“Acute abdomen” in human medicine is defined as sudden and severe abdominal pain of unclear etiology. In veterinary medicine, it is typically a more general term used to describe a patient with any acute significant abdominal signs. These patients often have abdominal pain, but may also present with vomiting, profuse diarrhea, abdominal distension, or sometimes even shock or dyspnea. As this description could potentially represent a large number of medical and surgical conditions in our profession, it is not surprising that acute abdomen patients account for a high percentage of cases that are triaged in emergency hospitals. It can sometimes be very challenging to determine the cause of the symptoms, as well as to rapidly stabilize these patients and to make a decision to proceed with either medical or surgical management.
Pathophysiology of acute abdomen
The basic mechanisms that lead to acute abdominal pain include distension of a hollow organ (or stretch of the capsule of a solid organ), obstruction, torsion, ischemia, and peritonitis (inflammatory, chemical, or septic). Any organ within the abdominal cavity can potentially be the origin of the pain. In addition, body wall or diaphragmatic herniation can lead to organ entrapment and sometimes strangulation. Extra-abdominal conditions can sometimes lead to referred abdominal pain and an incorrect diagnosis. This can occur with back pain (such as intervertebral disc disease), pelvic pain (fractures), or even with inflammatory thoracic conditions such as pyothorax or pneumonia. Vomiting and diarrhea can have a myriad of underlying causes, and are typically classified as either primary gastrointestinal (inflammatory, infectious, toxic, parasitic, neoplastic) or secondary to extra-gastrointestinal disease (including pancreatitis, hypoadrenocorticism, hepatic or renal disease, and others). Abdominal distension is usually the result of accumulation of ascites, but can also be seen with gastric dilation (+/- volvulus), as well as with significant organomegly or large abdominal masses. Shock can develop subsequent to large volume gastrointestinal fluid losses, hemorrhage, hypoadrenocorticism, or sepsis. Dyspnea can be seen due to primary thoracic disease, aspiration pneumonia, acute respiratory distress syndrome, or pressure on the diaphragm from severe abdominal distension.
Initial triage of patients with acute abdomen symptoms
The initial approach to the patient with acute abdominal signs should include a concise history to reveal any vomiting, diarrhea, urinary tract signs, access to toxin or foreign material, prior surgeries, current or recent medications, and the duration and progression of the symptoms. In addition, the signalment and vaccine history of the patient should be noted as this can sometimes help to narrow the differential list. For example, younger patients are more likely to have infectious diseases or to have ingested foreign or toxic items. Older patients are more likely to have neoplasia or metabolic disease. Intact patients could potentially have pyometra (female) or prostatitis (male) on the differential list. Large breed dogs with non-productive retching should be presumed to have gastric dilatation and volvulus (GDV) until this diagnosis is eliminated. In many cases, the stability of the patient will dictate the speed with which diagnostics and treatments must be performed.
Rapid evaluation of the patient presenting with acute abdominal signs should include obtaining basic vital signs (temperature, pulse, respiratory rate and effort, blood pressure, and pulse oximetry if indicated), as well as thoracic auscultation, abdominal palpation, and evaluation of hydration status. If an arrhythmia is ausculted, EKG should be performed as arrhythmias are not uncommonly seen as a manifestation of severe abdominal disease. Rapid evaluation of key laboratory parameters should be prioritized, including packed cell volume and total solids (PCV/TS), electrolytes, blood glucose, and lactate as these values most effectively help determine patient stability and guide immediate interventions. Ultrasound can be used to perform a Focused Assessment with Sonography for Triage (or Trauma), or “FAST” scan. This rapid 4-point evaluation can be learned quickly and performed with minimal stress to the patient, often simultaneously with initial therapeutic interventions. It can be invaluable in the rapid detection and sampling of abdominal or thoracic fluid.
Life-threatening abnormalities should be immediately addressed, including circulatory shock, hypoxemia, arrhythmias, and severe anemia. Initial stabilizing therapies may include oxygen supplementation (to help ensure adequate tissue oxygenation), fluid therapy, analgesia, anti-emetics, antibiotics, and even transfusion therapy. If the patient is hypotensive and/or tachycardic, an intravenous fluid bolus should be administered over 15-20 minutes (approximately 10mL/kg isotonic crystalloid in a cat or 20mL/kg in a dog, or 3-5mL/kg synthetic colloid) with re-evaluation of vital signs afterwards to assess for improvement. Patients with active hemorrhage (based on serial FAST scans and/or abdominal fluid sampling) may require transfusion therapy as part of resuscitation. Patients that are not responding to volume expansion may be candidates for vasopressor or inotropic therapy. Analgesia is commonly administered to evaluate for improvement in heart rate and comfort. For patients with severe abdominal pain, a pure mu opioid is ideal as it provides potent analgesia and is reversible if undesirable side effects are seen. Gastric decompression should be performed urgently in patients with gastric dilatation and volvulus (GDV), ideally via trocharization with a large bore over-the- needle catheter. Therapeutic abdominal drainage may be indicated during pre-surgical stabilization in patients with uroabdomen or bile peritonitis and large volume effusion. Patients with severe gastric fluid or gas distension (without torsion) may benefit from placement of a nasogastric tube for suctioning, which can also reduce regurgitation and aspiration risk.
Diagnostic evaluation of acute abdomen
While laboratory tests do not always reveal a precise diagnosis, they can sometimes assist with identifying the source of the problem, evaluate for pre-existing diseases, and help determine patient stability and appropriate immediate emergency interventions as well as ongoing supportive therapy. Testing typically includes a complete blood count, chemistry panel, electrolytes, lactate, and potentially blood gas analysis, urinalysis, and fecal testing (including parvovirus ELISA in appropriate patients). With acute hemorrhage, the total solids will drop first while the packed cell volume (PCV) will initially remain in the normal range due to splenic contraction. Anemia is typically unmasked with additional time, particularly following fluid resuscitation, so PCV/TS should be monitored closely in unstable patients.
Hypoproteinemia can also be seen with peritonitis and capillary leak or third spacing of fluid into the peritoneal cavity. Patients with hemorrhagic gastroenteritis or other large volume fluid losses will have an elevated PCV (hemoconcentration). Evaluation of a complete blood count (CBC) helps assess for adequacy of immune response, as well as the presence of disseminated intravascular coagulation (DIC) or other platelet consumptive conditions. The chemistry profile can help identify renal or hepatic disease, both of which can present with acute abdominal signs.
Hypoglycemia should raise concern for sepsis, hepatic failure, or hypoadrenocorticism. Hyperglycemia is commonly seen with stress of critical illness (particularly in cats) as well as with pancreatitis. Extremely elevated blood glucose measurements can be seen with cardiovascular collapse and impending arrest. Electrolyte concentrations are typically decreased secondary to losses (vomiting, diarrhea) and decreased intake. The combination of hyponatremia and hyperkalemia should prompt suspicion for hypoadrenocorticism.
While elevated serum lipase and amylase levels have not been found to be reliable for the diagnosis of pancreatitis, they can be suggestive and suspicion can be further supported if levels measured on the abdominal fluid are 4x the upper end of the reference range, or at least 2x the serum level. Additional tests such as the canine pancreatic lipase (cPL) test (available as both Spec cPL and a semi-quantitative cage side SNAP cPL) can be used to evaluate for pancreatitis, but results should be interpreted with caution as a recent study showed a fairly significant potential for false positive results in up to 40% of dogs presenting with acute abdominal disease. In addition, it is possible to have a potentially misleading positive test in a patient with an obstructive gastrointestinal foreign body and secondary pancreatitis. Urinalysis can assist with evaluation of renal function, hydration status, and presence of acute renal injury as well as pyelonephritis. Fecal testing may reveal the presence of parasites or other infectious organisms (including salmon poisoning disease). Parvovirus testing is indicated in any puppy with acute gastrointestinal signs, especially in the presence of an incomplete vaccine history. The test will often cross react with feline panleukopenia virus, and can also be used to screen for this infection. Venous blood gas analysis most commonly reveals a metabolic alkalosis due to repeated vomiting (and possible upper GI obstruction), but may alternatively show a metabolic acidosis due to lactate accumulation from impaired perfusion. Coagulation testing may be indicated in certain cases, such as patients with active hemorrhage and those that are critically ill and require emergent surgical intervention.
Imaging is an essential part of the evaluation of the acute abdomen patient. Radiography is commonly available, and abdominal radiographs are an excellent general screening test for ascites (indicated by loss of serosal detail), organ size and position, as well as overt gastrointestinal obstruction or perforation (indicated by the presence of free abdominal air in the absence of a history of recent surgery or open needle centesis). Free abdominal air is most readily detected between the stomach or liver and diaphragm on the lateral view. While there are rare reports in the literature of pneumoperitoneum secondary to blunt trauma and progression from pneumomediastinum, in general the finding of free abdominal air should prompt surgical exploration. Segmental dilation of loops of small intestine to greater than 2-3 times the width of a rib, or >1.6 times the narrowest width of the fifth lumbar vertebrae (L5) on a lateral radiograph is concerning for obstructive disease. Massive generalized gas dilation is concerning for mesenteric torsion. Generalized mild to moderate gas or fluid dilated small intestine is most consistent with enteritis, although partial obstruction with an occult foreign body can often be difficult to completely exclude. Plication of small intestines (indicated by presence of curvilinear or “C” shaped small intestinal gas pockets) is supportive of the presence of a linear foreign body. Abdominal radiographs should also be evaluated for the presence of urinary tract stones causing possible obstruction and/or a nidus for infection. Radiographs should also be scrutinized for the presence of extra-abdominal abnormalities, such as narrowing of intervertebral spaces, discospondylitis, destructive bony lesions, fractures, and subcutaneous emphysema. Thoracic radiographs are indicated in any acute abdomen patient with respiratory signs and/or cardiac abnormalities on auscultation, and should also be considered in any geriatric patient as a screen for metastatic disease prior to embarking on expensive or invasive therapies.
Ultrasound allows further evaluation of specific regions of the abdomen, can be very useful to detect small amounts of free fluid (and for guided sampling), and can also be used to confirm the presence and origin of a mass lesion, intestinal obstruction, or regional or generalized intestinal thickening. Ultrasound is also far more sensitive at detecting pancreatitis, lymphadenopathy, and biliary tract obstruction and/or rupture. Although pancreatitis cannot be ruled out in a cat with a sonographically normal pancreas, ultrasound can still be used to evaluate for other causes of acute abdominal signs, sometimes leading to a suspicion of pancreatitis as a diagnosis of exclusion. Even if full diagnostic ultrasound capability is not readily available, a FAST scan can be used to evaluate for and obtain abdominal or thoracic fluid. An abdominal fluid score (AFS) can be determined and monitored serially over time, and trends can be used to potential guide the decision to process with exploratory surgery in equivocal cases. While computed tomography (CT scan) is currently the gold standard diagnostic test for evaluating acute abdominal pain in human patients, there are significant limits to widespread use in veterinary patients, including cost, availability, need for general anesthesia, as well as typically requirement for interpretation by a radiologist.
Abdominal fluid sampling and analysis
Abdominal effusion (if present) can be collected via abdominocentesis (blind, ultrasound-guided, 4-quadrant) or potentially via diagnostic peritoneal lavage (DPL). DPL is a more involved and higher risk procedure, but theoretically can detect with 93-98% accuracy and retrieve smaller amounts of fluid (1-4.4mL/kg). These techniques can be very useful in further narrowing the differential list, as well as in making a decision on the need for surgery. Both biochemical and cytologic evaluation of abdominal fluid should be performed (Table 2), and samples should also be saved for bacterial culture if indicated. Following analysis, abdominal fluid samples can be broadly categorized as transudates, modified transudates, or exudates. Transudates most commonly occur secondary to hypoproteinemia and decreased osmotic pressure allowing third spacing of fluid, or due to portal venous obstruction. This type of effusion can also be seen initially following urinary bladder rupture, and this diagnosis is typically confirmed with biochemical testing of the effusion (Table 2). Modified transudates most commonly occur secondary to fluid and protein leakage from lymphatics or blood vessels, due to increased hydrostatic pressure or increased vascular permeability. This type of effusion is diagnostically the least specific, and can occur subsequent to a variety of disease processes including cardiovascular disease, liver disease, hemorrhage, and neoplasia. As uroabdomen progresses, the fluid becomes more consistent with a modified transudate. Cytologic evaluation of hemorrhagic exudates is typically low yield, and imaging and coagulation studies should be given priority. Exudates occur most commonly due to an inflammatory process and the presence of intra-abdominal chemotactant substances. They are further classified as septic (in the presence of bacterial organisms) or non-septic exudates. Non-septic exudates can be seen secondary to bile peritonitis, uroabdomen, acute pancreatitis, FIP, and intracavitary neoplasia. While overlap occurs between the main fluid type categories, they still provide a useful framework for initial evaluation and together with biochemical testing and the rest of the clinical picture, a tentative diagnosis can often be reached.
Medical vs surgical management of acute abdomen
Of paramount importance is the distinction between medical and surgical causes of acute abdominal signs, as this will impact considerably the cost associated with treatment, as well as the resources required to proceed. Some cases are straightforward and a diagnosis is reached following examination and initial diagnostics. However, in other cases, ongoing patient evaluation is required and sometimes a decision to proceed with surgical exploratory must be made in the absence of a concrete diagnosis. In these cases it is particularly important to have meticulous client communications and if surgical exploratory does not reveal an obvious cause for the signs, samples should be collected for histopathology and possible cultures, and consideration should be given to feeding tube placement for nutritional support post- operatively. Even for patients where the decision to proceed to surgery is made readily, there are some cases that must go urgently and some that will benefit from more extensive pre-operative evaluation and stabilization.
There are also a variety of causes of acute abdominal signs that will respond to aggressive medical therapy without the need for surgery. These include pancreatitis (in the absence of abscessation, or persistent/worsening extrahepatic biliary obstruction), gastroenteritis, non-perforated gastrointestinal ulcers, pyelonephritis, prostatitis, cholangiohepatitis or cholecystitis, food bloat, gastric dilatation (without volvulus), and many traumatic hemoabdomen patients. For patients undergoing attempted medical management, treatment typically includes a combination of fluid therapy, analgesia, anti-emetic medications, gastroprotectants, and a bland diet following a period of fasting to allow gut rest. Nasogastric tube placement may be recommended for gastric suctioning and enteral nutrition. Patients that are intolerant of enteral nutrition may require temporary parenteral nutritional support during recovery. Care should be taken with the use of anti-emetics in patients that have not had a gastrointestinal foreign body conclusively excluded, as these medications can potentially delay diagnosis and appropriate surgical intervention. Antibiotics are indicated in some cases – primarily to treat known bacterial infections, or to prevent infection in significantly leukopenic patients or those considered at high risk for bacterial translocation due to severe gastrointestinal insult, mucosal sloughing, and/or shock.
TABLE 1 – Abdominal effusion evaluation
*Because cytology has been estimated to be only ~60-80% sensitive for the presence of bacteria, additional testing (glucose, lactate) is strongly recommended to evaluate for septic abdomen if clinically indicated.
**Automated vs manual methods – not always available in-hospital and may require submission of fluid sample to an outside reference laboratory.
TABLE 2 – Stabilizing therapies for acute abdomen patients.
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