Schwannomatosis is an extremely rare genetic disorder closely related to the more-common disorder neurofibromatosis (NF). Originally described in Japanese patients, it consists of multiple cutaneous schwannomas, central nervous system tumors, and other neurological complications, excluding hallmark signs of NF. The exact frequency of schwannomatosis cases is unknown, although some populations have noted frequencies as few as 1 case per 1.7 million people.Schwannomas are mostly benign tumors that commonly occur in individuals with NF2 and schwannomatosis (sometimes called neurofibromatosis type III). Schwann cells are glial cells that myelinate the axons of nerve cells. Myelin is a lipid covering that speeds the conduction of action potentials. When Schwann cells proliferate out of control in an encapsulation it is called a schwannoma. Although schwannomas are benign they become detrimental when the growing tumor compresses the nerve. Schwannomas on sensory nerve axons cause chronic severe pain. Treatment options for schwannomas are to surgically remove them, have radiation, cyberknife or Intracapsular Enucleation. Previous designations for schwannomas include neurinoma and neurilemmoma. Cause
The candidate schwannomatosis gene, named SMARCB1, is a tumor suppressor gene that regulates cell cycle, growth and differentiation. An inactivating  germline mutation in exon 1 of the tumor suppressor gene SMARCB1 has been reported in patients with schwannomatosis. It is located on chromosome 22 a short distance from the NF2 gene.
SMARCB1 is mutated in additional tumors including malignant brain & kidney tumors in children. It seems that heterozygotes for mutations in the SMARCB1 gene have an increased risk to develop a malignant kidney tumor in early childhood but if they survive to adulthood, they may be predisposed to the development of schwannomas. One schwannomatosis patient had a mutation in exon 2 of the SMARCB1 gene. Another patient exhibited a novel germline deletion of the SMARCB1, because most SMARCB1 mutations are point or frameshift. In this patient genetic analysis from different schwannomas indicated  inactivation of both the SMARCB1 and NF2 genes. Schwannomatosis is known to be a genetic disorder. However, familial occurrence is inexplicably rare. Diagnosis
Prenatal
Schwannomatosis can be tested prenatally on the NHS. Postnatal
Ferner et al. give the following diagnostic criteria for Schwannomatosis:

Definite
Age >30 years and ≥2 nonintradermal schwannomas, at least one with histologic confirmation and no evidence of vestibular tumor on MRI scan and no known NF mutation, or
One nonvestibular schwannoma plus a first-degree relative with schwannomatosis
Possible
Age <30 and ≥2 nonintradermal schwannomas, at least one with histologic confirmation and no evidence of vestibular tumor on MRI scan and no known NF mutation, or
Age >45 and ≥2 nonintradermal schwannomas, at least one with histologic confirmation and no symptoms of 8th nerve dysfunction and no NF2, or
Nonvestibular schwannoma and first-degree relative with schwannomatosis
Segmental.