Appendicitis that causes peritonitis can cause signs of peritoneal irritation, abdominal muscle tension, and may be characterized by a hardening of the lower abdomen. If appendicitis is not diagnosed and treated in time, resulting in suppuration and perforation of the appendix, it may cause peritonitis and signs of peritoneal irritation: pressure pain, rebound pain and abdominal muscle tension. This is the defensive reaction of the peritoneum after being stimulated by inflammation, suggesting that the inflammation of the appendix is aggravated, and suppuration, gangrene, perforation, etc. may have occurred. In some people, the abdominal muscle tension is so heavy that the abdomen is as hard as a wooden board, which is called “plank abdomen”, and this is a manifestation of the severity of peritonitis. When peritonitis occurs in appendicitis, immediate surgery is needed to remove the diseased or even necrotic appendix. If peritonitis is present, seek medical attention immediately.