A squamous intraepithelial lesion (SIL) is a localised area of disorganised epithelial cell growth on the surface of the skin or mucosa, commonly affecting the cervix, vagina, vulva, anus, penis, or oropharynx. SILs are caused primarily by human papillomavirus (HPV), a sexually transmitted infection. While the lesions themselves are not infectious, HPV can be transmitted through sexual contact. The hallmark of SILs is that the abnormal cells remain confined to the epithelial layer, without invading deeper tissue, which is why they are considered precancerous. Early detection is crucial to prevent progression to invasive cancer.
SILs are classified as low-grade (LSIL) or high-grade (HSIL). LSILs, or mild dysplasia, exhibit only minor cellular abnormalities and may resolve spontaneously, whereas HSILs, comprising moderate to severe dysplasia and carcinoma, display pronounced cellular atypia and generally require prompt treatment to reduce the risk of malignant transformation. SILs are usually asymptomatic and are often identified through routine screenings, such as Pap smears, cervical, vaginal, or anal cytology, or anoscopy.
HPV is responsible for approximately 90% of SILs. High-risk HPV types, particularly 16 and 18, are most often associated with high-grade lesions, while types 6 and 11 commonly lead to low-grade lesions. Other high-risk strains can also contribute.
