A bathing trunk nevus is a darkly pigmented, disfiguring, often hairy patch of skin (nevus) present at birth, which may cover an extremely large area of the body. Sometimes the nevus covers most of the trunk, the upper arms, and thighs.

Bathing trunk nevi are thought to be caused by spontaneous mutations or other events during fetal development, but in some families, the frequent appearance of these lesions suggests that they may be genetically inherited. They may be associated with other birth defects.

The surface texture may vary from smooth to warty, and the color varies from brown to bluish black. Bathing trunk nevi may cause emotional problems because of their appearance.

They can also be dangerous. The nevus has a high risk of developing into melanoma (a type of skin cancer). The risk is thought to be between 3% and 7%.

The hallmark of this condition is a darkly pigmented lesion covering an extensive area of the trunk or extremities. The nevus may contain hair, and its surface texture may vary from smooth to warty and the color from brownish to bluish black.

Smaller lesions may exist near the margins of the primary lesion

All birthmarks should be evaluated by your health care provider. A biopsy of suspicious areas may be obtained for examination to determine if the cells have become cancerous. An MRI of the brain might be performed.

Treatment consists of surgery to remove of the nevus, with skin grafting where necessary. Extremely large nevi may be removed in several stages.

Psychological treatment may be needed to deal with the emotional impact of having a disfiguring disorder.

Except when the nevus transforms into malignant melanoma, bathing trunk nevi are compatible with a normal life span.

On rare occasion, bathing trunk nevi are associated with a condition called leptomeningeal melanocytosis (growth of pigment-producing cells in the head), which can produce hydrocephalus, motor abnormalities, and seizures.

Bathing trunk nevi can become melanoma. They can also be associated with spina bifida, meningocele, other nevi, lipomas, and neurofibromatosis.

Depression and other emotional problems may occur due to social difficulties related to bathing trunk nevi.

This condition is usually diagnosed at birth. Call for an appointment with your health care provider (or mention it during a well-baby exam) if your child has a large pigmented area anywhere on the skin.