They appear most frequently at the inferior temporal quadrant of the corneal limbus. However, they occasionally may present entirely within the cornea or may be confined to the conjunctiva. They may contain a variety of histologically aberrant tissues, including epidermal appendages, connective tissue, skin, fat, sweat gland, lacrimal gland, muscle, teeth, cartilage, bone, vascular structures, and neurologic tissue including the brain.
Malignant degeneration is extremely rare.
The most common system for classifying dermoids is based on the location of the lesion and separates the lesions into 3 broad categories. The most common dermoid is the limbal dermoid, in which the tumor straddles the limbus. These are usually superficial lesions but may involve deeper ocular structures. The second type involves only the superficial cornea, sparing the limbus, the Descemet membrane, and the endothelium. The third type of dermoid involves the entire anterior segment, replacing the cornea with a dermolipoma that may involve the iris, the ciliary body, and the lens.