Introduction

Spontaneous bacterial peritonitis (SBP) is a term used to describe acute infection of ascites, an abnormal accumulation of fluid in the abdomen, without a distinct or identifiable source of infection.[1][2] SBP virtually always occurs in the patients of cirrhosis and ascites and is suspected when the patients present with abdominal pain, fever, or altered mental status. There are not accepted diagnostic criteria for SBP, and a minority of patients also presents without apparent abdominal pain.

Etiology

SBP is most commonly (75%) caused by gram-negative aerobic organisms, with Klebsiella pneumonia accounting for 50% of these. Gram-positive aerobic microorganisms are responsible for the remainder of cases; the most common of these are Streptococcus pneumoniae or Viridans group streptococci.[3][4]

The ascitic fluid typically has a high oxygen tension. Therefore anaerobic organisms are not commonly seen. In the majority of cases of SBP, only one infecting organism is involved (92%), though a small number of cases have been reported as polymicrobial.

Typically, patients who experience SBP have chronic liver disease with a Child-Pugh classification, which assesses the prognosis of liver disease, of C. This ranking involves a high to a maximum score of 10 to 15 points (on the Child-Pugh scale), and measures 1-year patient survival at 45% and 2-year survival at 35%. Decompensated cirrhotic patients are those at the highest risk of developing SBP. Infecting organisms typically originate from the intestinal lumen, from where they pass via translocation to mesenteric lymph nodes.

Additional risk factors for SBP include a previous history of SBP, low complement levels, and reduced hepatic synthesis of proteins with (1) an associated prolonged PT time and reduced protein levels in ascitic fluid (less than 1 g/dL), and (2) long-term proton pump inhibitor (PPI) therapy such as increased gastric pH with PPI use, which promotes gut bacterial growth and translocation.

Though in adults, SBP is typically seen in patients with abdominal ascites, most children with SBP do not have ascites. The reason for this has not yet been elucidated.[5]

Epidemiology

SBP can occur in adults and children. In children, it most commonly occurs in neonates and those around five years of age. It is most common in patients with cirrhosis. However, it can occur as a complication of any disease that results in the accumulation of ascitic fluid, such as liver disease, Budd-Chiari syndrome, congestive heart failure, systemic lupus erythematosus, renal failure, or cancers, and has a poor prognosis. Approximately 10% to 25% of patients with ascites will develop SBP, and the condition is associated with a 20% in-hospital rate of mortality.

Patients with a prior incidence of SBP are more likely to encounter a subsequent infection with a drug-resistant organism. Additionally, the risk of developing SBP increases with age, the use of proton-pump inhibitors (PPIs), and when undergoing SBP prophylaxis, such as selective intestinal decontamination.[6]

Pathophysiology

The majority of the isolated organisms in SBP (90%) are gram-negative enteric organisms (e.g., Escherichia coli or Klebsiella pneumoniae), which suggests that the primary source of contamination is the gastrointestinal (GI) tract. Enterotoxin is also frequently isolated from ascitic fluid, further supporting the theory that bacteria involved in SBP migrate transmurally from the intestinal lumen (i.e., bacterial translocation).

An alternatively proposed mechanism of contamination involves hematogenous spread, from a distant source, such as a urinary tract infection, in individuals predisposed to the disease by a weakened immune system (i.e., immunocompromised).  Patients with cirrhosis typically have an elevated level of bacterial overgrowth in the GI tract, mainly due to a prolonged intestinal transit time. This, coupled with reduced protein production by a cirrhotic liver (e.g., low complement levels in both the serum and ascites) and poor phagocytic and reticuloendothelial system function, results in a decreased ability to clear microorganisms from the system, thereby further contributing to bacterial overgrowth, migration, and expansion within the ascites fluid.[7]

History and Physical

One should have a high index of suspicion for SBP in all patients presenting with ascites, and this is especially true if the patient has an acute history of clinical deterioration. The majority of patients with SBP will present with fever, chills, and abdominal pain, although some patients may be asymptomatic,, and SBP is an incidental finding. Temperature is the most common symptom encountered in patients with SBP, which is a particularly useful clinical symptom as patients with cirrhosis are typically hypothermic. Additional signs and symptoms include diarrhea, paralytic ileus, new-onset or worsening encephalopathy (e.g., altered mental status) without any other identifiable cause, new-onset or worsening renal failure, or presence of ascites that does not improve with the use of diuretic medications.

On physical examination, most patients will have a tender abdomen, although patient response can vary from mild discomfort to the presence of guarding and rebound tenderness. The absence of fever does not rule out SBP.

Evaluation

There is a short window of opportunity for treating SBP before it progresses to septic shock or multisystem organ failure; therefore, rapid assessment and diagnosis are critical (i.e., treatment is much more successful if antibiotics are started before the development of shock).[2][8][9]

Peritoneal fluid analysis (e.g., cell count, differential, culture, lactate level, pH) should be performed in all patients with suspected SBP. This may be achieved either by diagnostic paracentesis or withdrawal of fluid through a peritoneal catheter, which is sometimes present in patients undergoing peritoneal dialysis. In patients with only a small amount of ascites, ultrasonography should be employed to help guide the paracentesis procedure.

Additionally, blood and urine cultures should be obtained before initiation of antibiotic therapy as the results of these may help point towards a source of infection and guide antibiotic therapy.

The most accurate predictor of SBP is a polymorphonuclear leukocyte (PMN or granulocytes such as neutrophils, basophils, and eosinophils) count of greater than 500 cells/uL in a sample of ascitic fluid with a sensitivity and specificity of 86% and 98%, respectively. At a PMN count of higher than 250cells/uL, sensitivity increases to 93%, while specificity declines to 94%. This is the widely accepted number of PMNs needed to form a possible diagnosis of SBP, before beginning empiric antibiotic therapy.

If the perforation is suspected within the abdomen, imaging is warranted, and computed tomography (CT) should be strongly considered, as it is more sensitive for detecting smaller perforations than a plain radiograph.

More recently, proposed tests for SBP include a rapid reagent strip that evaluates for the presence of leukocyte esterase in ascitic fluid, which is shown to have a sensitivity of 100% in the diagnosis of SBP when compared to manual PMN counting. Though this may prove to be a much more efficient method of SBP diagnosis than those previously used, the test still needs to undergo a large-scale comprehensive evaluation. 

Patients who are at high risk for SBP include: 

• Patients with gastrointestinal bleed and cirrhosis.
• Patients who already had SBP one or more times in the past.
• Cirrhotic patients with ascites in which ascitic fluid protein is < 1.5 g/dl along with renal failure (Creatinine > 1.2mg/dl).
• Cirrhotic patients who are hospitalized for a cause other than SBP and have an ascitic protein concentration of < 1 g/dl (10g/L).

Differential Diagnosis

• Perforated viscus
• Perinephric abscess
• Pyelonephritis
• Diverticulitis
• Appendicitis
• Mesenteric ischemia

Prognosis

The infection-related mortality in SBP is very low with appropriate treatment. However, the mortality rate is quite high in those patients who develop sepsis. But with appropriate antibiotic therapy, better outcomes can be achieved. In hospital settings, the non-infection related mortality in SBP patients can be as high as 20-40 percent, and one to two years of mortality rates are 70 and 80 percent, respectively.[15] Regardless of the short term outcome of SBP, patients who have liver disease severe enough to develop SBP usually have a terrible long term prognosis. Moreover, liver transplantation should be considered for the survivors of SBP, who are otherwise good transplantation candidates.

Complications

• Renal failure 
• Sepsis
• Liver failure/insufficiency
• Tense ascites
• Bleeding after paracentesis 
• Bowel perforation after paracentesis 
• Spontaneous fungal peritonitis

Deterrence and Patient Education

• Treat Acute GI bleeding aggressively.
• Patients with ascitic fluid levels with protein concentration less than 1g/dl should be managed as inpatients.
• Patients who have a history of SBP in the past year should be given appropriate antibiotic prophylaxis for a longer duration.
• For patients with a history of SBP, prolonged out-patient treatment with either Trimethoprim-sulphamethoxazole or ciprofloxacin/norfloxacin should be used.

Enhancing Healthcare Team Outcomes

When patients develop ascites from whatever cause, the treatment usually involves a team of healthcare professionals, including nurses, pharmacists, physicians, and other allied healthcare workers. These patients not only need to be treated for the acute infection but also the primary disorder causing the fluid build-up. The patients are often emaciated and need a dietary consultation. A physical therapy consult is required in order to increase mobility and muscle function.  The pharmacist must ensure that the patient is on no medications that can worsen liver or renal function and must abstain from alcohol use. The nurse has to monitor the patient's abdomen circumference, body weight, and vital signs regularly to make sure that the treatment is working. The family has to be educated on the signs and symptoms of bacterial peritonitis and when to bring the patient back to the hospital. There is a small margin for errors, and any delay in seeking help can lead to mortality. [16][17] (Level V)

Outcomes

Spontaneous bacterial peritonitis carries a mortality rate of 30-70% in patients with end-stage liver and kidney disease. The mortality rates are lower in children with nephrosis. Recently the mortality rates have shown a slight decrease because of earlier diagnosis and advances in treatment. For the patient who develops septic shock secondary to bacterial peritonitis, the mortality increases each hour until antibiotic therapy is started. [18][7](Level V)