Imaging the emergency abdomen

THE ACUTE ABDOMEN ENCOMPASSES MANY DISEASE PROCESSES but is generally defined as any process causing severe abdominal pain or a rapid deterioration in the cardiovascular status of the patient. If left untreated these conditions will often prove fatal.

Imaging of these patients is required to identify surgical v. medical disease and to allow prompt treatment of the patient. However, imaging is only part of the process and stabilisation of the patient should always be considered the priority. When considering the emergency patient the important questions to answer are:

• Is there any gastrointestinal dilation?
• Is there any free fluid or free air within the peritoneum?
• Are there any mass lesions?
• Are there any concurrent injuries, for example pelvic or spinal?

It is also imperative to consider the body systems that can cause patients to rapidly destabilise, with the cardiovascular system, the urinary tract and to a lesser degree the gastrointestinal tract uppermost of these. Both radiography and ultrasound can be used to obtain this information and remain very complementary techniques.

Radiography

Radiography remains a mainstay of emergency imaging and offers a great overview of the abdomen but is negatively affected by the presence of fluid in the abdomen.

Abdominal contrast is due to the presence of intra-abdominal fat surrounding soft tissue abdominal organs, so to maximise contrast a low kVp technique (<70kVp typically) with high mAs is used to increase the difference between fat and soft tissue. Scatter should also be reduced by the use of a grid in patients where the body width is greater than 10cm.

Ultrasound

Ultrasound is now commonly available; it is particularly useful for identifying and guiding sampling of free fluid and for identifying gastrointestinal obstruction, where it has been shown to be more sensitive than radiography.

The Focused Assessment with Sonography in Trauma (FAST) scan technique can be used to rapidly determine the presence of free fluid by checking in four separate locations:

1. Subxiphoid process.
2. Midline, cranial to the bladder at the level of the pubis.
3. Between the right kidney and caudate lobe of the liver.
4. Between the left kidney and the spleen. If required, a full abdominal scan can then be performed when the patient is stable to assess the abdominal organs.

Specific aetiologies Ascites

• Free abdominal fluid is seen with many abdominal emergencies including haemoabdomen, uroabdomen and septic abdomen. Other causes include biliary rupture, liver lobe torsions or local fluid accumulation in pancreatitis.
• Recognition of the presence of free fluid radiographically is due to the loss of serosal detail, with the degree of loss reflecting the amount of fluid.
• Large fluid volumes result in a generalised loss of serosal detail.
• Small amounts of fluid give a streaky appearance to the peritoneal cavity and may appear similar to peritonitis.
• It is important to differentiate this from the normal appearance in thin or emaciated animals due to a loss of fat, where lack of abdominal detail is normally accompanied by a “tucked up” appearance to the abdomen and reduced fat over the spine. A similar appearance is often seen in juvenile patients where fat deposition in the abdomen has not yet occurred. In both of these cases, retroperitoneal detail is also lost.
• Ultrasound may help differentiate these fluids by the degree of speckling within the free fluid with more cellular fluid (septic peritonitis or haemoabdomens) having a more speckled appearance compared to transudates and modified transudates; however, sampling is required for confirmation and can be guided by ultrasound when fluid volume is small.
• Haemoabdomen may also be accompanied by hypovolaemia, with a reduced size of the caudal vena cava in the caudal thorax or in severe cases also a reduced aortic size.
• Uroabdomen is recognised by increased potassium and creatinine within the fluid. Rupture of the urethra or bladder requires a contrast urethrocystogram or vaginourethrocystogram for confirmation.

GDV

• Gastric dilation and volvulus is a common disease encountered in deep chested dogs (typically larger dogs) due to rotation of the stomach.
• The disease has a typical “double bubble” appearance due to the gas-filled pylorus being seen dorsal to the fundus.
• Rupture may result in pneumo-peritoneum; gastric pneumatosis may also be seen as gas accumulating in the ventral gastric wall, due to necrosis occurring to the mucosa.

Intestinal obstruction

• Intestinal obstruction is a common presentation, most often due to foreign bodies within the small intestine but may also be caused by intussusception (usually less than one year of age) and, less commonly, mesenteric torsion or neoplasia.
• Changes are visible both ultrasonographically and radiographically with ultrasound now considered the technique of choice.
• Findings include visualisation of the obstructive lesion (with a foreign body usually seen as a strongly shadowing interface within the lumen) or segmental intestinal dilation, due to the dilation of the intestine proximal to the lesion with either gas or fluid.
• Dilation can be determined radiographically by the diameter of the intestinal loop being greater than 1.6x the height of the L5 vertebral body in dogs or greater than 2x the height of the L2 vertebral body in cats.

Pneumoperitoneum

• Pneumoperitoneum is the presence of free air within the peritoneum and is seen secondary to gastrointestinal tract rupture (ulceration, foreign bodies, neoplasia) or due to external trauma to the abdomen.
• Gas is seen accumulating around the abdominal organs, highlighting the serosal detail. It is often easiest to detect craniodorsally, where accumulation adjacent to the diaphragm causes the crus of the diaphragm to become visible and borders of the stomach, liver and right kidney may become more evident.

References and further reading

1. Boag, A. and Hughes, D. (2004) Emergency management of the acute abdomen in dogs and cats 1 – Investigation and initial stabilisation. In Practice 26: 476-483.
2. Franks, J. N. and Howe, L. M. (2000) Evaluating and managing acute abdomen. Vet Med 95 (1): 56-69. Abdominal and thoracic focused assessment with sonography for trauma, triage, and monitoring in small animals. J Vet Emerg Crit Care (2011) 21 (2): 104-122. BSAVA manual of abdominal imaging. Eds Robert O’Brien and Frances Barr: BSAVA 2008.