Plexiform neurofibromas are tumors that grow along nerves. That’s how they get their name: "neuro" means nerves, and "fibroma" is a type of tumor. These growths have nerve tissue and many different types of cells in them. They can form deep inside the body or right under the skin.

Plexiform neurofibromas are benign tumors, but they can turn into cancer. When these tumors become cancerous, doctors call them malignant peripheral nerve sheath tumors (MPNST).

It helps to know how these tumors grow so that you can tell your or your child’s doctor about any changes you notice.

Where Do They Form?

Plexiform neurofibromas can grow on the inside or outside of the body, including on the:

  • Face, often around the eye
  • Neck
  • Arms and legs
  • Back or chest
  • Belly

Tumors can also form inside the body on organs. About the only places plexiform neurofibromas don't grow are in the brain and spinal cord. Tumors inside the body may only be visible with imaging tests, such as magnetic resonance image (MRI).

As plexiform tumors grow, they make the nerve thicker. Sometimes little clusters of tumors pop up along the same nerve. They can grow on a single nerve or on bundles of nerves and on large nerves or small ones. Plexiform neurofibromas often weave themselves into normal tissues as they grow, which makes them hard to remove with surgery.

Who Gets Them?

Neurofibromas affect many people with neurofibromatosis type 1 (NF1) -- a condition that causes tumors to grow along the nerves, among other features.

A change to a gene causes NF1. About half of the time, a parent passes that faulty gene to their child. The other half of the time, the gene change happens on its own and doesn't run in the family.

There are a few types of neurofibromas. But they don't always cause symptoms or require treatment.

Most of the time children who have plexiform tumors are born with them. But sometimes these tumors don't appear or cause problems for many years.

Why Do They Grow So Large?

Plexiform tumors often form early in life, or they are already there at birth. They start out as a soft lump under the skin. The tumors keep growing as the child gets older, although they usually grow slowly. Fast growth can be a sign that the tumor has turned into cancer.

Over time, plexiform neurofibromas can become so large that they press on and damage the bones, skin, muscles, and organs around them. That damage can cause pain, along with more serious problems, such as hearing loss, high blood pressure, and trouble moving.

An injury can make the tumor grow faster, too. The neurofibroma may suddenly swell up if the injury damages the blood vessels around it.

What Do They Look and Feel Like?

These tumors look like lumps under the skin. The skin over the lump may feel thicker and appear darker than the skin around it. The lump itself may feel like a bundle of thick cords or knots.

Plexiform neurofibromas have a type of cell that releases histamine, a chemical in the body that can cause itching. Histamine is the same chemical that makes you itch when you have an allergic reaction. For that reason, plexiform neurofibromas may itch.

These tumors can sometimes hurt as they grow and press on other tissues and structures inside the body causing significant pain. They may also turn into cancer. It’s important to stay in touch with your doctor when a tumor begins to hurt or changes in other ways.

© 2022 WebMD, LLC. All rights reserved.

Show Sources

Photo Credit: KATERYNA KON / SCIENCE PHOTO LIBRARY / Getty Images

SOURCES:

Boston Children's Hospital: "Neurofibromatosis: Symptoms & Causes."

Children's Tumor Foundation: "Plexiform Neurofibromas."

National Cancer Institute: "Plexiform Neurofibroma."

National Institute of Neurological Disorders and Stroke: "Neurofibromatosis Fact Sheet."

NF Midwest: "Be Informed about neurofibromatosis."

NHS: "Overview: Neurofibromatosis Type 1."

NTAP: "NF1 and Plexiform Neurofibromas."

NYU Langone Health: "Types of Neurofibromatosis."

Washington University Physicians: "Nerve Tumors."