A fistula is a tiny channel or tract that develops in the presence of inflammation and infection. The channel usually runs from the rectum to an opening in the skin around the anus. If the fistula does not respond to antibiotics, surgery is usually very effective. An abscess and fistula often occur concurrently. An abscess must be surgically drained to allow healing and relieve discomfort.
An anal fissure is a small tear in the lining of the anus, frequently caused by constipation, inflammation, injury to the anal area during childbirth or severe bouts of diarrhoea. If the fissure is not responding to treatment, it can be corrected with surgery. Over 90% of the patients who require surgical intervention have no further problems. Patients can help avoid the return of fissures by maintaining adequate fiber in their diet.
Lateral Internal Sphincterotomy is an operation performed on the internal anal sphincter muscle for the treatment of chronic anal fissure.
Further Reading:
https://cssanz.org/index.php/patients/anal-abscess-anal-fistula
https://cssanz.org/index.php/patients/anal-fissure
https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/anal-fissure