Microscopic description and IHC:

Absence of the appendiceal luminal architecture

  • lymphoid aggregate, intestinal-type epithelial layer, goblet cells

Presence of endometrial architecture

  • Endometrial glands, stroma, sclerosis, punctate hemorrhage
  • (IHC will be positive for estrogen receptor)

Acute inflammation

  • Neutrophils in muscularis propria and serosa of the appendix
Discussion:

Endometriosis is a condition characterized by the growth and proliferation of endometrial glands and stroma outside of the uterus. The GI tract is affected in 12 - 37% of cases, and appendix in 0.4-2.8%. Appendiceal endometriosis typically involves the serosa or muscularis propria of the appendix, the submucosal involvement is not very common.

The clinical presentation of appendiceal endometriosis is non-specific abdominal pain that wax and wane with menstrual cycle. It may mimics acute appendicitis, and therefore, definitive diagnosis is made by only postoperative histological examination.

The interesting fact about this case is that the endometrial glands and stroma have completely replaced the intestinal mucosa and submucosa of the appendix as if it was inoculated from the luminal side instead of the serosal side. This occurs very rarely. Possible mechanism is infiltration of the endometrial glands from the serosal side followed by period proliferation, sloughing of the intestinal mucosa, and eventual penetration and replacement of the intestinal mucosa. Also, there is acute inflammation of both the appendix and ectopic endometrial tissue.

References:

Odze, R. D. and goldblum, J. R. Surgical Pathology of the GI Tract, Liver, Biliary Tract, and Pancreas, 3rd ed.: Philadelphia, Elsevier Saunders, 2015.

Emre, A. et. al. An unusual cause of acute appendicitis: Appendiceal endometriosis. Int J Surg Case Rep. 2013; 4(1): 54-57.

Curbelo-Pena, Y. et. al. Endometriosis causing acute appendicitis complicated with hemoperitoneum. J Surg Case Rep. 2015; 8: 1-3.

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