A schwannoma is a benign tumor that grows from Schwann cells, the cells that wrap around and insulate your nerves. These tumors are well-contained, meaning they don’t spread into surrounding tissue, and they’re the most common type of nerve sheath tumor, accounting for roughly 89% of cases. Most schwannomas grow slowly over years and many never cause symptoms at all, but when they do, the effects depend entirely on which nerve they’re pressing against.
Where Schwannomas Grow
About 60% of all benign schwannomas are vestibular schwannomas, meaning they develop on the nerve that connects your inner ear to your brain. You might hear these called “acoustic neuromas,” which is an older name for the same tumor. Beyond the vestibular nerve, schwannomas can appear almost anywhere a nerve runs. The upper limbs are the next most common site, followed by the head, trunk, and lower limbs.
Less commonly, schwannomas develop along spinal nerve roots, in the chest cavity, or in the abdominal space behind the organs. They’ve been found in the digestive tract, pancreas, liver, thyroid, and adrenal glands. This wide distribution makes sense when you consider that Schwann cells exist along virtually every peripheral nerve in the body.
What Causes Them
Most schwannomas are linked to a loss of function in a tumor-suppressing protein called merlin, which is produced by the NF2 gene. In healthy cells, merlin acts as a brake on cell growth. It ramps up when cells become crowded, suppressing signals that tell cells to keep dividing. When merlin is absent or defective, that brake disappears. Cells lose their ability to stop growing when they contact neighboring cells, and several growth-promoting pathways switch on simultaneously.
Without merlin, cells also accumulate excess growth factor receptors on their surface, making them overly responsive to normal growth signals in the body. The result is a clonal population of Schwann cells, all descended from one cell that lost its growth controls, slowly forming a contained mass around the nerve.
Most schwannomas occur sporadically, meaning they arise from a random genetic change in a single cell. But people with a condition called neurofibromatosis type 2 (NF2) inherit a faulty copy of the NF2 gene in every cell, which makes them far more likely to develop multiple schwannomas. In NF2, tumors often grow on both vestibular nerves simultaneously, leading to hearing loss in both ears.
Symptoms Depend on Location
Because these tumors grow slowly, symptoms often take years to appear and can be easy to miss early on. What you eventually feel depends on which nerve the tumor sits on and how large it gets.
For vestibular schwannomas, the most common symptoms are gradual hearing loss on one side, ringing in the ear (tinnitus), and problems with balance and coordination. As the tumor grows, it can press on the facial nerve, causing numbness or weakness in the muscles on that side of the face. Very large tumors can compress the brainstem and block the normal flow of cerebrospinal fluid, leading to a dangerous buildup of pressure inside the skull. Rarely, if a vestibular schwannoma grows large enough to affect the lower cranial nerves, it can cause changes in swallowing or voice.
Schwannomas on peripheral nerves in the arms or legs typically present as a slow-growing lump, sometimes with a tingling or shooting pain when the area is touched or pressed. Spinal schwannomas can cause pain, numbness, or weakness in the areas served by the affected nerve root.
How Schwannomas Are Diagnosed
MRI is the primary imaging tool for identifying schwannomas. On an MRI scan, these tumors have a characteristic appearance: they typically show up as bright spots on fluid-sensitive sequences and enhance visibly when contrast dye is injected. A homogeneous, brightly enhancing mass attached to a nerve is a strong indicator. When the tumor has undergone internal changes like cyst formation or minor bleeding, the signal becomes more varied and patchy, but these features are still recognizable to radiologists familiar with nerve sheath tumors.
If the tumor is removed, a pathologist can confirm the diagnosis by looking at the tissue under a microscope. Schwannomas have a distinctive two-zone pattern. Dense, tightly packed areas (called Antoni A regions) alternate with loose, less cellular areas (Antoni B regions). Within the dense zones, cells often line up in a palisading pattern around small fiber-filled spaces known as Verocay bodies. This internal architecture is unique to schwannomas and clearly distinguishes them from neurofibromas, which are another type of nerve sheath tumor with a very different cellular structure and clinical behavior.
How Common Are They
Schwannomas are diagnosed more frequently now than in previous decades, largely because of improvements in imaging technology. A 40-year Danish study tracking vestibular schwannomas found the incidence rose from about 3 cases per million people per year in 1976 to 34 per million per year by 2015. That works out to roughly 3.4 cases per 100,000 people annually. The median age at diagnosis also shifted upward during that period, from about 49 years to 60 years, reflecting the fact that better scans are catching smaller tumors in older adults who might never have been diagnosed in earlier decades.
Treatment Options
Not every schwannoma needs immediate treatment. Small tumors that aren’t causing symptoms are often monitored with periodic MRI scans, an approach sometimes called “watch and wait.” If the tumor stays stable, no intervention may ever be needed.
When treatment is necessary, the two main options are surgery and focused radiation (stereotactic radiosurgery). The choice between them depends on tumor size, location, the patient’s age, hearing status, and overall health.
Surgical Removal
Surgery aims to remove the entire tumor in one procedure. For vestibular schwannomas, total removal is achieved in the vast majority of cases, around 94% in large surgical series. When complete removal is accomplished, recurrence rates are remarkably low, generally under 2% and sometimes well below 1%, depending on the surgical approach used. The main tradeoffs involve risks to the nerves near the tumor. For vestibular schwannomas, preserving hearing and facial nerve function during surgery is a central concern, and outcomes depend heavily on tumor size and the surgeon’s experience.
Stereotactic Radiosurgery
Stereotactic radiosurgery delivers a focused dose of radiation to the tumor in a single session or a small number of sessions. It doesn’t remove the tumor but aims to stop its growth. Five-year tumor control rates typically range from 92% to 96%, and ten-year control rates fall between 84% and 88%. A comparative study of over 900 patients found a recurrence rate of 7% after surgery versus 11% after radiosurgery over an average follow-up of seven years. Radiosurgery is often preferred for smaller tumors or for patients who aren’t good candidates for surgery.
Schwannomas vs. Neurofibromas
Both schwannomas and neurofibromas involve Schwann cells, but they behave differently. Schwannomas are made up entirely of Schwann cells, grow as a contained capsule attached to the outside of a nerve, and can usually be separated from the nerve during surgery. Neurofibromas, by contrast, grow within the nerve itself and contain a mix of cell types, making them harder to remove without damaging the nerve.
Schwannomas are almost always solitary unless you have NF2. Neurofibromas can also be solitary, but multiple neurofibromas are a hallmark of neurofibromatosis type 1 (NF1), a different genetic condition. Neurofibromas also carry a small but real risk of transforming into a malignant tumor over time. Schwannomas, with rare exceptions, remain benign throughout their existence.