Spontaneous Bacterial Peritonitis

What is spontaneous bacterial peritonitis?

Spontaneous bacterial peritonitis is a bacterial infection of the peritoneum, the lining of the abdominal cavity, and the fluid within it known as the peritoneal fluid. Peritonitis is a condition that occurs with injury or infection of the the abdominal organs. However, spontaneous bacterial peritonitis arises on it is own without preceding injury or infection. It is more likely to occur in conditions and diseases where there is greater fluid accumulation within the cavity but the mechanism by which bacteria are able to enter and contaminate this fluid is still unclear.

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Spontanous Bacterial Peritonitis Incidence

The condition affects adults and children with ascites equally. The incidence is highest in newborns and 5 year olds. Spontaneous bacterial peritonitis may arise in as many as 1 out of 5 cases of ascites. There risk of death is greatest among adults with a mortality rate of between 40% to 70% reported.

Spontaneous Bacterial Peritonitis Pathophysiology

Peritoneal cavity

The peritoneum is a thin membrane that lines the abdominal cavity and some of the organs. There are two layers – the parietal peritoneum and visceral peritoneum. The parietal peritoneum adheres to the inner wall of the abdomen. The visceral peritoneum surrounds the abdominal organs. Between the two layers of the peritoneum is a potential space known as the peritoneal cavity. A small amount of fluid, peritoneal fluid, is within this cavity and provides lubrication. In this way the abdominal organs are secured and do not rub against the abdominal wall during movement. Abdominal organs bound by peritoneum is known as intraperitoneal organs. Some organs may lie outside it and are therefore known as extraperitoneal organs.

Fluid accumulation

Normally there is as little as 20mL (milliliters) of peritoneal fluid within the cavity produced by the peritoneal lining. Sometimes excess fluid may be produced or can accumulate from leaking out of blood vessels. This is known as ascites. It can occur with various different diseases. Normally this fluid is sterile meaning that there are no microbes within it. Some cases of peritonitis can be due to an infection and microbes will therefore be found within the fluid. These microbes enter the peritoneal cavity through a penetrating abdominal injury, spread from surrounding organs or even through the bloodstream when there is an underlying infection.

Bacteria in the fluid

Ascites is present in spontaneous bacterial peritonitis due to conditions like liver cirrhosis. The route by which microorganisms gain entry into the fluid however, is not clear. Most of these microorganisms are gut bacteria although there is no communication point between the gastrointestinal tract and the peritoneal cavity. It was initially believed that the gut bacteria migrates through the gut wall and into the ascitic fluid. However, it appears that the bacteria most likely travel through the bloodstream to reach the peritoneal cavity. It is possible that the reason why these bacteria are able to enter the bloodstream is that there is an intestinal bacterial overgrowth in the bowels. This may arise with slower than normal bowel motility time along with impaired immunity in diseases like liver cirrhosis.

Spontaneous Bacterial Peritonitis Causes

Types of bacteria

Most cases of spontaneous bacterial peritonitis are due to a single type of bacteria. The most common bacteria found in the ascitic fluid is Escherichia coli (E.coli).  Overall gram-negative aerobic bacteria comprise 75% of microbes found in the ascitic fluid in this condition. Gram-positive aerobic bacteria comprise the remainder of cases with streptococci being a significant species involved. This bacteria are referred to as aerobic because it requires the presence of oxygen which is high in ascitic fluid. Therefore anaerobic bacteria, which do not require oxygen for growth, are rare in spontaneous bacterial peritonitis.

Risk factors

Debilitated patients, usually with a chronic disease, are at the greatest risk of developing spontaneous bacterial peritonitis. Specific risk factors include :

  • Peritoneal dialysis patients
  • Severe liver disease like cirrhosis or other liver conditions where blood proteins and prothrombin time are impaired.
  • Immune compromised
  • Low protein levels in ascitic fluid
  • Asymptomatic bacteruria

Spontaneous Bacterial Peritonitis Symptoms

Up to 1 out of 3 cases of spontaneous bacterial peritonitis can be asymptomatic meaning that no symptoms are present.

Abdomen

The main symptoms are within the abdomen. Depending on the severity of the ascites, the abdomen may be swollen (distended due to fluid). With spontaneous bacterial peritonitis, this swelling does not improve even though diuretics are being administered to increase water loss. Abdominal pain or discomfort is usually present as well as tenderness during palpation.

Bowels

Diarrhea is another symptom associated with spontaneous bacterial peritonitis. Due to the jaundice as a result of liver disease, the stools may be pale in color. Ileus may also be noted which is impaired movement of gut contents through certain segments of the gastrointestinal tract despite the lack of a mechanical obstruction. This can sometimes contribute to constipation but diarrhea is more prominent with spontaneous bacterial peritonitis.

Fever and chills

Since spontaneous bacterial peritonitis is an infection, fever and chills are almost always present. However, it is important to note that other factors such as the causative conditions may cause abnormal changes in body temperature.

Other symptoms

  • Hypotension (low blood pressure)
  • Jaundice and other symptoms of liver failure
  • Signs of kidney failure
  • Encephalopathy

Spontaneous Bacterial Peritonitis Diagnosis

Spontaneous bacterial peritonitis is suspected when ascites is worsening despite treatment and the overall of condition of the patient is declining However, more conclusive tests need to be conducted on the ascitic fluid, blood and urine.

Ascitic fluid

Diagnostic paracentesis is the procedure of collecting a sample of ascitic fluid. A thin needle is inserted into the abdomen and a small amount of fluid is withdrawn. Patients undergoing peritoneal dialysis have an indwelling catheter which provides easier access for tapping the fluid. Various tests are performed on this extracted fluid to verify the cause of ascites and diagnose spontaneous bacterial peritonitis.

Blood and urine

Blood culture and urine culture are both useful tests although not always accurate for diagnosing spontaneous bacterial peritonitis. Here the sample is used to grow the bacteria in the laboratory. The exact bacteria can then be identified and sensitivity tests can be conducted to ascertain which antibiotic may be the most effective in the treatment of the condition.

Spontaneous Bacterial Peritonitis Treatment

Treatment with antibiotics is necessary. The choice of antibiotic may depend on the underlying disease as certain types of antibiotics may be contraindicated. The results of the cultures will also determine the choice of antibiotic in treating spontaneous bacterial peritonitis. This may be administered intravenously or orally. Antibiotics that are used include :

  • Cefotaxime
  • Gentamicin
  • Ampicillin
  • Norfloxacin
  • Ciprofloxacin
  • Sulfamethoxazole and trimethoprim

Therapeutic parencentesis, where the fluid is removed from the abdomen to treat the condition, is not necessary.

References :

http://emedicine.medscape.com/article/789105-overview

http://www.merckmanuals.com/professional/hepatic_and_biliary_disorders/approach_to_the_patient_with_liver_disease/ascites.html

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