The term acute abdomen represents 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, in children, in the immunocompromised, and in the last trimester of pregnancy. Acute abdominal complaints are common.    Estimates vary, but a prevalence of around 50% was identified in one study of upper abdominal pain,  while one large telephone survey found that 45% of people experienced at least one upper GI symptom in the previous 3 months. 
Acute abdominal pain is a frequent emergency department presentation. Pain may be located in any quadrant of the abdomen and may be intermittent, sharp or dull, achy or piercing; it may radiate from a focal site and there may be associated symptoms such as nausea and vomiting. Immediate assessment should focus on distinguishing those cases of true acute abdomen that require urgent surgical intervention from those that do not, which can initially be managed conservatively.  Data from the UK suggest that availability and/or access to an experienced surgeon reduces unnecessary admissions while appropriately managing or triaging patients. 
Abdominal pain lasting more than 48 hours is less likely to require surgery than pain of shorter duration.  A patient with acute surgical pathology may deteriorate rapidly; thus, severe, unremitting symptoms in the first few hours warrant vigorous investigation and close monitoring. Additionally, for patients who are clinically stable and have not undergone surgery due to the lack of an obvious cause, diagnostic laparoscopy may be considered.   
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 examinations by an experienced physician may help determine the underlying cause.  Alternatively, diagnostic laparoscopy can be considered in selected patients.    Laparoscopy not only is a useful tool for diagnosis, but is increasingly used as a therapeutic modality of choice for conditions such as appendicitis, cholecystitis, lysis of adhesions, hernia repair, and many gynaecological causes of an acute abdomen.
Assessment of acute abdominal pain in terms of diagnostic accuracy may be improved with the use of algorithms or decision tools. These approaches have been evaluated and noted to improve the specificity for diagnostic accuracy.  Specific aetiologies, such as an acute appendicitis, continue to undergo assessment for novel clinical triage tools, such as the appendicitis inflammatory response (AIR) score, in an attempt to more accurately stratify patients upon presentation. 
Traditionally, the use of narcotic analgesia in patients with an acute abdomen had been discouraged until a definitive diagnosis could be made.   Concerns existed about analgesia making patients unstable, hindering subsequent physical examinations, and ultimately delaying a definitive diagnosis. However, more recent reviews suggest that the use of narcotic analgesia is reasonable and does not hinder subsequent diagnosis.   [ ] Fentanyl or one of its analogues can be a useful agent in this situation due to the combination of potency and short half-life.
Abdominal pain in populations such as older people, the immunocompromised, and pregnant patients deserves special consideration. These populations frequently present atypically compared with the general population, and because of the lack of understanding of physiological changes in their response to certain diseases, there are often delays in diagnosis and treatment, with consequently higher complication and mortality rates. 
Abdominal pain in older patients is a common complaint and is challenging to diagnose due to frequent atypical presentation and lack of overt clinical features.  Older patients also usually have more long-standing comorbidities than younger adults, which can also affect their ability to mount characteristic physiological responses - this may be due either to the direct effects of a comorbid condition (e.g., diabetes) or to medicines used to treat the comorbid conditions (e.g., beta-blockers that blunt tachycardia or narcotics that may blunt sensation of pain). Older patients are also at higher risk for more severe disease due to decreased immune function.  Central and peripheral nervous systems are also affected by ageing. Conditions such as dementia can restrict an older patient's ability to communicate problems, and decreased peripheral nervous system function can alter perception of pain and temperature, making diagnosis and management more difficult. For example, one study that reviewed patients with perforated ulcers found that only 21% of older patients presented with peritonitis. 
An awareness that older patients may have atypical and/or delayed presentations due to pathophysiologic, immunologic, pharmacological, or neurological effects of ageing combined with a high index of suspicion helps prevent delay in diagnosis of intra-abdominal emergencies in these patients.
Pregnant patients are also challenging to diagnose and treat, due to the physical and physiological changes involved. 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, together with the laxity of the abdominal wall, makes it difficult to localise pain and can blunt peritoneal signs.  Pregnant patients may sometimes have a mild physiological leukocytosis, so this finding is non-specific in these patients. 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 ionising 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 CT 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 minimise exposure. Importantly, the use of ultrasound for diagnosis is clearly safe in pregnancy, although it should be utilised to evaluate and answer a defined clinical problem. 
It is important to counsel the patients as to the risks and benefits prior to obtaining radiographs, as any risk there may be must be carefully balanced against the fact that a delay in diagnosis and treatment can increase the risk of fetal and maternal loss. Alternatives to ionising radiation imaging, such as ultrasound and MRI, 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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