Acoustic neuroma (schwannoma)

Definitions & Key takeaways

Acoustic neuroma is a benign tumor that arises from the Schwann cells that surround the eighth cranial (vestibulocochlear) nerve. The symptoms of acoustic neuroma vary depending on the size and location of the tumor, but they can include hearing loss, tinnitus (ringing in the ears), dizziness, and facial weakness. The cause of acoustic neuroma is not fully understood, but it appears to be due to a combination of environmental and genetic factors.

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Introduction0:00–0:01

Schwannoma, sometimes called neurilemmoma, is a tumor that develops from Schwann cells. Schwann cells belong to a category of cells called glial cells, which surround and support the neurons of the nervous system, and get their name from Theodor Schwann, a nineteenth-century physicist who first discovered them.

Introduction0:01–0:22

Physiology0:22–1:46

Neurons are made up of three main parts. The dendrites, which are little branches off of the neuron that receive signals, which is essentially an electrical impulse, from other neurons; the soma, or cell body, which has all of the neuron’s main organelles like the nucleus; and the long axon, which transmits the signal to the next neuron in the series.
In the peripheral nervous system, Schwann cells synthesize a fatty substance made of lipoproteins called myelin, which forms insulating sheaths at along parts of the axon.
The myelin sheath plays an important role in conducting electrical impulses or action potentials. Action potentials propagate along the axon when sodium ions move into the cell through ion channels.
In sections with a myelin sheath, there are no ion channels, but sections in between, called nodes of Ranvier, have a ton of ion channels.
The action potential therefore doesn’t have to move along each section of the neuron, but rather from node to node, resulting in super fast saltatory conduction.
Also, the Schwann cells express a gene called neurofibromin 2, or NF2, which encodes a protein called merlin. In schwann cells, myelin acts as a tumor suppressor, meaning it prevents the Schwann cells from dividing uncontrollably.

Pathology1:46–4:23

And that’s exactly what happens with a schwannoma, the schwann cells start dividing uncontrollably. No one knows what causes most schwannomas from developing.
The majority of schwannomas are solitary tumors of Schwann cells which are found around peripheral nerves. They are usually benign meaning that the cells don’t invade surrounding tissue structures, and schwannomas therefore don’t metastasize to distant locations.
Most often, they arise around cranial nerve 8, the vestibulocochlear nerve, and are known as vestibular schwannomas, or acoustic neuromas, though acoustic neuroma is not that precise a description.
Less frequently, schwannomas can arise in the nerves of the trunk, arms, or legs, compressing the nerves around which the tumor is located.
Histologically, schwannomas have a biphasic appearance, meaning they have alternating regions that show two different cell patterns.
These patterns are called Antoni A and Antoni B. Areas with an Antoni A pattern are hypercellular, meaning they have lots of tightly packed Schwann cells with elongated or spindle-shaped nuclei and little surrounding cytoplasm.
Antoni A areas may have Verocay bodies, which are fibrous cell processes, or projections, between two rows of palisading, or lined-up, Schwann cells.
On the other hand, areas with an Antoni B patterns are hypocellular, meaning they have fewer, more loosely packed Schwann cells that appear scattered.
Schwannomas stain positive for S100 proteins, meaning proteins that are soluble in 100% ammonium sulfate at a neutral pH, and as a result S100 proteins are a marker of Schwann cells.
Schwannomas grow very slowly, and don’t typically spread to other tissues, but once they do grow enough, they can compress nearby nerves, obstruct the passage for cerebrospinal fluid, or even obstruct an artery or vein blood supplying and removing blood from the area.
Now, a small number of schwannomas are related to a disease called neurofibromatosis type 2. In neurofibromatosis type 2 there’s a deletion on chromosome 22, which results in a mutation in NF2.
This NF2 mutation inactivates merlin, and that allows the Schwann cells to divide uncontrollably. As a consequence, those with neurofibromatosis type 2 develop what’s called schwannomatosis.
Unlike schwannomas, which are solitary tumors, in schwannomatosis there are usually several schwannomas in multiple locations.

Symptoms4:23–4:56

The symptoms of schwannoma vary according to tumor location, and relate mainly to nerve compression. For the most part, with vestibular schwannoma, symptoms include hearing loss on one side, tinnitus, meaning a ringing in the ears, problems with balance, and in cases of pressure on the nearby facial nerve, facial weakness and paralysis.
Compression of the spinal cord can lead to weakness, trouble with bladder, bowel control, and paralysis. Other symptoms include pain around the tumor area.

Diagnosis and treatment4:56–5:09

A diagnosis of Schwannoma is usually made via a CT or MRI scan. A biopsy, or microscopic examination of a tumor tissue sample, can confirm the tumor type.
Treatment of schwannoma involves surgical removal of the tumor. All right, as a quick recap, a schwannoma is a benign tumor composed of Schwann cells, a type of glial myelinates the neurons of the peripheral nervous system.

Review5:09–5:26

Schwannoma arises most commonly on the eighth cranial nerve, resulting in difficulty with hearing and balance.