An ACS may be of a primary or secondary cause: Primary ACS refers to the syndrome arising in the presence of an inciting condition associated with injury or disease in the abdomino-pelvic region, while secondary ACS occurs in conditions not originating in the abdomino-pelvic region. In secondary ACS, the pathophysiological processes in the abdominal cavity appear to be the consequence and result of events elsewhere [9, 11].

Patients affected by ACS are typically critically ill and admitted in an intensive care environment. While all such patients are potentially at risk of the syndrome, patients usually exhibit one or more of the following risk factors: (1) reduced abdominal compliance (e.g. major burns, trauma, intra-abdominal inflammation), (2) increased intraluminal contents (e.g. intestinal ileus), (3) increased intra-abdominal contents (e.g. intra-abdominal sepsis, ascites, etc.), (4) local or generalised oedema due to permeability changes secondary to inflammation and infection and (5) some potential iatrogenic effects from resuscitation, intensive therapy or surgical interventions [2]. In addition, age, obesity, abdominal surgery, certain patient positions, mechanical ventilation and other manoeuvres resulting in increased intra-thoracic pressure, bacteriaemia, sepsis, metabolic derangement and shock have all been linked with the development of ACS [9, 12]. A recent meta-analysis identified large volume crystalloid resuscitation, the respiratory status of the patient and shock as common risk factors for ACS among a heterogeneous group of inciting pathologies. In this review, obesity, sepsis, abdominal surgery, and large volume fluid resuscitation were risk factors achieving statistical significance.

Fundamental in the development of an ACS, both primary and secondary, is a complex cellular and tissue oedema and hypoxia [11]. In turn, organ dysfunction ensues. With the deteriorating organ function, a viscous cycle develops, further adding to the cellular injury and inflammatory response. Excessive and injudicious crystalloid resuscitation has been particularly implicated in further compounding this cycle [13, 14].

The effects of the ACS on the body are extensive and widespread [11]. In the brain, increased venous pressure is measured. In the heart, reduced cardiac return due to vena cava compression in association with peripheral venous pooling in the lower extremities and torso is seen. Cardiac contractility is reduced by direct compression, and increased cardiac effort is required due to changes in afterload. The thoracic volume is reduced and pulmonary ventilation impaired. The precarious ventilator situation is further complicated by the high risk of an acute lung injury/adult respiratory distress syndrome. Hepatic dysfunction and renal impairment are the result of direct vascular and tissue compression, as well as the inflammatory organ crosstalk. Altered and dysfunctional neuro-hormonal axes are also implicated. In the kidney, the post-renal urinary obstruction does not completely account for the organ’s dysfunction; ureteric stenting and renal pelvis decompression does not lead to major clinical improvement. In the intestine, numerous changes in blood flow, mucosal integrity and bacterial translocation are seen. All these changes by the ACS effectively perpetuate the viscous cycle responsible for the secondary ACS.

Most non-traumatic, emergency general surgical conditions leading to an ACS are of a primary nature (i.e. abdomino-pelvic pathological origin) [15]. The location of the primary stimulus necessitates effective source control and potential removal of pathological volume as well as the management of the ACS within the same anatomical region. Rapid attention to both these endeavours is critical to facilitate interruption of the futile cycle. Secondary ACS could present in acute general surgical cases when extra-abdominal bleeding or sepsis requires massive resuscitation.

In initial studies, the incidence of ACS in major trauma patients was higher than one-third [16–18], though more recent experience suggests this has much reduced during the last two decades [11]. The combination of modern resuscitation strategies, management of coagulopathy, damage control surgical strategies and modern anaesthesia and intensive care has been credited with this reduction in incidence. However, while the incidence has reduced, if untreated, the condition remains highly lethal, with mortalities in excess of 50 % [8]. Reported reductions in the mortality rate of ACS reflect the more timely diagnosis and treatment.

Early studies during the 2000s, into the prevalence of ACS among non-traumatic emergencies (both abdominal and elsewhere), suggested the presence of intra-abdominal hypertension in greater than 50 % of intensive care patients, and ACS around 2–5 % of patients. Similar to trauma patients, a high mortality rate was reported [19, 20]. Approximately two-thirds of patients experienced a primary ACS [15]. While the benefits of modern critical care and resuscitative strategies may predict a reducing incidence in recent years, alarming reports of continued high prevalence may be found. A recent series from the Netherlands on pancreatitis reports all patients at their institution experiencing intra-abdominal hypertension, and almost half fulfilling the diagnostic criteria for an ACS [21], and almost equally high incidences were reported in a Belgian series on severe burns patients [22].

As most patients developing an ACS are unable to communicate symptoms due to their critical illness, or sedation for ventilation, symptoms are rarely reported. On examination, most patients will have a distended abdomen, though the sensitivity and specificity of this sign is poor [23–25]. Signs of organ dysfunction, particularly of renal and pulmonary, are common. Imaging is largely unhelpful (though there may be evidence of the secondary effects of the increased intra-abdominal pressures: vena cava compression, renal compression, bowel oedema, abdominal wall herniation, lung atelectasis and reduced volume, etc.) [26].

An ACS may rapidly develop (over hours), and a high clinical level of suspicion is required for its timely diagnosis. Ideally, continuous screening on all patients at risk could be obtained; unless active screening for ACS is undertaken, the diagnosis will likely be missed early in its clinical course. Due to practical and operational constraints, most centres will rely on intermittent measurement, using the standardised technique of urinary bladder pressure, on patients clinically thought to be at risk. Abnormal measurement results must then trigger a timely clinical response. Regretfully, delayed decision-making and tardy decompression are common, and associated with poor outcomes [8].

The precise clinical course will vary depending on the underlying cause of the ACS. For example, the clinical course of an ACS that develops in the setting of severe acute pancreatitis is different to that seen if an ACS develops in the setting of a small bowel obstruction. In acute pancreatitis, the ACS may develop as a primary phenomenon due to the pancreatic phlegmon and oedema in the retroperitoneal tissues. It may also occur as a secondary ACS, due to the systemic inflammatory response that develops during the first day or so of severe acute pancreatitis, in combination with the chosen fluid resuscitation strategy. In an adhesive small bowel obstruction, an ACS may develop from critical distension, and provide an absolute surgical indication, even though the adhesive small bowel obstruction may have otherwise settled. As another example, a septic phlegmon from acute sigmoid diverticulitis may cause a primary ACS both from the inflammatory phlegmon, associated retroperitoneal oedema, potential space occupied by abscess formation, and from the possible large bowel obstruction, or due to secondary events due to injudicious fluid resuscitation, or an associated severe systemic inflammatory phenomenon. Each of these clinical cases is fundamentally different, and the early presentation will be dominated in the underlying pathology. However, in all cases, an acute deterioration occurs with the onset of physiologic compromise associated with the ACS. In all cases, an acute surgical indication to deal with the ACS develops, though in addition to the abdominal decompression, the patient will need a tailored strategy to address the underlying cause. In most cases, the patient’s acute physiologic compromise will dictate a damage control operative strategy [10]. The type of abdominal closure will also depend on the clinical situation, and is influenced by the primary pathology and its treatment strategy, the severity (and success of treatment) of the ACS, as well as patient and situational factors.