Neurosurgeons are experts in not only in the central nervous system (the brain and spine) but also the peripheral nervous system, which branches all over the body. These nerves conduct movement signals from the brain and spinal cord to the arms and legs, and they relay sensation from the rest of the body back to the brain via the spinal cord. Some of these tumors appear sporadically and as single lesions, while others emerge as multiple tumors.
Peripheral nerves are bundles of fibers wrapped in layers of a fatty, insulating material called myelin and connective tissue. Together these layers are known as the nerve sheath, and they can develop tumors. There are several types of peripheral nerve sheath tumors, but most are either schwannomas or neurofibromas. These tumors are usually benign and grow slowly. In some cases, however, they can cause debilitating pain, sensory issues, and weakness or paralysis of a particular muscle group if left untreated. (See Symptoms of Peripheral Nerve Sheath Tumors.) Rarely, these can be a more aggressive subtype known as a malignant peripheral nerve sheath tumor (MPNST).
Types of Peripheral Nerve Sheath Tumors
- Schwannoma: A schwannoma is a benign tumor that usually forms from the Schwann cells of the nerve sheath. It can affect cranial nerves, peripheral nerves, and nerves within the spinal canal (intradural tumors). It can be associated with syndromes such as Neurofibromatosis Type II and Schwannomatosis (also referred to as Neurofibromatosis Type III).
- Neurofibroma: A neurofibroma is another benign tumor. It is similar to schwannoma, but it arises not only from Schwann cells, but also from axons, perineurial cells, and fibroblasts (all parts of the peripheral nervous system). These tumors are a defining characteristic of Neurofibromatosis Type I (NF1). When only one peripheral nerve is involved, these tumors can appear as bumps/growths on the skin and can number from a few to hundreds. In other situations, however, they can occur along multiple bundles of nerves and form a plexiform neurofibroma. These can be large, painful, and difficult to treat, and also carry a higher risk of becoming malignant.
- Perineuriomas: A rare peripheral nerve sheath tumor, these benign tumors arise from the perineurium, which forms the protective covering of nerve fascicles (bundles of neurons). There are even rare extraneural variants of this tumor, which do not occur in the nerve sheath, but rather in the soft tissue and skin.
- Malignant Peripheral Nerve Sheath Tumors (MPNST): MPNSTs are a rare subtype of sarcomas that arise from the cells surrounding nerves. These are aggressive, cancerous growths. These most commonly are observed in individuals with NF1, but they can also form in patients with no history of the disease. These tumors can grow rapidly. Treatment may consist of a combination of surgery, chemotherapy, and radiation therapy. Even with optimal and timely therapy, these tumors can metastasize to other parts of the body and have a 5-year survival rate of 23-69%.
Non-neoplastic Pathologies that Mimic Peripheral Nerve Sheath Tumors
- Neuroma: A neuroma is a non-cancerous overgrowth of the normal cells surrounding nerves. These typically occur in the ball of the foot between the third and fourth toes (called Morton’s neuroma) and cause pain in the ball of the foot and toes.
- Ganglion cysts: Ganglion cysts arefluid-filled benign lumps that commonly appear on the back or front of the wrist over tendons or joints.
- Baker’s cyst (also called popliteal cyst): These arefluid-filled benign lumps that commonly appear on the back surface of the knee. Is usually associated with history of trauma to the knee, osteoarthritis, or gout.
- Lipomas (fatty tumors): Lipomas are very common benign tumors arising from fat cells that form outside the nerves in the soft tissue, and show up as soft, movable lumps just under the skin. They can occur anywhere in the body where there’s fat. In rare situations, these can form in the context of a genetic syndrome known as lipomatosis, where multiple lipomas form through the body.
What Causes a Peripheral Nerve Sheath Tumor?
Individuals may be predisposed to developing peripheral nerve sheath tumors if they have a family history of them. Even in those without a family history, these tumors can form due to certain genetic mutations that result in an overactivation of nerve sheath cells. Those who have had radiation therapy to treat cancer may be at risk for MPNSTs between 10 and 20 years after treatment.