What causes anal fistula?

Anal fistula is a common anorectal disease. In the early stages, anal fistula usually causes itching and pain in the anal area, which has a great impact on people's daily lives. If anal fistula is not treated in time, it can easily lead to pus discharge from the affected area, and it can also cause the condition to become more and more serious, seriously affecting the patient's health. Generally speaking, there are many causes of anal fistula, and the cause needs to be identified before treatment.

How does anal fistula form?

1. Abscess around the anus

It is mainly caused by the feeling of dirt and feces retained in the anal crypts, causing anal gland inflammation and leading to perianal abscesses, which is the main cause of anal fistula formation.

2. Rectal and anal injury

Trauma, slippery foreign bodies from the digestive tract, metal, anal table and anoscope examination can damage the anal canal and rectum. Bacteria invading the wound can cause perianal abscesses.

3. Fistulas often pass between the anal sphincters

Because the sphincter contracts and relaxes constantly, it presses on the fistula tract. Affects the discharge of pus. It is easy to accumulate pus and become infected and difficult to heal.

4. After the anorectal abscess ruptures

The pus is discharged, the abscess cavity gradually shrinks, the external ulcers and wounds also shrink, and the cavity wall forms a hard tubular wall of connective tissue hyperplasia, so it cannot close naturally.

5. Poor anal venous return

Local congestion and tissue malnutrition often occur, affecting healing.

6. Abscesses caused by tuberculosis, actinomycetes, etc.

Diseases such as Crohn's disease are difficult to heal on their own and may form special anal fistulas.

7. Abscess

There are two major types of anorectal abscesses: one is related to anal glands and anal fistulas, called primary acute anal gland intermuscular fistula abscess, or fistula abscess for short, which is more common.

One type has nothing to do with anal glands and anal fistulas, called acute non-anal glandular non-fistulous abscess, abbreviated as non-fistulous abscess, which is less common; anal fistulas mostly develop from the former. Anal fistula is mostly caused by general purulent infection, and a few are caused by specific infections, such as tuberculosis, Crohn's disease, and ulcerative colitis.

Anal fistula can also be caused by secondary infection of rectal and anal canal trauma, and malignant tumors of the rectum and anal canal can also rupture into fistulas, but both are rare and are significantly different from general purulent anal fistulas.

8. Sex hormones

Some people speculate that the influence of sex hormones is the main cause of anal fistula. During puberty, the body's own sex hormones begin to become active, and then some sebaceous glands, especially the anal glands, begin to develop and proliferate.

The anal glands in men are more proliferative than in women. Due to the vigorous secretion of the anal glands, if the anal glands are not excreted smoothly or the anal gland ducts are blocked, they are prone to infection and cause anal gland inflammation. This can explain why the incidence of anal fistula is higher in young and middle-aged men, and why the anal gland ducts in women are straighter and not as curved as those in men.

The secretions are not easy to accumulate, so the incidence of anal fistula in women is lower. When people get old, the anal glands atrophy along with other sebaceous glands, so anal fistula is rare in the elderly.

9. Pathogens

Anal fistula has a primary internal opening, fistula tract, branches and secondary external opening. The internal opening is the entrance of the infection source, which is mostly located in and near the anal sinus, on both sides of the posterior midline, but can also be found in the lower rectum or any part of the anal canal.

Fistulas can be straight or curved, and a few have branches. The external opening is the site of abscess rupture or incision drainage, mostly located in the skin around the anal canal. Because pathogens continuously enter the canal through the internal opening, and the canal is tortuous and runs near the internal and external sphincters, the canal wall is composed of fibrous tissue, and there is granulation tissue inside the canal, it takes a long time to heal.

The lower end of the rectum and the anal canal have some special anatomical structures, such as the rectal columns, anal valves, and crypts, which are often rubbed by feces during defecation, and there is a great chance of injury.

The crypts contain the openings of the anal glands, which are bag-shaped and easily accumulate feces. The mucus secreted by the anal glands is also easily drained.

There are usually a large number of bacteria in the rectum and anal canal. There are many potential anatomical spaces around the lower rectum and anal canal that are filled with fat and loose connective tissue.