We present the case of a 30-month-old girl who came to the emergency department for presenting in the last two days with slight gait instability and intention tremor. The parents reported that three weeks earlier a mild generalised rash without fever had appeared, which resolved spontaneously in a few days. His personal history included no known allergies or previous trauma and his vaccination schedule was up to date, including varicella (chickenpox). Her psychomotor development was normal. She was re-evaluated 48 hours later, highlighting a worsening of ataxia that prevented her from standing upright, associated with severe irritability and irregular sleep. Physical examination on admission showed the patient to be in good nutritional condition and there were no meningeal signs, external signs of trauma or other pathological findings. The neurological examination revealed slurred but intelligible speech appropriate to the context, cerebellar ataxia, and intense and generalised intention tremor. Cranial nerves, strength, muscle tone and osteotendinous reflexes were normal and symmetrical.
On admission, electroencephalogram (EEG) and brain tomography (CT) with iodine contrast were performed, which revealed no electroencephalographic or structural abnormalities. Urine toxins were negative. Lumbar puncture showed mild lymphorrhoea with negative biochemistry and cultures, including Herpes group, neurotropic virus and Borrelia. Serology for Herpes simplex virus, Epstein-Barr, Varicella-zoster, enterovirus, Salmonella sp., Streptococcus pyogenes and Borrelia burgdoferi showed no pathological findings.
On the fifth day after symptom onset, the patient presented with intention tremor and severe truncal ataxia that made sitting difficult. The tremor was aggravated by probable intentionally reinforcing myoclonus and the irritability was extreme. Rapid, saccadic, multidirectional, chaotic but conjugated eye movements then appeared, which were also visible with the eyes closed, and which decreased when the patient was able to fixate his gaze. Given the clinical diagnosis of opsoclonus (SOMA), the search for a possible tumour was initiated. An abdominal ultrasound was requested, with normal results, and a simple chest X-ray showed a left parasternal thoracic mass. A thoracic CT scan showed a left paravertebral intrathoracic mass, compatible with NCT, confirmed after surgery as localised ganglioneuroblastoma. Neuron-specific enolase in blood and 24-hour urine catecholamines were also requested and found to be normal.

The opsoclonus decreased after initiation of high-dose oral prednisone (2 mg/kg/day), and was no longer observed 15 days after surgery. Ataxia and irritability improved markedly, but one year later she still requires treatment with oral prednisone at minimal daily doses for relapses of mild gait instability and irritability, with no evidence of tumour recurrence to date. His psychomotor development continues to progress normally.