The anal and cervical canal share embryologic, histologic, and pathologic characteristics. Both develop from the embryonic cloacal membrane, and are sites of fusions of endodermal and ectodermal tissue to form a squamocolumnar epithelial junction. Both areas may display normal metaplastic change and abnormal dysplastic change related to infection with human papillomavirus (HPV) (figure 1). (See "Virology of human papillomavirus infections and the link to cancer".)
The biologic consequences of anal intraepithelial neoplasia (AIN), also termed anal squamous intraepithelial lesions (ASIL) and anal dysplasia, are considered analogous to those of cervical dysplasia. As with cervical intraepithelial neoplasia, AIN may be further subdivided into low-grade AIN and high-grade AIN. Anal high-grade AIN is considered premalignant and may progress to anal cancer, similar to the progression of cervical high-grade cervical intraepithelial neoplasia to cervical cancer . Although low-grade AIN is not considered to be a direct precursor of anal cancer, it may progress to high-grade AIN. (See "Invasive cervical cancer: Epidemiology, risk factors, clinical manifestations, and diagnosis".)
The risk factors, pathology, clinical manifestations, screening, prevention, and treatment of AIN are discussed here. Anal cancer is discussed separately. (See "Clinical features, staging, and treatment of anal cancer".)
A number of risk factors have been identified in the development of AIN; these include anal HPV infection [2,3], receptive anal intercourse , HIV infection , and lower CD4 levels .
HPV infection — HPV infection of the anal canal and perianal region may be latent, subclinical, or clinically apparent as condylomata. Latent infection occurs after acquisition of HPV but before any clinical evidence of infection. This period may last eight months or longer, and some individuals never develop clinically apparent lesions. Subclinical anal infections, such as the presence of high-grade AIN, may be identified by high-resolution anoscopy (HRA), a method of examining the anal canal that uses the application of acetic acid and magnification . Condylomata, when present, are usually clinically obvious and often have a frond-like or plaque-like appearance (picture 1). (See "Condylomata acuminata (anogenital warts)".)