Inclusion Criteria:

- Children with newly diagnosed OMS/DES either NB-pos or NB-neg.

Three out of the following four components are necessary for the diagnosis of OMS/DES:

- Opsoclonus or ocular flutter (but not nystagmus)

- Ataxia and/or myoclonus

- Behavioural change and/or sleep disturbance

- Neuroblastoma The diagnosis of OMS/DES may be difficult in some patients. Opsoclonus,
in particular, may be intermittent or late in onset. A video example will be
available at www.dancingeyes.org.uk. If uncertain, please contact the national
coordinator for support in interpreting clinical features.

- Age 6 months or over up to less than 8 years (< 8th birthday) The date of
diagnosis of OMS/DES is the date on which a doctor confirms the condition to be
OMS/DES. The date of symptom onset needs also to be documented.

- Treatment start with the standard corticosteroid treatment with dexamethasone
pulses as proposed by the guidelines given in this trial protocol (see 11.10,
page 71).

- In patients with presumed NB-neg OMS/DES, neuroblastoma must be excluded
according the guidelines of this trial (see chapter 4.4.1.4, page 30, and
appendix 11.9, page 70)

- Documented informed consent for treatment and enrolment in the trial by parents
/ legal representatives.

Exclusion Criteria:

•Patients with opsoclonus, myoclonus or ataxia caused by other identified disease (e.g.
current active CNS infection, neurometabolic disorder or demyelination).

An identified viral precursor is not an exclusion criterion.

- prior or parallel use of chemotherapy (other than required for treatment of the
neuroblastoma)

- Corticoid steroid for OMS/DES or other reasons lasting 14 days or more immediately
before treatment start according the standard treatment proposed (treatment with
corticosteroids for less than 14 days will be allowed)

- contre-indication of use of one of the experimental study drug (cf Summary of Product
Characteristics used in this study)