The evaluation of abdominal pain requires an understanding of the possible mechanisms responsible for pain, a broad differential of common causes, and recognition of typical patterns and clinical presentations. All patients do not have classic presentations. Thus, unusual causes of abdominal pain must also be considered, especially in older adult and immunocompromised patients.
An epidemiologic assessment of acute abdominal pain found that 10 diagnostic groups could be classified in outpatients complaining of abdominal pain on their first visit to primary care physicians: whole abdominal, epigastric, right subcostal, left subcostal, right flank, left flank, periumbilical, right-lower, mid-lower, and left-lower (table 1) . The overall sensitivity of history taking and physical examination was poor. Specificity was highest in patients with epigastric pain caused by gastroduodenal diseases, right subcostal pain caused by hepatobiliary diseases, and mid-lower pain caused by gynecologic diseases.
This topic review will provide an overview of the mechanisms and differential diagnosis in patients with abdominal pain. Detailed discussions of the specific causes of abdominal pain and initial management are presented separately. A detailed history and physical examination in adults with abdominal pain is also discussed separately. (See "History and physical examination in adults with abdominal pain".)
Many patients with abdominal pain will have a functional disorder such as irritable bowel syndrome or functional dyspepsia. A diagnostic approach appropriate for most patients with abdominal pain and aimed at appropriately distinguishing functional disorders from more serious etiologies of abdominal pain is also presented separately. (See "Diagnostic approach to abdominal pain in adults".)
NEUROLOGIC BASIS OF ABDOMINAL PAIN
Pain receptors in the abdomen respond to mechanical and chemical stimuli. Stretch is the principal mechanical stimulus involved in visceral nociception, although distention, contraction, traction, compression, and torsion are also perceived . Visceral receptors responsible for these sensations are located on serosal surfaces, within the mesentery, and within the walls of hollow viscera, in which they exist between the muscularis mucosa and submucosa.