AIN 1: nuclear abnormalities affect the lower one-third of the epithelium.
AIN 2: nuclear abnormalities affect the lower two-thirds of the epithelium.
AIN 3: nuclear abnormalities involving the full thickness of the epithelium.
The term anal squamous intraepithelial lesions is also used to describe AIN and is classified into low-grade anal squamous intraepithelial lesions (LSIL) which is equivalent to AIN grade 1 and high-grade anal squamous intraepithelial lesions (HSIL) which is equivalent to AIN grade 2 or 3.
AIN is more common in the immune-suppressed patient and in particular in HIV-positive patients. The prevalence of AIN in HIV-positive men who have sex with men (MSM) is estimated to be around 26–36 %. The prevalence of high-grade AIN in this subgroup is around 5 % [3, 4].
AIN is associated with the human papillomavirus (HPV), which is usually transmitted by genital contact during sexual intercourse . The risk of persistence and progression of the HPV infection to precancerous lesions varies by the HPV type . There are over 100 HPV types identified and about 40 of these can cause anogenital disease in humans . They are graded according to their association with cervical cancer into low-, intermediate- and high-risk subgroups:
Low-risk HPV Types
6, 11, 40, 42, 43, 44, 54, 61, 70, 72
Medium-risk HPV Types
26, 53, 66
High-risk HPV Types
16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 68, 73, 82
HPV types 16 and 18 are most commonly isolated in cervical, anal, vulvar and penile cancer. Among men who have sex with men (MSM) and women (commercial sex workers and intravenous drug users) who are HIV positive, the incidence of HPV infection, multiplicity of HPV types and abnormal anal or cervical cytology and the prevalence of AIN are higher than in HIV-negative men and women . The progression from low- to high-grade AIN is related to HIV status and occurs in more than 50 % of HIV-positive patients within 2 years . The rate of progression to squamous cell carcinoma is probably <1 % per annum . It is thought that HPV infections occur in areas of trauma and result in cellular hyperplasia of the basal layer of the squamous, transitional and cuboidal epithelial cells. Viral genomes have been cloned from infected tissue. HPV E6 and E7gene products inactivate p53 and Rb protein products of tumour suppressor genes. This is postulated to result in inactivation of their tumour suppressor function under appropriate circumstances, such as deficient immune state, thereby allowing malignant transformation and tumorigenesis [10, 11].
Diagnosis of AIN
HPV-associated premalignant conditions are often asymptomatic. The diagnosis of AIN should always be considered in high-risk patients who include all men who have sex with men (MSM) regardless of HIV infection; women with cervical intraepithelial neoplasia (CIN), high-grade vulva disease or cancer; all HIV-positive men and women regardless of sexual orientation; individuals with perianal condylomata; and transplant recipients. Kreuter et al. have classified commonly seen lesions into bowenoid, erythroplakia and verrucous . Examination of the external genitalia should be performed in both sexes and women should be offered colposcopy. As AIN is often multifocal, patients should have mapping biopsies performed. Mapping involves performing four quadrant biopsies of the anal canal, anal margin and perianal skin and also biopsying any suspicious lesions. Scholefield et al. first described the use of high-resolution anoscopy with acetowhitening of the anal mucosa in diagnosing AIN . Subsequently Palefsky described in detail the colposcopic appearances of anal squamous intraepithelial lesions . High-grade AIN is usually smooth and flat with vascular punctuation. It is uncommon for high-grade AIN to have papillae, warty vessels or mosaicism. However, it is advised that any abnormal lesion is biopsied to confirm its histology.
Screening for AIN
A protocol for AIN screening was proposed by Palefsky et al. . The first step involved is taking anal cytology followed by confirmation of the disease stage by high-resolution anoscopy and biopsy. HRA testing has a high predictive value for normal mucosa and for AIN 3 but not for AIN 1 or 2 . Patients with normal cytology or normal HRA are rescreened in 1 year if they are HIV positive or 2–3 years if HIV negative. Patients with AIN 1 are screened every 6 months. AIN 2 and 3 will require treatment.
Treatment of AIN
The rationale for treating AIN is to prevent the progression into high-grade and invasive cancer. Sceptics argue that involvement of AIN 2 or 3 of deeper areas of the perianal skin and its appendages in up to 50 % of cases and sweat glands in 25 % of biopsy specimens may not be accessible to anal colposcopy, and ablative techniques such as cryotherapy, laser vaporisation and electrocautery are inadequate forms of treatment . HPV is epitheliotropic, the entire genital area is likely to be involved, and eliminating the virus may not be possible. AIN 1 and 2 could be treated expectantly since their natural course is unclear and occasionally regress spontaneously; furthermore, they are frequently associated with HPV 6 and 11 which is less oncogenic than HPV 16 .
Small perianal lesions are treated by topical agents that include podophyllotoxin, Imiquimod, Cidofovir, 80 % trichloroacetic acid and liquid nitrogen. Small intra-anal lesions can be treated with 80 % trichloroacetic acid. Medium and larger lesions are treated by surgical excision, electrocautery, laser ablation, photodynamic therapy or infrared coagulation. The advantage of surgical excision is that the tissue can be sent for histological analysis. Regardless of the mode of treatment, local recurrence is common. Chang et al. reported local recurrence in two-thirds of patients treated with HRA-directed surgical excision of intra-anal lesions . Goldstone et al. reported a similar recurrence rate for treating AIN using infrared coagulation under local anaesthesia . Ablative techniques must ensure a depth of penetration 2.2 mm below the basement membrane to adequately treat the disease.
Imiquimod cream as a treatment option has the advantage of being applied by the patient and also it can be used for extensive disease. It is an immune stimulant with minimal side effects. Systemic absorption can cause transient flulike symptoms the day after application. Imiquimod cream has been shown to be effective in treating vulva intraepithelial neoplasia. Van Seters et al. demonstrated a 35 % clearance rate of VIN 2/3 after a 12-month treatment period and an 81 % improvement in symptoms . Imiquimod cream is not licensed for use in or around the anus; however, initial studies have shown considerable promise. In an uncontrolled study by Wieland et al., 77 % of lesions resolved after 16 weeks of treatment and were associated with a reduction of HPV viral load. However, approximately half of the patients in the study had low-grade AIN . More recently Fox et al., in a double-blind randomised placebo-controlled trial, demonstrated that Imiquimod cream has a role in treating high-grade AIN. It can be used as an adjuvant to or alternating with other treatment for high-grade multifocal AIN. High-grade AIN resolved in 4 out of 28 patients and was downgraded in another 8 patients .
A 1 % gel used topically for 5 days a week for up to 6 weeks has been shown to be more effective than electrosurgery in the treatment of anogenital warts in HIV-positive patients 
Treating lesions in the anal canal with electrocautery has been disappointing with a high recurrence rate (79 % in 1 year and 100 % in 50 months) .
This procedure is better tolerated than electrocautery. Goldstone pioneered this technique and showed an initial response rate of 72 % per treatment cycle . Infrared coagulation can be performed in the outpatient setting with administration of local anaesthetic. It provides good haemostasis and no smoke plume. It uses an infrared beam of light pulsed at 0.5–3.0 s intervals and prevents damage to the deeper tissues. The depth of penetration in millimetres is approximately equal to the length of pulse applied in seconds. For example, a 1 s pulse penetrates approximately 1 mm of tissue. Typically a lesion is treated by touching it with IRC and firing for 1.5 s and repeating as necessary to remove the lesion. Patients can expect 1–2 weeks of pain and bleeding with bowel movements.
Photodiagnosis and Photodynamic Therapy for AIN
There are a few studies showing the feasibility of photodynamic therapy in treating AIN. 5-Aminolevulinc acid (ALA), when added to many tissues, results in the accumulation of sufficient quantities, especially in tumour and dysplastic cells, of the endogenous photosensitizer protoporphyrin IX, which exhibits red fluorescence with blue light and produces a photodynamic effect when exposed to activating light by generating cytotoxic singlet oxygen. Topical application of ALA followed by exposure to activating light has been used as a treatment for AIN.
Human Papillomavirus Vaccination
HPV vaccination is safe and well tolerated. There are two vaccines available:
Gardasil—a quadrivalent vaccine which protects against HPV 6, 11, 16 and 18
Cervarix—a bivalent vaccine which protects against HPV types 16 and 18
However, these vaccines do not exert therapeutic effects on existing lesions and are unlikely to have an immediate impact on the prevalence of anogenital cancer and its precursor lesions. It is suggested that therapeutic HPV vaccines could result in the elimination of pre-existing lesions and infections by generating cellular immunity against HPV-infected cells. These vaccines could play a major role in controlling HPV-associated premalignant lesions in the future .
Bowen’s Disease of the Anus
Bowen’s disease was first described by John T. Bowen in 1912 . It is an intraepithelial squamous cell carcinoma of the skin. While it is more common on the trunk, it can also occur in the perianal region and perineum where it is associated with the human papillomavirus types 16 and 18 [27–30]. Bowen’s disease is more likely to present in female patients than males and it commonly presents in the fifth decade [30, 31]. In comparison with extramammary Paget’s disease, the incidence of internal malignancy in Bowen’s disease is low (5 vs. 52–73 %), and the incidence of invasive carcinoma at the site of the lesion is also low (2–6 vs. 50 %). Bowen’s disease is often asymptomatic and found coincidentally in haemorrhoidectomy specimens (25–40 %). Often there is a delay in diagnosis of months to years due to its slow-growing nature and the paucity of symptoms [31, 32]. Patients who do develop symptoms tend to complain of pruritus, perianal bleeding and anal discharge. On clinical examination, there may be a dry scaly raised lesion with an irregular margin and brownish-red eczematoid appearance [27, 30, 33]. Microscopically Bowen’s disease is characterised by hyperchromatic enlarged nuclei with easily identifiable mitotic figures, parakeratosis and hyperkeratosis in the superficial surface layers, loss of cellular polarity, marked epidermal hyperplasia with elongation and thickening of the rete ridges and randomly distributed dyskeratotic cells with densely prematurely enlarged eosinophilic cytoplasm . Large haloed hyperchromatic nuclei are present and stain negative with periodic acid-Schiff stain for glycogen [27, 31]. This is in contrast to Paget’s disease which stains for carcinoembryonic antigen and usually mucin [30, 33]. It is thought that the natural history of Bowen’s disease is relatively benign with an estimated 2–6 % conversion to invasive squamous cell carcinoma [28, 31, 34]. The treatment options available for Bowen’s disease are similar to those for AIN and include wide local excision with primary closure [35, 36], wide local excision with split-thickness skin graft [37, 38], wide local excision with advancement cutaneous anoplasty [30, 39, 40], local excision of macroscopic lesions alone [35, 36], cautery fulguration , CO2 laser ablation [31, 41], cryotherapy [10, 42, 43], topical 5-fluorouracil cream , photodynamic therapy , argon beam laser therapy [41, 45] and observation alone .
Paget’s Disease of the Anus
In 1874 Sir James Paget described 15 patients in whom the nipple and the surrounding skin of the breast was diseased, and within 2 years all patients developed an ipsilateral breast carcinoma. Extramammary Paget’s disease (EMPD) was first described by Crocker in 1889 , and in 1893 Darier and Coulillaud described EMPD of the anogenital area.