Acute appendicitis

What is acute appendicitis?

Acute appendicitis is a non-cancerous medical condition caused by acute inflammation in the appendix. It can occur in any age group, but most commonly affects adolescents and young adults.

The inflammation seen in acute 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.

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.

Visit the Ottawa Pathology Atlas to see microscopic images of acute appendicitis.

The appendix

The appendix is 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.

The appendix and the large bowel have a similar structure. Both the appendix and the large bowel are lined by cells called goblet cells. These cells produce mucin which helps food move smoothly through your large bowel. The appendix also contains cells that are a part of your immune system and these cells help to form a protective barrier between your body and the inside of the bowel.

What causes acute appendicitis?

In acute 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 acute 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 do pathologists make this diagnosis?

If your doctor suspects acute 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. Acute appendicitis is most commonly treated by removing the appendix. Your appendix will then be sent for examination by a pathologist.

The diagnosis is made after the appendix has been surgically removed and sent to a pathologist for examination under a microscope. The procedure used to remove the appendix is called an appendectomy. 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 pus. Neutrophils may also be seen on the outer surface of the appendix. Pathologists describe this as peri-appendicitis.


A perforation is a hole that opens up in your appendix as a result of severe inflammation and a type of tissue death called necrosis. Perforation can lead to the development of an abscess near the appendix.


A margin is any tissue that must be cut by a surgeon so that the injured tissue can be removed from your body. The appendix is connected to the large bowel, and generally has two areas that must be cut to remove it from the body. The end that attaches to the large bowel and is in direct communication with it is called the proximal margin.

The appendix has an area of fat that contains blood vessels and occasionally lymph nodes. This area must be cut to free the appendix and it is called the mesoappendix margin. In the setting of acute appendicitis, the inflammation can also extend into this area of surrounding fat, which pathologists describe as mesoappendix involvement.


Dysplasia is an abnormal pattern of growth that can turn into cancer over time. Your pathologist will carefully examine your tissue for any sign of dysplasia. However, it is very rare to find dysplasia in an appendix removed for acute appendicitis.

by Madeline Fitzpatrick MD and Stephanie Reid, MD FRCPC, updated March 7, 2021
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