An anal fissure is a small crack or tear of the skin around the back passage. The tear is usually very small but can be extremely painful. A fissure may also result in small amounts of bright red bleeding from your back passage. The cause of an anal fissure is usually from being constipated resulting in straining to pass a bowel motion. Usually an anal fissure will heal within 1‐2 weeks without any treatment although it can take longer. Your doctor may suggest an E.U.A. of your rectum (back passage) to assess and treat the fissure.

A EUA is an examination under anaesthetic. You may need a general anaesthetic to allow your doctor examine your back passage as it may be too uncomfortable to do so in the outpatient clinic without sedation or a general anaesthetic. 

The aim of treating the fissure is to relax the muscle around the back passage to allow the tear to heal.

This can be achieved by

• Giving an injection of Botox (botulinum toxin) into the anal sphincter muscle.
• Making a small cut in the muscle around the anus (internal sphincterotomy). This permanently reduces the tone around the back passage to allow the fissure to heal. This procedure is only recommended for certain patients.

In the case of Botox this is transient and wears off after 3 months.  If a sphincterotomy is performed the problem may be more permanent.

Lateral internal sphincterotomy is an operation performed on the internal anal sphincter muscle for the treatment of chronic anal fissure. The internal anal sphincter is one of two muscles that comprise the anal sphincter which controls the passage of feces. The procedure helps by lowering the resting pressure of the internal anal sphincter, which improves blood supply to the fissure and allows faster healing.

https://en.wikipedia.org/wiki/Lateral_internal_sphincterotomy