AssessmentsAcoustic neuroma (schwannoma)
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 35-year-old woman comes to the clinic because of moderate hearing loss that she first noticed several months ago. She also complains of a ringing noise in her right ear that is very disturbing. Physical examination shows right-sided sensorineural hearing loss, an asymmetric smile, and decreased corneal reflex in her right eye. This patient’s condition suggests an intracranial mass located in which of the following locations?
Content Reviewers:Rishi Desai, MD, MPH
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.
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, merlin acts as a tumor suppressor, meaning it prevents the Schwann cells from dividing uncontrollably.
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.