**2.5 Opsoclonus-myoclonus syndrome**

Opsoclonus-myoclonus syndrome (OMS) is a rare autoimmune neurological disorder that can occur in both children and adults. In childhood OMS, the main age of onset is approximately 18 months [23, 24]. Classic manifestations include an acute or subacute onset of ataxia, opsoclonus and myoclonic jerks associated with irritability and sleep disturbance [25]. All the main features may not be present initially and the diagnosis may be delayed weeks or months from the onset of symptoms.

OMS can be idiopathic, parainfectious or occur as a paraneoplastic syndrome. The most frequent neoplasia associated with childhood OMS in neuroblastoma. Neuroblastoma is present in at least 50% of affected children, and OMS presents as a paraneoplastic syndrome in 2–3% of all children with neuroblastomas [26]. Children with neuroblastoma and OMS have a better prognosis for their oncologic disease since they are usually diagnosed at earlier tumour stages and neuroblastoma is better differentiated [25].

The management and outcome depend on the aetiology and the spread of the disease. A regular response of neurological symptoms to immunosuppressive treatment is described. However, although the main neurological symptoms may show a good response, most children remain severe neuropsychological alterations [23].
