Your peripheral nerves link your brain and spinal cord to other parts of your body. These nerves control your muscles so that you can walk, blink, swallow, pick things up and do other activities.

Several types of nerve tumors occur. Though their cause is usually unknown, some are hereditary.

Most tumors aren't cancerous (malignant), but they can lead to nerve damage and loss of muscle control. That's why it's important to see your doctor when you have any unusual lump, pain, tingling or numbness, or muscle weakness.

Types

Peripheral nerve tumors affect nerves by growing within them (intraneural tumors) or by pressing against them (extraneural tumors). Most are benign. Different types of benign peripheral nerve tumors include:

• Schwannoma. The most common benign peripheral nerve tumor in adults, a schwannoma can occur anywhere. It typically comes from a single bundle (fascicle) within the main nerve and displaces the rest of the nerve.

  Some schwannomas enlarge and assume unusual shapes within your spine or pelvis, such as dumbbell tumors. If you develop a schwannoma in an arm or leg, you might notice a mass. When a schwannoma enlarges, more fascicles are at risk, making removal more difficult. In general, these tumors grow slowly.

  Schwannomas most commonly occur alone, though occasionally some people have several of them in the arms, legs or body, a condition known as schwannomatosis.

  If you have a rare schwannoma near your brainstem, known as an acoustic neuroma, you might have trouble with your balance or hearing. Also known as a vestibular schwannoma, this type of tumor usually forms without being associated with a syndrome, but a few acoustic neuromas are diagnosed in people with neurofibromatosis 2 (NF2).

  Untreated acoustic neuromas that continue to grow can affect nearby nerves and compress the brainstem.

• Neurofibroma. This common type of benign nerve tumor tends to form in the center of the nerve. A neurofibroma might arise from several nerve bundles and tends to cause mild symptoms. This tumor most commonly develops in people who have neurofibromatosis 1 (NF1), a genetic disorder that causes tumors to grow on nerves.

  Doctors usually diagnose people who have NF1 by examining discolorations or benign tumors on their skin. Some people who have NF1 develop other conditions, such as curvature of the spine (scoliosis), another bone deformity or an eye nerve tumor (optic glioma). People with NF1 are at risk of developing a malignant peripheral nerve sheath tumor.

• Perineurioma. This rare benign peripheral nerve tumor can also develop as an extraneural tumor. An intraneural perineurioma occurs most commonly in children and young adults and typically causes gradually progressive weakness and sensory loss involving an arm or leg.
• Lipoma. Slow-growing fat cells cause these benign, soft lumps that usually appear under the skin on the neck, shoulders, back or arms. A lipoma near a nerve can compress it. Usually, though, they cause no pain or other problems. Your doctor might want to monitor it through regular checkups.
• Ganglion cyst. Some of these cysts come from an injury, but most have no known cause. They commonly form near joints, such as your wrist, and can cause pain and interfere with everyday activities. Some go away without treatment, but ganglion cysts that compress neighboring nerves should be removed.

The cause of benign peripheral nerve tumors usually isn't known. Some are inherited. The most common type is schwannoma. It typically starts in a single bundle inside the main nerve, displacing the rest of the nerve.

Another common type is neurofibroma. These tumors often form inside the nerve. Sometimes they arise from several nerve bundles.

Perineuriomas are rare and can grow from inside or outside the nerve. Tumors outside a nerve can cause problems when they press on the nerve. Other benign tumors that form outside nerves are soft lumps of slow-growing fat cells called lipomas and ganglion cysts.

Your doctor will run tests to pinpoint where your peripheral nerve tumor is situated and what type of tumor it is.

Tests

You might undergo one or more of the following tests.

• MRI. This is the preferred method for imaging peripheral nerve tumors. This scan uses a magnet and radio waves to produce a detailed 3D view of your nerves and surrounding tissue. It can help determine whether you have a tumor and whether the tumor is inside or outside the nerve.
• CT scan. A CT scanner rotates around your body to record a series of images. Although this test is not as useful as an MRI in diagnosing a peripheral nerve tumor, your doctor might recommend it if you can't have an MRI.
• Electromyogram (EMG). For this test, your doctor places small needles in your muscles so an electromyography instrument can record the electrical activity in your muscle as you try to move it.
• Nerve conduction study. You're likely to have this test along with your EMG. It measures how quickly your nerves carry electrical signals to your muscles.
• Tumor biopsy. If imaging tests identify a nerve tumor, your doctor might remove and analyze a small sample of cells (biopsy) from your tumor. Depending on the tumor's size and location, you might need local or general anesthesia during the biopsy.
• Nerve biopsy. If you have a condition such as a progressive peripheral neuropathy or enlarged nerves that mimic nerve tumors, your doctor may take a nerve biopsy.

Treatment of peripheral nerve tumors involves either surgical removal or observation. If there's a low likelihood that the tumor will become cancerous and if it isn't causing you problems, you might not need surgery.

Your doctor might also recommend observation if your tumor is in a place that makes removal difficult. Observation includes regular checkups and imaging tests to see if your tumor is growing.