Where does a normal anal fissure usually appear?
Picture the anal opening as a clock face. Whatever position the patient is lying in, surgeons describe two lines: the back midline (towards the tailbone) and the front midline (towards the genitals). The large majority of ordinary fissures sit in the back midline. A smaller share sit in the front midline, and those are seen more often in women, especially after childbirth; we cover them in Anterior anal fissure in women: why it happens and what changes in treatment.
Why the midline? The anoderm at the back of the canal has the poorest blood supply and takes the most stretch when a hard stool passes. When the internal sphincter is tight, the blood flow there drops further. That combination is what makes a "primary" fissure, the one caused by a hard or explosive stool in an otherwise healthy person. If you need the basics first, start with What is an anal fissure? A surgeon explains it with a drawing of the anal canal.
A fissure that sits at 3 or 9 o’clock (on the sides), or several tears at once, does not follow that logic. Something other than simple stretching is usually going on, and that is why location is one of the first things a colorectal surgeon checks.
What makes a fissure "atypical"?
Location is the most visible clue, but it is not the only one. Surgeons call a fissure atypical, or "secondary", when one or more of these features appear:
- Off the midline: on the left or right side of the canal.
- More than one fissure at the same time.
- Little or no pain, which is unusual: a classic fissure hurts sharply with every bowel movement and for minutes or hours after.
- Wide, deep or with undermined edges, closer to an ulcer than to a thin tear.
- Large, swollen, purplish skin tags around it, different from the small sentinel pile of a chronic fissure (see Sentinel pile: what is the lump next to a chronic anal fissure?).
- No healing after 8 weeks of correct treatment with a sphincter-relaxing ointment and soft stools.
- Other symptoms: diarrhea, weight loss, fever, mouth ulcers, a rash, discharge, swollen glands in the groin.
None of these features proves a specific disease. They simply mean the "hard stool plus tight sphincter" explanation is not enough, and that the next step is to find the cause.
Which diseases can cause a lateral or multiple fissure?
This is the list a colorectal surgeon goes through when a fissure does not look typical. Most patients turn out to have one of the first three; cancer is uncommon but must not be missed.
| Cause | What raises the suspicion | How it is confirmed |
|---|---|---|
| Crohn’s disease | Wide, painless or mildly painful fissures, often multiple; swollen skin tags; diarrhea, abdominal pain, weight loss | Biopsy, colonoscopy, blood and stool tests, sometimes pelvic MRI |
| HIV | Deep, broad ulcer, often higher in the canal and very painful; may coexist with other infections | HIV blood test; biopsy and swabs of the ulcer |
| Syphilis | Ulcer off the midline, sometimes two facing each other; swollen glands in the groin; often after receptive anal sex | Blood tests for syphilis; swab of the lesion |
| Herpes and other sexually transmitted infections | Several small, very painful sores; burning; discharge or proctitis (chlamydia, gonorrhea) | Swab with PCR testing |
| Tuberculosis | Chronic ulcer with irregular, undermined edges; past or present lung tuberculosis; night sweats | Biopsy with culture and specific stains |
| Anal cancer and other tumors | Hard, irregular edges; a lump; bleeding that keeps coming back; no healing | Biopsy |
Crohn’s disease deserves a special mention because it is not rare in this setting: a 2025 review estimates that around 45 % of people with Crohn’s develop non-fistulizing perianal lesions such as fissures, ulcers, strictures or skin tags (Abreu Marques, J Clin Med 2025). Other, rarer causes include blood diseases such as leukemia, Behçet’s disease, trauma, and previous anal surgery or radiotherapy. The ASCRS 2023 and ACPGBI 2023 guidelines both state the same principle: a fissure that is atypical in position or appearance needs further evaluation before routine treatment.
Why would a doctor test for HIV or syphilis because of a fissure?
Because both can produce an anal ulcer that looks very much like a fissure, and both are treatable. A syphilis sore in the anal canal can be mistaken for a fissure for weeks; it heals with the right antibiotic, not with an ointment. An HIV-related ulcer needs the person to be on effective antiretroviral treatment, and sometimes needs testing for other infections such as herpes or cytomegalovirus.
Asking about sexual practices, including receptive anal sex, is part of a normal medical history in proctology. It is asked of everyone, it is confidential, and the answer changes which tests are useful. You do not need to have had anal sex to get syphilis or herpes in this area, and having had it does not mean you have an infection. The point is simply not to miss a diagnosis with a clear treatment.
If you already live with HIV and have a well-controlled viral load, an ordinary midline fissure is still the most likely explanation for a painful tear. The difference is that your doctor will have a lower threshold for biopsy and swabs if the fissure is atypical or slow to heal. For questions about sex while a fissure heals, see All guides.
What tests should you expect for an atypical fissure?
The work-up depends on what the surgeon sees and hears, but a typical sequence looks like this:
- Careful inspection and history: location, number, depth, pain, bowel habits, weight, other symptoms, sexual history, travel and tuberculosis exposure.
- Blood tests: HIV and syphilis serology, blood count, inflammation markers; others according to suspicion.
- Swabs: PCR for herpes, chlamydia and gonorrhea when there are sores or discharge.
- Examination under anesthesia with biopsy: if the fissure is very painful, deep or does not heal. A short anesthetic allows a full look at the canal and samples from the edge of the fissure, without pain.
- Colonoscopy: if Crohn’s disease is suspected, or if there is bleeding mixed with stool, a change in bowel habit or you are over 45 and have never had one.
- Pelvic MRI: if an abscess or fistula is suspected, which is more common in Crohn’s (see Fissure, fistula or abscess? How to tell them apart).
A biopsy is a small sample, usually a few millimeters. It does not damage the sphincter and is the only way to rule out cancer with certainty.
Is the treatment different for a lateral fissure?
Yes, in two ways. First, the treatment is aimed at the cause: antibiotics for syphilis, antivirals for herpes, antiretroviral therapy for HIV, anti-tuberculosis drugs, or medical treatment of Crohn’s disease led by a gastroenterologist. Often the fissure improves once the underlying disease is controlled. We explain the Crohn’s side in detail in Anal fissure and inflammatory bowel disease: how is a Crohn’s fissure different?.
Second, the surgical steps for an ordinary chronic fissure are not applied blindly. A lateral internal sphincterotomy should not be done on an atypical fissure until the diagnosis is clear. Cutting the sphincter in someone with active perianal Crohn’s disease, or with an ulcer caused by an infection, can leave a wound that does not heal and a weaker sphincter. Meanwhile, soft stools, sitz baths and local pain relief remain useful in almost every case.
Sphincter-relaxing ointments may still have a place once a specialist has examined you and the cause is known, but they are not a substitute for that examination.
When should you see a doctor without delay?
Any fissure that does not fit the typical picture deserves a consultation with a colorectal surgeon or a gastroenterologist. Do not wait if you notice:
- A fissure off the midline, several fissures at once, or a fissure that hardly hurts.
- Bleeding after the age of 45, or blood mixed with the stool.
- Fever, pus, or a painful swelling next to the anus.
- A lump that grows, or an ulcer with hard, irregular edges.
- Unintended weight loss, persistent diarrhea, night sweats, or swollen glands in the groin.
Our page on anal fissure symptoms explains how a typical fissure behaves, and All guides describes what happens at the appointment.
Where does FISURIN fit?
FISURIN is a cream for the typical fissure: nifedipine (a sphincter relaxant) plus lidocaine in a water-based hydrogel, prescribed by the surgeons of Clínica Proctológica de Lima with a written 6-week protocol. It is not the first step for a lateral or multiple fissure. In that case our surgeons examine first, request the tests needed and treat the cause; a sphincter relaxant is considered only once the diagnosis is clear. FISURIN is a magistral formula prepared under prescription in Peru and is not shipped abroad; FISURIN Hydrogel and FISURIN Relief are in preparation, with a waiting list. See what is inside FISURIN or how to get it where you live.
Frequently asked questions
Is a lateral anal fissure dangerous?
Not necessarily, but it is unusual. Most fissures sit in the back or front midline. A lateral fissure is more often linked to Crohn’s disease, an infection or, rarely, a tumor, so it should be examined and, if needed, biopsied.
Can a normal fissure be on the side?
Occasionally, yes. A primary fissure can sit slightly off the midline. The surgeon decides whether it looks typical; if there is any doubt, or it does not heal in 8 weeks, further tests are done.
Do multiple anal fissures mean Crohn’s disease?
Not always, but Crohn’s is one of the first diagnoses to consider, together with infections such as herpes and syphilis. Diarrhea, abdominal pain or weight loss make Crohn’s more likely.
Should I get an HIV test if I have a fissure?
For a typical midline fissure, the test is not needed because of the fissure itself. For an atypical, deep or non-healing ulcer, HIV and syphilis tests are a routine part of the work-up.
Does a biopsy of a fissure hurt?
It is usually done under local or general anesthesia, often during an examination under anesthesia. The sample is small and does not damage the sphincter.