What is the diagnosis and treatment for an 8-year-old female presenting with sudden persistent myoclonic jerks, horizontal eye movements, and intact sensorium?

Opsoclonus-Myoclonus Syndrome (OMS)

This 8-year-old girl presenting with sudden persistent myoclonic jerks, horizontal eye movements (opsoclonus), and intact consciousness most likely has Opsoclonus-Myoclonus Syndrome, which requires urgent workup for neuroblastoma and immunotherapy treatment.

Clinical Diagnosis

The combination of myoclonic jerks with horizontal eye movements (opsoclonus) and preserved consciousness is pathognomonic for Opsoclonus-Myoclonus Syndrome, not a primary seizure disorder. This presentation differs from typical myoclonic epilepsy in several critical ways:

• Preserved consciousness during episodes distinguishes this from myoclonic seizures, which typically occur with or shortly after loss of consciousness 1
• The horizontal eye movements (opsoclonus) are chaotic, multidirectional saccades that are not seen in epileptic myoclonus 1
• Age of onset at 8 years with sudden presentation raises concern for secondary causes rather than idiopathic epilepsy 1

Urgent Diagnostic Workup Required

Immediate evaluation for neuroblastoma is mandatory, as 50% of pediatric OMS cases are paraneoplastic:

• Obtain chest/abdominal/pelvic CT or MRI to identify occult neuroblastoma 1
• Measure urine catecholamines (VMA, HVA) 1
• Consider MIBG scan if initial imaging is negative but clinical suspicion remains high 1
• Brain MRI to exclude structural lesions, though typically normal in OMS 1
• EEG should be performed but will likely show no epileptiform activity, helping distinguish from epileptic myoclonus 1

Treatment Approach

First-Line Immunotherapy (Not Antiepileptics)

OMS is an immune-mediated disorder requiring immunosuppression, not antiepileptic drugs as primary therapy:

• ACTH or high-dose corticosteroids are first-line treatment for OMS
• IVIG (intravenous immunoglobulin) is often used in combination with steroids
• Rituximab may be considered for refractory cases

Symptomatic Management

While immunotherapy is being initiated, symptomatic control of myoclonus may be attempted:

• Clonazepam can provide symptomatic relief for myoclonus of any origin, including non-epileptic myoclonus 2, 3
• Levetiracetam is less effective for non-cortical myoclonus but may be tried 4, 3
• However, these medications address symptoms only and do not treat the underlying immune pathology

Critical Distinguishing Features from Epileptic Myoclonus

This case has multiple red flags suggesting secondary (non-epileptic) myoclonus rather than primary myoclonic epilepsy:

• Horizontal eye movements (opsoclonus) are not a feature of any epileptic syndrome 5
• Intact sensorium during episodes argues strongly against epileptic myoclonus 1
• Sudden onset at age 8 without prior seizure history is atypical for juvenile myoclonic epilepsy, which typically presents in adolescence 6
• The combination of myoclonus with abnormal eye movements points to a brainstem/cerebellar process, not cortical epilepsy 1

Common Pitfalls to Avoid

• Do not treat empirically with antiepileptic drugs without first obtaining EEG and considering OMS, as this delays appropriate immunotherapy
• Do not dismiss the case as benign myoclonus based on preserved consciousness—the eye movements are the critical distinguishing feature 1
• Do not delay neuroblastoma workup—even if neuroimaging is planned, obtain urine catecholamines immediately as neuroblastoma may be occult 1
• Do not confuse opsoclonus with nystagmus—opsoclonus consists of chaotic, multidirectional saccades without a slow phase, unlike the conjugate horizontal jerk oscillations of latent nystagmus 5

Prognosis and Follow-up

• OMS associated with neuroblastoma paradoxically has better oncologic outcomes than neuroblastoma without OMS
• However, long-term neurodevelopmental sequelae (cognitive, behavioral, motor) occur in up to 70% of cases despite treatment
• Early aggressive immunotherapy improves neurologic outcomes
• Long-term monitoring for developmental delays and learning disabilities is essential