Primary anal fissure
It is not known exactly what causes primary anal fissures to occur, but it is thought that most cases first develop when a particularly hard or large stool damages the anal canal. This type of damage is common and usually heals within a few days.
However, some people may have underlying problems with the ring of muscle that surrounds their anal canal, known as the internal sphincter. These problems can prevent the anal canal from healing in the normal way, leading to the development of acute and (less commonly) chronic anal fissures.
A leading theory is that some people have a sphincter muscle that is larger than normal, which produces muscle tension that is stronger than normal. The increased muscle tension causes increased pressure in the anal canal, making it more susceptible to tearing.
The increased pressure in the anal canal also restricts the blood flow to the anus. Reduced blood flow prevents your body from being able to heal itself as well as it usually can, making it difficult for your fissure to heal naturally. Furthermore, each time that you pass a stool, the injured tissue inside the anal canal is further damaged.
The reduced blood supply can also sometimes cause your sphincter muscle to spasm (contract tightly and painfully) when you are passing a stool, causing pain and further restricting the blood supply.
This may be why some people develop chronic anal fissures. The increased anal pressure leads to a reduction in blood supply, which in turn causes the sphincter muscles to spasm, leading to a further reduction in blood supply. Therefore, the anal fissure never has an opportunity to heal properly.
Constipation
Constipation is the most common cause of a secondary anal fissure. Constipation is a very common condition where you do not pass stools as regularly as normal. It can cause your stools to become dry and hard, which increases the risk of the stool tearing the lining of your anal canal.
Another common problem is that some people try to avoid passing stools due to the pain of an anal fissure. This can trigger the symptoms of constipation, which means that the fissure will not have a chance to heal.
Inflammatory bowel disease (IBD)
Inflammatory bowel disease (IBD) is a term that is used to refer to a number of conditions that cause intestinal inflammation (swelling), such as Crohn’s disease and ulcerative colitis.
Sometimes IBD can cause ulcers to form on the lining of the anus. This can lead to an anal fissure developing.
Sexually transmitted infections (STIs)
If the lining of the anus becomes infected with a sexually transmitted infection (STI), it can cause the tissue to break down and result in an anal fissure developing.
STIs that are known to affect the anus include:
- syphilis – a bacterial infection that causes a wide range of symptoms
- human papilloma virus (HPV) – the virus that causes genital warts
- herpes – a viral infection that can cause blistering around the genitals
- chlamydia – a bacterial infection that causes pain when passing urine
Pregnancy
During the third trimester of pregnancy (week 29 to birth), your risk of developing an anal fissure is increased. You may also develop an anal fissure during childbirth because the continual straining of your muscles can sometimes cause the lining of your anus to tear.
Cancer of the anus
In very rare cases, an anal fissure can be a sign of cancer. Your GP will be able to tell if your fissure is a sign of a more serious illness from your symptoms and the appearance of your fissure. They will refer you for further testing if they think it is necessary. It is important to remember that in the majority of cases, anal fissures are not caused by cancer.
Macmillan Cancer Support’s website has more information about anal cancer.
