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The central nervous system is comprised of the brain and spinal cord. Myriads of nerves run from the spinal cord to all parts of the body in order to gather information from the environment (internal and external) which is then relayed to the brain. One group of nerves is known as the cranial nerves. There are twelve cranial nerves but the one of interest for acoustic neuromas is the eighth (8th) cranial nerve, called the auditory nerve or the vestibulocochlear nerve.

An acoustic neuroma, sometimes called a vestibular schwannoma is a benign tumor that is located on the 8th cranial nerve which leads from the inner ear to the brain. The auditory nerve cells, like many nerve cells in the brain and spinal cord, are wrapped or insulated by layers of specialized cells called Schwann cells. A gene on chromosome 22 directs Schwann cells to produce merlin, a protein which is also called schwannomin. Merlin acts as a tumor suppressor preventing Schwann cells from rapidly dividing in an uncontrolled manner. When there is an abnormality or mutation of the gene, the merlin which is produced is nonfunctional and this enables the Schwann cells to rapidly multiply and form a tumor.

Acoustic neuromas are typically slow growing (approximately 2 mm to 4 mm per year) and can usually develop over a period of years - although growth rates can vary. These rounded tumors typically appear as a single mass and are encapsulated. At least 95% of acoustic neuromas are unilateral (occur on one side only). Unlike many other types of tumors, acoustic neuromas do not metastasize (spread) to other parts of the brain or the body.