An erosion occurs when the cervical or vaginal epithelium is peeled off. This is often confused with ectropion, because both conditions appear as red patches on the cervix. The importance of recognizing an erosion is that it may mislead the colposcopist in identifying the position of the SCJ, because the margins of an erosion may look like the SCJ.
An erosion appears as a red patch on any part of the cervix (ectropion will always surround the external os) and has a sharp margin, with the peeled-off epithelium sometimes visible at the edge.
The naked stroma is visible on colposcopy at the base of the erosion, with fine vessels (as opposed to the features of columnar epithelium seen in ectropion).
Erosion may be seen in the postmenopausal atrophic cervix. The thin epithelium tends to peel off easily.
Erosion can be an after-effect of inflammation. Inflamed epithelium is soft and oedematous and tends to peel off easily, especially during speculum insertion.
Erosion is also seen in a prolapsed cervix because of mechanical friction. The eroded area may become infected and ulcerated, giving rise to a decubitus ulcer.
In a high-grade precancerous lesion or in cervical cancer, the epithelium may peel off easily because of poor cohesiveness between the neoplastic cells. Erosion is frequently seen in such conditions. The peeled-off epithelium seen hanging loosely is known as the rag sign.
The erosion does not take up iodine, because there is no epithelium to be stained, or it stains a light yellow. The eroded area retains the red colour of the stroma.