by Madeline Fitzpatrick MD and Stephanie Reid, MD FRCPC
November 25, 2023

Acute appendicitis is a non-cancerous medical condition caused by acute inflammation in the appendix, a small finger-shaped organ that connects with your large bowel (colon) by a small opening. In most people, the appendix sits in the lower right abdomen just above your hip bone. Acute appendicitis can occur in any age group, but most commonly affects adolescents and young adults.

What causes acute appendicitis?

In appendicitis, the small opening to the appendix becomes blocked, often by a piece of digested food called a fecalith. When the opening is blocked, bacteria that normally live in your colon, fill the appendix. Your immune system responds by sending specialized immune cells called neutrophils to surround and destroy the bacteria. The combination of bacteria and neutrophils produces pus.

What are the symptoms of acute appendicitis?

Patients with appendicitis usually experience a gradual, constant pain that starts near the belly button and moves to the right lower side of the abdomen.  Other possible symptoms may include nausea, vomiting, loss of appetite, diarrhea, and fever.

How is the diagnosis of acute appendicitis made?

The diagnosis is made after the appendix has been surgically removed and sent to a pathologist for examination under a microscope. If your doctor suspects appendicitis they will order bloodwork and radiology imaging such as an ultrasound or CT scan. Your bloodwork will commonly show high numbers of immune cells called white blood cells (WBC). An ultrasound or CT scan will usually show an enlarged appendix. Appendicitis is most commonly treated by removing the appendix.

What does acute appendicitis look like under the microscope?

When your pathologist examines your appendix under the microscope, they will look for specialized immune cells called neutrophils within the wall of the appendix. These neutrophils often combine with bacteria to form an abscess (pus). Neutrophils may also be seen on the outer surface of the appendix. Pathologists describe this as peri-appendicitis.

The inflammation seen in appendicitis starts on the inside of the appendix but quickly spreads to involve the wall and even the outside of the appendix. The appendix can also become swollen which blocks blood from getting in or out of the appendix. When this happens, parts of the appendix can start to die by a process called necrosis.

What does perforated appendicitis mean?

If left untreated, a hole or perforation can develop in the appendix and the material inside the appendix can be released into the abdominal cavity. This release can cause an abscess to form around the appendix.

What does peri-appendicitis mean?

Peri-appendicitis means inflammation on the outside surface of the appendix. Peri-appendicitis is a very common finding in appendicitis.

About this article

This article was written by doctors to help you read and understand your pathology report. Contact us if you have any questions about this article or your pathology report. Read this article for a more general introduction to the parts of a typical pathology report.

Other helpful resources

Atlas of Pathology