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What is Opsoclonus myoclonus syndrome and explain in detail?
Opsoclonus myoclonus syndrome (OMS), also known as opsoclonus-myoclonus-ataxia (OMA), is a rare neurological disorder of unknown cause which appears to be the result of an autoimmune process involving the nervous system. It is an extremely rare condition, affecting as few as 1 in 10,000,000 people per year. It affects 2 to 3% of children with neuroblastoma and has been reported to occur with celiac disease and diseases of neurologic and autonomic dysfunction.

Signs and symptoms
Symptoms include:
opsoclonus (rapid, involuntary, multivectorial (horizontal and vertical), unpredictable, conjugate fast eye movements without intersaccadic [quick rotation of the eyes] intervals)
myoclonus (brief, involuntary twitching of a muscle or a group of muscles)
cerebellar ataxia, both truncal and appendicular
aphasia (a language disorder in which there is an impairment of speech and of comprehension of speech, caused by brain damage)
mutism (a language disorder in which a person does not speak despite evidence of speech ability in the past, often part of a larger neurological or psychiatric disorder)
lethargy
irritability or malaise
drooling
strabismus (a condition in which the eyes are not properly aligned with each other)
vomiting
sleep disturbances
emotional disturbances (including fits of rage)About half of all OMS cases occur in association with neuroblastoma (a cancer of the sympathetic nervous system usually occurring in infants and children).

Disease course and clinical subtypes
In most cases, OMS starts with an acute flare-up of physical symptoms within days or weeks, but some less obvious symptoms such as irritability and malaise may begin weeks or months earlier.

Cause
In children, most cases are associated with neuroblastoma and most of the others are suspected to be associated with a low-grade neuroblastoma that spontaneously regressed before detection. In adults, most cases are associated with breast carcinoma or small-cell lung carcinoma. It is one of the few paraneoplastic (meaning indirectly caused by cancer) syndromes that occurs in both children and adults, although the mechanism of immune dysfunction underlying the adult syndrome is probably quite different.It is hypothesized that a viral infection (perhaps St. Louis encephalitis, Chikungunya, Epstein-Barr, Coxsackie B, enterovirus, or just a flu) causes the remaining cases, though a direct connection has not been proven. Rare cases of Opsoclonus myoclonus syndrome associated with Lyme disease have also been reported.OMS is not generally considered an infectious disease. OMS is not passed on genetically.

Diagnosis
Because OMS is so rare and occurs at an average age of 19 months (6 to 36 months), a diagnosis can be slow. Some cases have been diagnosed as having been caused by a virus. After a diagnosis of OMS is made, an associated neuroblastoma is discovered in half of cases, with median delay of 3 months.The interictal EEG pattern is usually normal.

Treatment
There is no known definitive cure for OMS. However, several drugs have proven to be effective in their treatment.
Some of medication used to treat the symptoms are:

ACTH has shown improvements in symptoms but can result in an incomplete recovery with residual deficits.
Corticosteroids (such as prednisone or methylprednisolone) used at high dosages (500 mg - 2 g per day intravenously for a course of 3 to 5 days) can accelerate regression of symptoms. Subsequent very gradual tapering with pills generally follows. Most patients require high doses for months to years before tapering.
Intravenous Immunoglobulins (IVIg) are often used with varying results.
Several other immunosuppressive drugs, such as cyclophosphamide and azathioprine, may be helpful in some cases.
Chemotherapy for neuroblastoma may be effective, although data is contradictory and unconvincing at this point in time.
Rituximab has been used with encouraging results.
Other medications are used to treat symptoms without influencing the nature of the disease (symptomatic treatment):
Trazodone can be useful against irritability and sleep problems
Additional treatment options include plasmapheresis for severe, steroid-unresponsive relapses.The National Organization for Rare Disorders (NORD) recommends FLAIR therapy consisting of a three-agent protocol involving front-loaded high-dose ACTH, IVIg, and rituximab that was developed by the National Pediatric Myoclonus Center, and has the best-documented outcomes. Almost all patients (80-90%) show improvement with this treatment and the relapse rate appears to be about 20%.A more detailed summary of current treatment options can be found at Treatment Options
The following medications should probably be avoided:

Midazolam - Can cause irritability.
Melatonin - Is known to stimulate the immune system.
Also, see "An Innovative Approach to the Problem of Sedating Children with Opsoclonus-Myoclonus Syndrome". Pranzatelli Abstracts. for more details

Prognosis
Currently, there are no clinically established laboratory investigations available to predict prognosis or therapeutic response.Tumors in children who develop OMS tend to be more mature, showing favorable histology and absence of n-myc oncogene amplification than similar tumors in children without symptoms of OMS. Involvement of local lymph nodes is common, but these children rarely have distant metastases and their prognosis, in terms of direct morbidity and mortality effects from the tumor, is excellent