The term acute abdomen refers to the rapid onset of severe symptoms that may indicate potentially life-threatening intra-abdominal pathology that requires urgent surgical intervention.
Abdominal pain is usually a feature, but a pain-free acute abdomen can occur, particularly in older people, children, and the immunocompromised, and in the last trimester of pregnancy. Acute abdominal symptoms are common.    Estimates vary, but one study of upper abdominal pain identified a prevalence of around 50%,  while one large telephone survey found that 45% of people had experienced at least one upper gastrointestinal symptom in the previous 3 months. 
Acute abdominal pain is a frequent presentation in the emergency department. Pain may:
Be located in any quadrant of the abdomen
Be intermittent, sharp or dull, achy or piercing
Radiate from a focal site
Be accompanied by nausea and vomiting.
Immediate assessment should focus on distinguishing patients with true acute abdomen that requires urgent surgical intervention from patients who can initially be managed conservatively.  Data from the UK suggest that access to an experienced surgeon reduces unnecessary admissions. 
A patient with acute surgical pathology may deteriorate rapidly; therefore, patients with severe, unremitting symptoms in the first few hours warrant vigorous investigation and close monitoring.
An acute abdomen is diagnosed by a combination of history, physical examination, radiography, and laboratory results. When symptoms do not necessitate immediate surgery and when imaging has not led to a definitive diagnosis, further abdominal examination by an experienced physician may help to determine the underlying cause.  Alternatively, diagnostic laparoscopy can be considered in selected patients.    Laparoscopy is not only a useful tool for diagnosis but is increasingly used as a therapeutic measure for appendicitis, cholecystitis, lysis of adhesions, hernia repair, and many gynecologic causes of an acute abdomen.
Diagnostic accuracy may be improved by using algorithms or decision tools. For example, the Appendicitis Inflammatory Response (AIR) score has been shown to help stratify risk of appendicitis in patients presenting with acute abdominal pain. 
Use of narcotic analgesia in undiagnosed patients with an acute abdomen has traditionally been discouraged.   This is because of concerns that symptoms would be masked, the examination hindered, and, therefore, the correct diagnosis missed. Recent reviews, however, suggest that narcotic analgesia does not hinder management and improves patient comfort.  [ ] Fentanyl or one of its analogs can be a useful agent in this situation due to the combination of potency and short half-life.
Abdominal pain in older people, the immunocompromised, and pregnant women often presents atypically. Combined with a lack of understanding of the physiologic changes in these groups’ responses to certain diseases, this means their diagnosis and treatment is often delayed, and complication and mortality rates are consequently higher. 
Older people usually have more long-standing comorbidities than younger adults, which can further affect their ability to mount characteristic physiologic responses (this may be due to direct effects of a comorbid condition or to medications used to treat the comorbid condition). Older patients are also at higher risk for more severe disease due to decreased immune function. 
Central and peripheral nervous systems are affected by aging. Conditions such as dementia can restrict an older person's ability to communicate problems, and decreased peripheral nervous system function can alter perception of pain and temperature, making diagnosis and management more difficult. One study of patients with perforated ulcers found that only 21% of older patients presented with peritonitis. 
The physical and physiologic changes associated with pregnancy may present a challenge to diagnosis and treatment. A prompt and thorough evaluation is essential as a delay in diagnosis and treatment can lead to poor outcomes for both mother and fetus. 
The enlargement of the uterus, which displaces and compresses intra-abdominal organs, and the laxity of the abdominal wall makes it difficult to localize pain and can blunt peritoneal signs.  Pregnant women may sometimes have a mild physiologic leukocytosis, so this finding is nonspecific in pregnant women presenting with an acute abdomen. If there is a high index of suspicion for intra-abdominal pathology, further studies are warranted.
There is a lot of concern among healthcare providers and patients around the issue of obtaining radiographs on pregnant patients. Although it is known that ionizing radiation exposure can lead to cell death, mutation of germ cells, and carcinogenesis, there is no modern radiographic procedure that results in radiation exposure to a level that threatens embryo or fetal well-being.  Radiation exposure of <5 rads (a computed tomographic abdomen/pelvis study carries an exposure of 3.5 rads) has not been associated with fetal defects or loss.  Careful shielding of the patient can also minimize exposure. Importantly, the use of ultrasound for diagnosis is clearly safe in pregnancy, although it should be used to evaluate and answer a defined clinical problem. 
It is important to discuss the risks and benefits with patients before obtaining radiographs. Any risk must be carefully balanced against the increased risk of fetal and maternal death resulting from a delay in diagnosis and treatment. Alternatives to ionizing radiation imaging, such as ultrasound and magnetic resonance imaging, may also have important roles in these patients.
Division of Gynecologic Oncology
The University of Alabama
CAL declares that he has no competing interests.
Walter Reed National Military Medical Center
WJL declares that he has no competing interests.
Dr Charles A. Leath III and Dr William J. Lowery would like to gratefully acknowledge Dr Aaron W. Campbell and Dr Jin H. Ra, previous contributors to this monograph. AWC and JHR declare that they have no competing interests.
Northwest Cancer Specialists
Rose Quarter Cancer Center
WW declares that he has no competing interests.
Professor and Vice Chair
UCSF Department of Surgery
Chief of Surgery
San Francisco General Hospital
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