When fissures are found laterally, tuberculosis, occult abscesses, leukemic infiltrates, carcinoma, acquired immunodeficiency syndrome (AIDS) or inflammatory bowel disease should be considered as causes. Some sexually transmitted infections can promote the breakdown of tissue resulting in a fissure. Examples of sexually transmitted infections that may affect the anorectal area are syphilis, herpes, chlamydia and human papilloma virus.Other common causes of anal fissures include:

childbirth trauma in women
anal sex
Crohns disease
ulcerative colitis
poor toileting in young children. Diagnosing
External anal fissures on the anal verge can be diagnosed by visual inspection.Internal anal fissures in adults on anterior side, posterior side or within any part of the inner circumference of the anal sphincter muscle can be diagnosed with beak proctoscope 23mm diameter, Chelsea Eaton anal speculum 23mm diameter, Park anal retractor or by digital rectal examination with a finger inside the anal sphincter muscle. Narrow anal fissures might not be felt by finger during rectal examination due to the glove.Note that colonoscopy, sigmoidoscopy, or normal proctoscopy is for diagnosing internal hemorrhoids and other internal rectal diseases and not for diagnosing anal fissures. Prevention
For adults, the following may help prevent anal fissures:

Avoiding straining when defecating. This includes treating and preventing constipation by eating food rich in dietary fiber, drinking enough water, occasional use of a stool softener, and avoiding constipating agents. Similarly, prompt treatment of diarrhea may reduce anal strain. Careful anal hygiene after defecation, including using soft toilet paper and cleaning with water, plus the use of sanitary wipes.
Lateral internal sphincterotomy
Lateral internal sphincterotomy (LIS) is the surgical procedure of choice for anal fissures due to its simplicity and its high success rate (~95%). In this procedure the internal anal sphincter is partially divided in order to reduce spasming and thus improve the blood supply to the perianal area. This improvement in the blood supply helps to heal the fissure, and the weakening of the sphincter is also believed to reduce the potential for recurrence. The procedure is generally performed as a day surgery after the patient is given general anesthesia. The pain from the sphincterotomy is usually mild and is often less than the pain of the fissure itself. Patients often return to normal activity within one week. LIS does, however, have a number of potential side effects including problems with incision site healing and incontinence to flatus and faeces (some surveys of surgical results suggest incontinence rates of up to 36%).Though lateral internal sphincterotomy (LIS) is considered safe on a short-term basis, there are concerns about its long-term safety. Pankaj Garg et al. published a systematic review and meta-analysis in which they analyzed the long-term continence disturbance two years after the LIS procedure. They found the incidence of long-term continence disturbance to be 14%, so caution and careful patient selection are needed before undergoing LIS. Anal dilation
Anal dilation, or stretching of the anal canal (Lords operation), has fallen out of favour in recent years, primarily due to the unacceptably high incidence of fecal incontinence.