Causes of an anal fistula

• Fistulas in the anus are one of the most common and annoying problems of the digestive system.
• The cause of anal fistula is usually the formation of an abscess in the rectum that does not heal.
• Other causes of anal fistula are related to inflammatory diseases of the digestive tract such as Crohn's disease.

A fistula is understood as a pathway or route through different parts of the body and the skin that connects two spaces. Like any pathway, it has a point of exit and a point of arrival, in this case a primary orifice being the origin of the pathway and a secondary orifice where it ends. 

The anal fistula pathway is formed and surrounded by epithelialising and granulation tissue that forms its structure, gives it solidity and makes it difficult to resolve.

The formation of a fistulous tract or fistula can appear in any location of the human body where the origin of a chronic inflammatory and infectious process can be lodged as the start of an abscess and the beginning of the formation of a fistulous tract or fistula. 

Generally, most of these types of processes form spontaneously in the region around the buttocks and perineum, as the final third of the digestive tract (colon and rectum) and its relationship with the anus are frequent sources of infections that can develop into abscesses and complications of secondary processes such as inflammatory bowel diseases.

When the fistulous tract or fistula is in this region it is known as anorectal fistula.

Generally the cause of anal fistula formation begins with infection and inflammation of a glandular portion or component of the colon, rectum or anus (mainly at the mouth of the anal glands of Hermann and Desfosses, which form the anal crypts which, in turn, form part of the anal canal or anus). 

Cryptoglandular fistula accounts for 90% of all anal fistulas.

When this infectious and inflammatory formation becomes chronic over time, it turns into an abscess. An abscess is the accumulation of cellular detritus and protein and fibrous content resulting from the attack of the infection by our defence system.

The contents of the abscess, when not adequately treated, attempt to continue their expansion by the progressive accumulation of the previously mentioned detritus (mainly following the ducts of the affected gland to the space surrounding the anal sphincter and from there to the fatty spaces and the skin) in such a way that a small trajectory is generated in search of an exit to the exterior or to the interior of one of the intra-abdominal organs. 

This pathway that connects the anal or rectal abscess with the exterior is known as the fistulous pathway or anal fistula.

The exit of the secondary orifice of the anorectal or anal fistula is usually found in some of the patient's buttocks, although it is possible that it may appear in any of the locations near the end of the intestinal system such as the rectal ampulla or in the perineum.

The other causes of an anal or anorectal fistula are medical problems that cause abscesses as part of the development of the disease and, as a consequence, form a fistulous tract or fistula. 

There are a large number of diseases that can cause fistulas secondarily, the most important of which are: inflammatory bowel diseases such as Crohn's disease and ulcerative colitis, diverticulitis or inflammation and infection of diverticula in the colon, rectal cancer, etc.

In each and every one of them, the discourse of the fistulous tract or fistula of the anus has the same characteristics as explained above. In each and every one of them, the origin must be an inflammatory and/or infectious process that degenerates into an abscess. This abscess, over time, will degenerate into a fistulous tract in search of an exit to the outside.

The fundamental difference between primary fistulas and those secondary to other inflammatory processes such as Crohn's disease is the presence of other alterations along the entire digestive tract. Crohn's disease is characterised by the presence of perianal or anal ulcers, anal fistulas and stenosis or narrowing of the rectal ampulla or anal canal due to this generalised inflammation of that specific portion of the large intestine.

For the classification of anal or anorectal fistulas, it is very important to delimit their primary and secondary orifices and their trajectory in order to be able to include them and relate them to the anal sphincters, which can be divided into two portions (internal sphincter and external sphincter). 

The sphincters of the perineum are the voluntary and involuntary muscles that allow the evacuation of faeces, urination and ejaculation in a controlled manner by the human being. 

This relationship between anal fistulas and the sphincters is important for general surgery specialists as they will indicate the most effective type of surgical treatment that will keep the sphincters intact to the greatest extent possible.

Therefore, depending on the trajectories of the anal fistula and its relation to the anal sphincters, anorectal fistulas can be classified into several groups:

• Low transsphincteric anal fistula: These are fistulas whose trajectory passes through the muscular and fatty contents below the external sphincter, so that the function of none of the sphincters is compromised.
• High transsphincteric anal fistula: In this type of fistula, the fistulous path touches part of the external sphincter but less than 50% of its extension.
• Intersphincteric or intramural fistula: The path of these fistulas extends into the intersphincteric space, i.e. between the internal and external sphincter, and may or may not open through a secondary orifice in the rectal ampulla.
• Suprasphincteric fistula: These are fistulas whose trajectory projects above both the lower and upper sphincters.
• Horseshoe fistula: It has a main tract and another that communicates between both sides of the anal canal and the buttocks.
• Double and triple fistulas: In these fistulas, 2 or 3 primary orifices and as many tracts are found in the same patient at the same time.

The only definitive option for the treatment of anal fistulas is surgery. The best option is to choose an experienced surgeon to achieve the highest success rate and the fewest complications. 

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