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What is Schwannomatosis and explain in detail?
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. However, molecular analysis of the NF2 gene in schwannomatosis patients has shown the presence of inactivating mutations in the tumor cells, but no evidence of the germline mutations that are found in NF2 patients.A mechanism involving both the SMARCB1 and NF2 genes may be responsible for the development of the disease because tumor analysis of schwannomas indicates the presence of inactivating mutations in both the SMARCB1 and NF2 genes. However, there is speculation about the involvement of an unidentified schwannomatosis gene(s) in most cases. This is because one study found no SMARCB1 germinal mutations in patients with familial schwannomatosis. Some schwannomatosis patients do not have SMARCB1 or NF2 mutations. Furthermore, many patients exhibit somatic mosaicism for mutations in the NF2 or SMARCB1 gene, which means that some somatic cells have the mutation and some do not in the same patient. Ultimately, the tumorigenesis of schwannomas is not solely dependent on one gene locus alone. In regards to the SMARCB1 and NF2 genes, it is important to understand constitutional mutations and somatic mutations. Constitutional mutations are the first inactivation events that are often small mutations, such as point mutations and deletion/insertion of single base pairs. Somatic mutations are the second mutations that occur and may also be another small mutation or the loss of the remaining allele of the gene. Schwannomas from one patient share the same constitutional mutations but have distinct somatic mutations. In addition, the constitutional mutation may be present in non-tumor.SMARCB1 is also known as INI1, hSNF5, or BAF47. 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.  Diagnosed as definite or possible but limited to one limb or ≤5 contiguous segments of spine.Another set of criteria are:

Two or more nonintradermal (cutaneous) schwannomas
No evidence of vestibular tumor
No known NF-2 mutationor

One pathologically confirmed nonvestibular schwannoma plus a first degree relative who meets the above criteria.

Treatment
Schwannomatosis patients represent 2.4% to 5% of patients undergoing surgical resection of their schwannomas.
In isolated regions of the body schwannomas are small and difficult to locate. Intraoperative sonography offers invaluable assistance in such cases by localizing small schwannomas and decreasing operative time and extent of the surgical incision.
If feasible, the schwannomas can be surgically removed. Any tumor-associated pain usually subsides after tumor removal.  Damaged nerves and scar tissue can be a result of surgery and pain can be an ongoing problem.
Sometimes, a tumor will reappear at the same site after surgery.
If surgery is unfeasible, then pain management will have to be used