Ruffoni E, Stewart S, Khanna C, et al. A prospective observational study of 345 canines with ruptured splenic tumors suggests benign lesions are more common than previously reported. J Am Vet Med Assoc. 2025;263(8):985-990. PMID 40334697 · doi:10.2460/javma.25.01.0044
  7. Buob S, Johnston AN, Webster CR. Portal hypertension: pathophysiology, diagnosis, and treatment. J Vet Intern Med. 2011;25(2):169-186. PMID 21382073 · doi:10.1111/j.1939-1676.2011.00691.x
  8. Campbell BG, O'Marra SK. Diagnosis and surgical management of septic peritonitis in small animals: A review. Vet Surg. 2026;55(1):13-31. PMID 41109949 · doi:10.1111/vsu.70024
  9. Iimori Y, Johnson PA, Thomovsky EJ, Brooks AC, Aghili A. Emergency management of hyperkalemia in dogs and cats, Part 1: Pathophysiology and etiology. Can Vet J. 2026;67(4):448-457. PMID 41929733
  10. Tasker S. Diagnosis of feline infectious peritonitis: Update on evidence supporting available tests. J Feline Med Surg. 2018;20(3):228-243. PMID 29478397 · doi:10.1177/1098612X18758592
  11. Hagman R. Pyometra in Small Animals. Vet Clin North Am Small Anim Pract. 2018;48(4):639-661. PMID 29933767 · doi:10.1016/j.cvsm.2018.03.001