13. Acute Abdomen / S. Meban 

The term acute abdomen describes an urgent situation in which abdominal symptoms onset suddenly and are sufficiently severe to suggest a potentially lethal condition. Pain is usually the predominant feature. Since many "acute abdomens" require prompt treatment, it is important to make a diagnosis as soon as possible. 

Acute abdominal pain may be referred to the abdominal wall from the intra-abdominal organs or may involve direct stimulation of the somatic nerves in the abdominal wall. Less commonly, pain may be referred to the abdomen from extra-abdominal sites. On occasion, acute abdominal pain is a feature of systemic disease. 

Visceral pain. This type of pain is carried by the sympathetic autonomic nerves and enters the spinal cord from T6 to L2. The parasympathetic system also carries pain sensation from the pelvic organs via S2, 3 and 4. The nerve supply to viscera arising from the primitive gut is bilateral, and pain is usually experienced in the midline. Foregut pain is epigastric in location. Midgut pain is umbilical, and hindgut pain is felt in the hypogastrium. Organs that are bilateral give rise to pain that is confined to one or the other side of the body. 

Somatic pain. Somatic afferents supplying the abdominal wall enter the spinal cord between T5 and L2. Additionally, the undersurface of the diaphragm has innervation from the phrenic nerve (C3, 4 and 5). Thus, irritation of the diaphragm may refer pain to the shoulder. 

The stimulus to pain may be chemical irritation from a perforated peptic ulcer or bacterial contamination from perforation of the colon. Other stimuli include ischemia and distention (or stretching) of the gut or parietal peritoneum. Direct pressure on a nerve by a prolapsed intervertebral disc or tumor may result in abdominal pain. 

Pain perception may be altered by aberrant function of the pain-conducting pathways. There is also marked variation in the pain "threshold" of individuals. 

Severe pain of sudden onset suggests a catastrophic event ^_ e.g., perforation of an ulcer, mesenteric embolism or rupture of aortic aneurysm. The level of referral gives a clue to the organ in which the pain originates. Visceral pain is less precise in location than somatic pain. 

Steady, severe pain is usually more ominous than colicky pain. Biliary colic is a misnomer, in that the pain is often steady (unlike the true colic of bowel obstruction or a ureteric stone). The latter corresponds to peristaltic waves and eases or disappears between waves. 

Radiation patterns are important clues. Irritation of the diaphragm from blood in the peritoneal cavity may cause shoulder tip pain. Biliary pain may radiate to the right scapular region. 

Anorexia, nausea and vomiting are nonspecific but more common in disease of the GI tract. Abdominal distention and change in bowel habit suggest obstruction. Blood in the stool may come from ulceration, tumor or infarction. In women an accurate menstrual history aids the diagnosis of ovarian disease, ectopic pregnancy and pelvic inflammatory disease. 

Examination is carried out with the patient in the supine position. Preferably, analgesia should be delayed until a diagnosis is made to avoid masking physical signs. 

Inspection should note any abdominal distention or local masses. The patient with peritonitis lies immobile, as any movement increases peritoneal irritation. With colic the patient may be restless, seeking a more comfortable position. 

Gentle palpation may detect masses. It also detects tenderness and muscle guarding or rigidity, which might suggest peritoneal irritation. 

Percussion is useful to assess the nature of abdominal distention or to outline masses. Percussion is also helpful as a "mini rebound" test that more accurately localizes the point of maximum tenderness. It is also much less distressing to the patient with peritonitis. 

Auscultation may reveal a range of bowel sounds, from the silent abdomen of peritonitis to the hyperactive sounds of bowel obstruction. Bruits suggest vascular disease, but an epigastric bruit may also be found normally. 

Rectal examination should be carried out and recorded by at least one examiner. Tenderness above the peritoneal reflection indicates pelvic peritonitis (e.g., appendicitis or diverticulitis). 

Pelvic examination may be necessary to help exclude a gynecological cause of abdominal pain. 

Apart from abdominal and extra-abdominal causes of abdominal pain, one should also consider nonsurgical causes such as diabetic ketoacidosis. 

In most cases a good history and a thorough physical examination provide the clinical diagnosis. Complete blood count (CBC) and urinalysis are standard, in addition to serum amylase and electrolytes when indicated. A serum lactate level should be done if bowel ischemia is suspected. Chest x-ray and views of the abdomen help narrow the differential diagnosis. 

More sophisticated tests may be necessary. Ultrasound is very useful in the diagnosis of biliary tract disease. Plain films of the abdomen will show large bowel obstruction, but a barium enema may be required to show the level of obstruction and to exclude pseudo-obstruction. Intravenous pyelography can demonstrate kidney nonfunction or hydroureter in suspected renal pain. An opaque calculus may be seen on plain abdominal x-rays. In suspected bowel ischemia, mesenteric angiography is essential. In diverticulitis a Gastrografin® enema can be helpful. Increasingly the abdominal CT scan is being used for diagnosis of the acute abdomen. The most appropriate investigation should be discussed with a radiologist. 

This will depend on diagnosis. In the early stages it may be impossible to distinguish the colic of acute enteritis from appendicitis. Only careful observation and repeated examination allow differentiation. Many abdominal pains settle without a confirmed diagnosis. Occasionally peritonitis requires laparotomy without a clear-cut preoperative diagnosis.