Peritonitis
Bacteria or gut contents reach the peritoneal cavity, which cannot clear them, so infection spreads across its huge surface and drives ileus, third-space fluid loss and sepsis.
Definition
Inflammation of the peritoneal membrane. It is primary (spontaneous) when ascitic fluid is infected without a perforated abdominal organ. It is secondary when contamination or transmural infection arises from an abdominal source. It is tertiary when infection persists or returns after apparently adequate treatment. Peritoneal dialysis peritonitis is infection of the dialysate, usually introduced through the catheter.
First principles
Peritonism is a sign that the parietal peritoneum is involved
Pain from the gut itself travels in visceral nerves and is vague and midline. The parietal peritoneum lining the abdominal wall has somatic nerves, so pus, bile, acid or faeces touching it produces sharp, well-localised pain, reflex guarding and pain on movement or coughing. The change from intermittent colic to constant pain that hurts to move is the clinical moment the lining became involved. That is why the patient lies still and why the abdominal wall goes rigid.
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