Can Opsoclonus-Myoclonus-Ataxia Syndrome Present Without Myoclonus?
Yes, opsoclonus-myoclonus-ataxia syndrome can present without myoclonus, and this incomplete presentation is increasingly recognized as a diagnostic pitfall that delays treatment and worsens outcomes.
Clinical Reality of Incomplete Presentations
The classic triad of opsoclonus, myoclonus, and ataxia does not always present simultaneously or completely. Recent evidence demonstrates that patients can have opsoclonus and ataxia without clinically apparent myoclonus, particularly early in the disease course 1, 2.
Key Evidence:
A 2024 case report documented a 52-year-old man with small-cell lung cancer who presented with opsoclonus, severe truncal and gait ataxia, and dysarthria explicitly "without myoclonus" 1. This patient achieved complete clinical remission with immunotherapy, confirming the diagnosis despite the absent feature.
A 2026 pediatric series found that ataxia was the most common initial symptom (85.7%), while opsoclonus was present in only 2 of 7 patients at initial presentation, though all but one eventually developed it during the disease course 2.
Another 2026 case described "isolated opsoclonus without significant myoclonus" in a patient with gastric cancer who responded to immunotherapy 3.
Diagnostic Implications
The absence of myoclonus should NOT exclude the diagnosis of opsoclonus-myoclonus-ataxia syndrome. The guideline literature characterizes OMS as "a rare disorder characterised by chaotic saccadic eye movements, myoclonus, ataxia and encephalopathy" 4, but this represents the full syndrome, not mandatory criteria.
Critical Diagnostic Pitfalls:
Delayed recognition of opsoclonus: A 2025 diagnostic odyssey study found that while nystagmus appeared simultaneously in early- and delayed-diagnosed groups, opsoclonus was recognized much later in the delayed group (11.5 days vs 96 days, p < 0.01) 5.
Misdiagnosis as other conditions: Patients are commonly misdiagnosed with acute cerebellar ataxia, acute disseminated encephalomyelitis, or acute cerebellitis when myoclonus is absent 5.
Opsoclonus misidentified as nystagmus: In the pediatric series, opsoclonus was initially misdiagnosed as nystagmus in some patients 2.
Practical Approach
When evaluating a patient with ataxia and behavioral/sleep disturbances, actively search for opsoclonus even if myoclonus is absent:
- Examine for chaotic, multidirectional saccadic eye movements (opsoclonus) distinct from rhythmic nystagmus
- Look for behavioral changes and sleep disorders (present in 71.4% of pediatric cases) 2
- Consider OMS in any patient with prolonged or relapsing ataxia 5
- Proceed with tumor screening (neuroblastoma in children, lung/breast/gynecological cancers in adults) regardless of myoclonus presence
- Initiate early immunotherapy if clinical suspicion is high, as delayed treatment increases risk of permanent neurological sequelae
The key principle: Myoclonus may be subtle, absent at presentation, or develop later in the disease course. Do not wait for the complete triad before initiating workup and treatment.