Histological features of resolving acute, non-complicated phlegmonous appendicitis.

The histological features of resolving acute appendicitis are described. Formalin-fixed, paraffin-embedded appendices of 200 cases with acute, non-complicated phlegmonous appendicitis were reviewed. In 80 out of 200 cases, a histological picture characterized by a predominantly lymphocytic infiltrate of the subserosa and muscularis propria or the subserosa alone was found. In the affected muscularis propria, eosinophils were admixed with lymphocytes, and the cellular infiltrate showed a lesser degree than that of the classic phlegmonous appendicitis. A multifocal, rather than a diffuse pattern of infiltration was observed. Cases were divided into three groups. Group 1: appendices with the typical features of phlegmonous appendicitis: 120 cases, 60%. Group 2: appendices with a predominantly lymphocytic infiltrate in the muscularis propria, subserosa, or both, and no granulation tissue: 65 cases, 32.5%. Group 3: appendices with granulation tissue: 15 cases, 7.5%. Complicated appendicitis was excluded. Data on the duration of the clinical symptoms were derived from the clinical history. The differences between the mean duration time of groups 1 and 2, and of groups 2 and 3 were statistically significant. The findings support the contention that a mixed infiltrate of lymphocytes and eosinophils represents a regression phase of acute appendicitis.