Low-Grade Squamous Intraepithelial Lesion

The LSIL category in the Bethesda System includes both HPV effects and CIN 1 (i.e. mild dysplasia). Most cytologists consider the cytopathic effects of HPV, including multinucleation, perinuclear halos and nuclear atypia with irregular nuclear outlines and hyperchromasia, to overlap the cytological features of CIN 1. These features are referred to as ‘koilocytosis’, a term derived from the Greek koilos, meaning hollow. The classical studies of Reagan and others identified the key cytological features of CIN 1 (Table 18) (Reagan & Hamomic, 1956).

The cells are of the superficial or intermediate cell type. They are classically described as having nuclei 4–6 times the size of a normal intermediate-cell nucleus (Figure 33).

However, nuclei may vary in size and, in many cases of LSIL that are characterized by marked HPV cytopathic effects, are only twice the size of a normal intermediate-cell nucleus. The nuclei are usually hyperchromatic, and multinucleation is common. The chromatin is finely granular and uniformly distributed. The cells typically occur as individual cells or as sheets of cells with well defined cell borders.