Congenital nevus

by Robyn Ndikumana MD BScN and Allison Osmond, MD FRCPC
April 28, 2022


What is a congenital nevus?

A congenital nevus is a common non-cancerous tumour made up of melanocytes. A congenital nevus looks dark because it is made up of melanocytes that are producing a lot of melanin. Congenital nevi (nevi is the plural of nevus) are more common in people with light coloured skin. They can be found anywhere on the body, but the most common locations are the trunk and limbs.

Melanocytes

Melanocytes are specialized cells that are normally found at the very bottom of the epidermis. They are responsible for producing a dark pigment called melanin that helps protect our skin from the sun’s ultraviolet light. The amount of melanin in a person’s skin determines their skin colour – people with light skin produce little melanin and people with darker skin produce more melanin.

Melanocyte

What does a congenital nevus look like without a microscopic?

Congenital nevi range in size from very small to very large. A congenital nevus is called a giant congenital nevus when it is larger than 20 cm. Many congenital nevi have an irregular shape and they can range in colour from light brown to black. It is not uncommon to find a hair growing out of a congenital nevus.

How does a pathologist make the diagnosis of congenital nevus?

The diagnosis of congenital nevus is usually made after the tumour has been surgically removed and sent to a pathologist for examination. When examined under the microscope, the tumour is made up of melanocytes. Dark melanin pigment is often seen throughout the tumour.

The tumour cells grow in the layer of skin just below the surface called the dermis. As they grow, they can wrap around different normal structures like hair follicles, sebaceous glands, nerves, and blood vessels. Sometimes they can grow into the fat (subcutaneous tissue) below the dermis.

Congenital nevus

What are the ABCDEs?

In general, when your doctor is looking at a nevus on your skin, they are looking for some basic features known as ABCDE.

  • A stands for asymmetry which means one side of the nevus looks different from the other.
  • B stands for border. A nevus should have a smooth border.
  • C stands for colour. You should talk to your doctor if your nevus changes colour or if it starts to show multiple colours.
  • D stands for diameter. This is the size of the nevus measured from side to side. You should talk to your doctor if your nevus suddenly starts to grow.
  • E stands for evolving or ‘everything else’. This can include sudden changes in size or shape or a nevus that has started to bleed.

A nevus may be removed by your doctor if there are any changes in ABCDE, or for cosmetic reasons.  Some nevi will grow back after being removed and this is known as a recurrent or persistent nevus.  If you see any colour in a scar after nevus is removed, you should see your family doctor.

While a congenital nevus is a non-cancerous skin tumour, it can change over time into a type of skin cancer called melanoma. You should let your doctor know if your nevus suddenly changes in size or colour or if it starts to bleed.

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