Describe cervical intraepithelial neoplasia — its aetiology, clinical features and management.

Definition

Cervical intraepithelial neoplasia (CIN) is a premalignant lesion of the cervix in which atypical (dysplastic) squamous cells are confined to the epithelium above the basement membrane (no invasion). It is the precursor of invasive squamous-cell carcinoma.

Grading

• CIN I — mild dysplasia; atypical cells in the lower one-third of the epithelium.
• CIN II — moderate dysplasia; lower two-thirds.
• CIN III — severe dysplasia/carcinoma in situ; full thickness involved.

(The Bethesda cytology system groups these as LSIL = CIN I and HSIL = CIN II–III.)

Diagnosis

An abnormal Pap smear leads to colposcopy (acetowhite areas, abnormal vessels) and a directed/punch biopsy for histological confirmation.

Management

• CIN I — often regresses; may be observed with follow-up cytology/colposcopy.
• CIN II–III — treated by excision (LEEP/LLETZ or cold-knife conisation) or ablation (cryotherapy); conisation also provides histology.

Natural History

CIN is caused by persistent high-risk HPV (16, 18) infection. Low-grade lesions (CIN I) frequently regress spontaneously, whereas high-grade lesions (CIN II–III) are more likely to persist and progress to invasive cancer over 10–15 years if untreated — the long latent phase that makes screening effective.

Colposcopy Findings

After acetic acid, CIN shows acetowhite epithelium with punctation, mosaicism and atypical vessels, and is iodine-negative on Schiller's test; the most abnormal area is biopsied.

Prevention and Follow-up

HPV vaccination and regular screening prevent and detect CIN early. After treatment, women remain under cytology/HPV "test-of-cure" follow-up because the lesion can recur or persist.

A Note on WHY Cin Is Graded BY the Depth of the Epithelium

A useful concept is understanding why cervical intraepithelial neoplasia is graded by how much of the epithelial thickness is occupied by abnormal cells — and why this predicts behaviour. Normal squamous epithelium matures as it rises from the basal layer to the surface: the dividing, immature cells lie at the bottom, and as they move up they flatten, differentiate and lose their nuclei. In CIN, this orderly maturation is progressively lost from the bottom upwards. CIN 1 (mild dysplasia) affects the lower third — the epithelium still matures above it, and most such lesions regress spontaneously (they largely represent a productive HPV infection rather than true neoplasia), so they are usually followed rather than treated. CIN 2 involves the lower two-thirds, and CIN 3 (severe dysplasia/carcinoma in situ) involves the full thickness — with no maturation at all; these are true precancers and carry a substantial risk of progression to invasion, so they are treated. Crucially, in all grades the basement membrane remains intact — the disease is 'intraepithelial', which is why it cannot metastasise and why local destruction or excision is curative. Understanding that the grade reflects how far the failure of maturation has ascended explains both the natural history and the management.

CIN 1 usually regresses; CIN 3 needs treatment.

• CIN = dysplastic change confined to the cervical epithelium (basement membrane intact — so it cannot metastasise and is fully curable).
• Graded by the epithelial thickness involved: CIN 1 (lower third — most regress), CIN 2 (two-thirds), CIN 3 (full thickness = carcinoma in situ).
• Caused by persistent high-risk HPV (16, 18); it arises in the transformation zone and is asymptomatic — detected only by screening.
• Diagnosed by colposcopy (acetowhite areas, mosaicism, punctation, atypical vessels) with directed biopsy.
• Treatment: CIN 1 — follow up; CIN 2/3 — ablate (cryotherapy) or excise (LEEP/LLETZ, cone biopsy), which prevents progression to invasive cancer.