Acute abdominal pain is one of the most common symptoms that prompt patients to seek care in the emergency department (ED). Appendicitis is easily missed clinical presentation isn t classic in almost half the cases and except for CT scanning, routine investigations lack the sensitivity and specificity to help clinch the diagnosis. While the overall mortality for appendicitis is low (0.2 deaths per 100,000 cases), delay can lead to perforation, with illness and an increased risk of death. Missed cases have been a source of medicolegal actions against emergency physicians. When evaluating a patient with abdominal pain, an organized and evidence-based approach is therefore in order.

Diagnosis of exclusion
Acute abdominal pain of < 1-2 weeks duration accounts for up to 10% of admissions to the ED. Of those, some 20-40% are admitted to hospital for further investigation and symptom management, yet the etiology remains undetermined in about half.

Sorting out patients with abdominal pain is a tedious task and it poses risks for both the patient and physician. The spectrum of diseases that present in this way ranges from life-threatening to benign. Often the diagnosis can't be established in a single encounter. So it's probably more important to exclude life-threatening etiologies than to make a specific diagnosis.

Types of abdominal pain
Classic surgical teaching separates abdominal pain into 3 categories, based on underlying pathophysiology visceral, parietal and referred.

Visceral pain arises from distension, inflammation or ischemia near afferent autonomic nerve fibres of solid or hollow viscous organs i.e. kidney, liver, spleen, bowel, gallbladder and urinary bladder. To localize the pain, you have to consider the embryologic origin of the organ. The foregut is referred to the epigastrium (i.e. the upper abdomen, near the stomach region), the midgut to the umbilicus and the hindgut to the hypogastric region (i.e. lower abdomen). When an organ is innervated bilaterally, pain is perceived in the midline.

Parietal pain arises from irritation of the peritoneal membrane from a regional organ or tissue. The pain is localized to the dermatome above the site of stimulus. Initially, parietal pain is perceived as unilateral.

Referred pain produces symptoms, but not physical signs, and it's usually lateralized. Patterns of referred pain are based on embryologic sharing of dermatomes. For example, the testes and ureter share the lower thoracic and upper lumbar dermatomes, so pain of renal colic localizes to the flank.

Another conceptual framework is to determine whether the source of pain is intra- or extra-abdominal. Intra-abdominal causes are presumed to arise from gastrointestinal, genitourinary, gynecologic and vascular catastrophes, whereas extra-abdominal causes include cardiac, pulmonary, abdominal wall, toxic, metabolic and neurologic disorders (see Table 1).

Unfortunately, all of the textbook approaches to abdominal pain have their limitations, including one based on the relationship between overlying tenderness and underlying surgical disease. In one study of 600 patients, in whom diagnoses were confirmed intraoperatively, typical clinical findings were detected in only 60-70% of subjects.1 Any classic presentation of diagnosis for abdominal pain should therefore be considered a gift to the physicians on the front lines.

Focused assessment
A multicentre study of over 10,000 patients with abdominal pain, who presented to 200 EDs over a 10-year period, provides the most realistic ranking of actual diagnoses (Table 2). At the top of the list is an entity called nonspecific abdominal pain. This default diagnosis is made when the cause remains elusive at the time of discharge from the ED or after admission to hospital and later discharge. When a cause is actually found, the main diagnoses are appendicitis, biliary tract disease, bowel obstruction, gynecologic disease and renal colic, in descending order of frequency.

As with any acute condition, the clinician must follow a focused assessment. The critical information relates to the characteristics of the pain, including location, quality (sharp or dull), severity, pattern of onset (sudden or gradual), radiation and aggravating or relieving factors. it's important to note the presence or absence of associated symptoms that are systemic (e.g. fever), as well as those that are organ-specific, like gastrointestinal (e.g. nausea and vomiting) and gynecologic in the case of female patients (e.g. vaginal bleeding).

A general physical examination is always in order, as are vital signs blood pressure, heart rate, respiratory rate, temperature and pulse oximetry. Beware of volume depletion in elderly patients. Because of the use of beta-blockers or autonomic neuropathy, they may not show symptoms such as tachycardia. Moreover, blood pressure is preserved early on in hypovolemic shock and a postural drop is a late sign.

The specific physical exam involves inspection, auscultation, percussion and palpation of the abdomen and external genitalia. In selected women, a speculum and bimanual pelvic examination may be indicated.

Myths
While the physical examination is of paramount importance, research has shown that there are many myths in classic surgical teaching. The first is that rebound tenderness is a good indicator of peritonitis. In one study, this symptom had a 25% false-positive rate.2 Clinical findings more highly associated with peritonitis are rigidity, referred tenderness and cough pain.

The second myth is that all patients with abdominal pain should undergo a digital rectal examination.3 Studies have consistently demonstrated that this uncomfortable manoeuvre adds little to the clinical assessment, except for the detection of blood or melena stool.4 Despite this evidence, the failure to perform a rectal exam is a recurring theme in American malpractice literature.5

The third myth is that administration of an opioid analgesic contaminates the examination. Trials consistently demonstrate that giving morphine doesn't alter the physical exam,6,7,8 and in one study at Brigham and Women's Hospital in Boston, administration of intravenous morphine actually enhanced diagnostic accuracy.

Routine investigations
The purpose of a diagnostic test is to distinguish patients who have a disease from those who don't. A positive likelihood ratio (LR+) indicates how much a positive test result is associated with the disease in question, whereas a negative likelihood ratio (LR-) connotes absence of the illness. The higher the LR, the more powerful the test is at discriminating whether to rule the disease in or out.

The WBC count is one of the screens most frequently ordered on patients with abdominal pain, yet study after study demonstrates that it's poor at separating those who have serious disease from the others. In one report that looked at 241 patients with undifferentiated abdominal pain, an elevated WBC was found in about 25% of subjects. The outcomes, however, were no different from patients whose WBC was normal.9 In a case series where appendicitis was diagnosed by laparotomy, the WBC count had a sensitivity of 76% and a specificity of only 52%.10

Plain abdominal x-rays also have limited use. Most researchers conclude that they give a low diagnostic yield, generate incidental and misleading findings and don't change patient management.11 Even among radiologists, there's poor inter-observer reliability. A clear indication, though, is a suspected bowel obstruction.

Urinalysis is cheap, simple and readily available. Either the dipstick test or routine analysis with microscopy exhibits high yield when the results fit with the clinical scenario. A screening urine pregnancy test is recommended for all women of child-bearing potential.

Clearly, not every patient with abdominal pain needs laboratory investigation. The decision to subject someone to testing ought to be driven by the severity or persistence of the pain and the presence of high-risk findings, e.g. advanced age, abnormal vital signs, right lower quadrant tenderness, etc.

Likelihood ratios
The incidence of appendicitis among all patients presenting to the ED with acute abdominal pain of < 1 week s duration is in the range of 12-26%. it's higher, though, in younger individuals < age 50. Most patients with appendicitis don't have the classic history or physical findings and, unfortunately, there are no pathognomonic features. Fortunately, clinicians don't usually rely on a single symptom or physical sign, but must use a combination of the two.

On history, the two features that have the highest positive LRs are pain in the RLQ, LR+ = 7.3-8.5, and migration of initial periumbilical pain, LR+ = 3.2. See Table 3 for other signs and symptoms.12 Only one physical finding showed a relatively high LR+ across all studies, and this was rigidity (LR+ = 3.8). Rebound tenderness had too wide a range of variance to make it a useful discriminating physical sign.

In ruling out appendicitis, a few features of the history proved to be useful absence of RLQ pain and having experienced previous episodes of the same pain, LR-: 0.0-0.3 and 0.3, respectively.

Microscopic hematuria and pyuria are present in 20-30% of patients with appendicitis, but they also occur in many other conditions and asymptomatic individuals as well. Elevated WBC (> 11.0 x 109/L) has poor sensitivity and specificity for the diagnosis.

Contrast-enhanced CT
Contrast-enhanced CT of the abdomen has become the king of tests to diagnose most intra-abdominal surgical conditions. Though it's less accurate than ultrasound in the evaluation of hepatobiliary or adnexal disorders, it's highly sensitive and specific for detecting appendicitis, diverticulitis, perforation, abdominal aortic aneurysm, abscess formation and mesenteric ischemia. Appendiceal CT has a sensitivity approaching 100% in certain hands and LR+ values are as high as 26.13

The problem with routine use of CT is that the radiation exposure increases the risk of cancer, especially in younger patients. Ultrasound has similar sensitivity to CT in children and it's almost as good as CT in adults. Consider this option when radiation exposure is a concern.