This is the commonest cause of the acute abdomen. It usually occurs when there is an obstruction in the lumen of the appendix either by a faecolith or foreign body or by enlargement of lymphoid follicles in its wall. It most often affects children, teenagers and young adults. It is rare at the extremes of life. In the infant, the lumen of the appendix is wide in relation to the remainder of the bowel and the diet is soft and hence, obstruction within the lumen is less likely. In the elderly, the lumen tends to be obliterated.
Rarer causes of appendicitis include carcinoma of the caecum obstructing the appendiceal lumen, carcinoid tumour and obstructing fibrous bands. Occasionally a carcinoma obstructing the lumen of the appendix will cause it to distend and fill with mucus, i.e. a mucocele of the appendix.
Appendicitis may resolve spontaneously. The appendix may
become surrounded by adjacent small bowel and omentum and give rise to an appendix mass. It may perforate giving rise to generalized peritonitis or it may perforate amidst local adhesions giving rise to an appendix abscess. Often it is difficult to diagnose appendicitis. If the symptoms have been present for 48 h (‘48 h rule’) and the diagnosis is truly appendicitis, then the patient should either have developed an appendix mass or generalized peritonitis. If neither of these two is present, then the diagnosis of appendicitis should be reviewed.