A vestibular schwannoma (VS), also called acoustic neuroma, is a benign tumor that develops on the vestibulocochlear nerve that passes from the inner ear to the brain. The tumor originates when Schwann cells that form the insulating myelin sheath on the nerve malfunction. Normally, Schwann cells function beneficially to protect the nerves which transmit balance and sound information to the brain. However, sometimes a mutation in the tumor suppressor gene, NF2, located on chromosome 22, results in abnormal production of the cell protein named Merlin, and Schwann cells multiply to form a tumor. The tumor originates mostly on the vestibular division of the nerve rather than the cochlear division, but hearing as well as balance will be affected  as the tumor enlarges. The great majority of these VSs (95%) are unilateral, in one ear only. They are called "sporadic" (i.e., by-chance, non-hereditary). Although non-cancerous, they can do harm or even become life-threatening if they grow to press on other cranial nerves and vital structures such as the brainstem. Variations in the mutation determine the nature of the tumors development. The only environmental exposure that has been definitely associated with the growth of a VS is therapeutic radiation exposure to the head. Symptoms of Sporadic VS
Sporadic VSs originate within the confining bony walls of the small (ca. 2 cm long) internal auditory canal.
The incidence of NF2 is approximately 1 per 60,000 people. Tumor sizes
Patient surveys in the U.S. by the national Acoustic Neuroma Association (1998, 2007–08, 2012, 2014) showed that the percentage of diagnosed tumors 1.5 cm or less increased significantly from 23% to 47%. Researchers in Denmark reported (2004): "The size of diagnosed tumors has decreased from a median of 35 mm in 1979 to 10 mm in 2001." In general, tumor size (diameter) is described as small (less than 1.5 cm), medium (1.5 to 2.5 cm), large (2.5 to 4.0 cm) and giant (more than 4.0 cm). (Note: 1 inch = 2.54 cm) Radiologists reporting on MRI scans use the Koos Grading Scale which relates tumor size to its proximity to the brainstem and nearby cranial nerves. Thus Koos grade 1 is a purely intrameatal (IAC) tumor, 1–10 mm in size; Koos grade 2, 10–20 mm, has extended into the cerebellopontine angle (CPA), but with no brainstem contact; Koos grade 3, 20–30 mm, fills the CPA space and touches on the brainstem, but without compression; and Koos grade 4, more than 30 mm in size, compresses the brainstem and nearby nerves, and displaces critical arteries. Diagnosis
Preliminary diagnostic procedures include ear examination, hearing and vestibular testing.