What is a congenital melanocytic nevus?

A congenital melanocytic nevus is a type of brown birthmark that looks similar to a mole or large freckle. More than one nevus is called nevi. These birthmarks are made up of pigment (color) cells called melanocytes. Sometimes, the birthmarks are hairy or bumpy. Congenital melanocytic nevi range is size, from small to very large. A very large nevus is called a giant congenital nevus.

Why does my child with a congenital nevus need an MRI scan of their brain and spine?

Some birthmarks put children at higher risk of having abnormalities (unusual or not typical physical features) in the brain and spine. Your doctor will help you decide if your child’s birthmark is high or low risk. Some features to consider are the size and location of the birthmark, and how many birthmarks your child was born with. You can ask your doctor why they recommend imaging or not.

Abnormalities in the brain and spine that can happen with high-risk birthmarks include simple cysts (fluid-filled pockets or sacs) and pigment deposits (spots in the brain or spine that are similar to moles on the skin).

Rarely, there are more serious problems. Very rarely, cancer of the brain or spine can happen. It is important to catch abnormalities early. This is because there are treatments for many of these problems.

What will an MRI be like for my child?

For the magnetic resonance imaging test (MRI), your child needs to lie still in the MRI machine for an hour or more. Laying still helps the MRI takes very clear images of the body. The MRI machine looks like a bed that is attached to a large, round, white camera. The camera will make whirring or knocking sounds while it takes pictures, but it will not touch your child’s body.

It is common for babies or children to move during the scan. To help them stay still, some children may need general anesthesia (medicine to help your child sleep safely and comfortably during a medical procedure) or sedation (medicine to help your child feel more relaxed, but not completely asleep, during a medical procedure). Overall, sedation and general anesthesia are very safe.

Does an MRI use radiation to take pictures?

An MRI scan takes pictures with magnets. There is no radiation from MRI scans, so they are very safe.

What will happen if my child’s MRI shows an abnormality?

Most MRIs in children with congenital nevi are normal. If the MRI shows an abnormality, you will see a pediatric neurologist (doctor who cares for children with problems of the brain and nerves) to better understand the results. Some abnormalities only need to be watched, with regular follow ups with your child’s doctor. Other abnormalities might need more treatment, like surgery. The care team will discuss which treatments, if any, may be best for your child.

Where can I learn more?

Rev. 9/2021. Mass General for Children and Massachusetts General Hospital do not endorse any of the brands listed on this handout. This handout is intended to provide health information so that you can be better informed. It is not a substitute for medical advice and should not be used to treatment of any medical conditions.