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Anal Intraepithelial Neoplasia (AIN)

What is Anal Intraepithelial Neoplasia (AIN)?

Anal Intraepithelial Neoplasia (AIN) is a pre cancerous change in the skin in the perianal area caused by exposure to human papilloma virus (HPV). The most common HPV types associated with AIN are HPV 16 and HPV 18.

In women, the cause of AIN is similar to cervical changes (cervical intraepithelial  neoplasia or CIN) detected on cervical smear tests. Women who have had previous abnormal smear tests may be at risk of similar changes in the skin in the perianal area.

Other high risk populations for this condition are:

  • Anyone who has had genital warts at any time in their life
  • Men who have sex with men (MSM)
  • People with immune suppression related to HIV
  • People who have previously had an organ transplant
  • People with Inflammatory Bowel Disease (IBD)
  • Those using drugs such as steroids.

AIN is important because in some cases over time it can develop into anal cancer. When AIN is looked at under the microscope, it progresses through three stages (I, II and II) before becoming cancer. If you have stage III AIN it is more likely to progress to cancer.

What are the symptoms of AIN?

AIN is often asymptomatic but it can be found in warts and other presumed benign skin tags/haemorrhoids and lesions in the anal canal or on the skin around the anus.

Sometimes patients complain of skin changes around the bottom or symptoms of bleeding or itching. If you are at high risk of AIN your doctor will investigate you for this.

How is AIN diagnosed?  

High resolution anoscopy is a procedure where the skin is examined under a microscope. This is the gold diagnostic standard for diagnosis and surveillance of AIN in people at risk of AIN.

AIN may also be identified incidentally in 3-10% of presumed benign perianal skin lesions, which have been removed.

How is AIN treated?

If you are diagnosed with AIN, your doctor may enter you into a surveillance programme to monitor and treat the AIN to ensure it does not progress to anal cancer. Surveillance may also involve taking small biopsies of the area affected at regular intervals.

Up to 25% of AIN cases regress over time but in some people it will persist, probably due to persisting HPV infection.

Approximately 9-10% of people with AIN may progress to anal cancer.

Topical treatments such as Imiquimod, Cidofovir and 5-Fluorouracil and ablative treatments such as electrocautery, laser therapy and radiofrequency ablation can all be used to treat AIN.

People diagnosed with HPV associated AIN may also be considered for vaccination against multiple strains of HPV.