What is on the differential diagnosis for fragmentary myoclonus?

Differential Diagnosis of Fragmentary Myoclonus

Fragmentary myoclonus is primarily a sleep-related phenomenon characterized by brief muscle potentials on polysomnography, and its differential diagnosis centers on distinguishing it from other forms of myoclonus and movement disorders, with particular attention to neurodegenerative conditions, epileptic syndromes, and physiologic variants. 1, 2

Primary Diagnostic Considerations

Neurodegenerative Disorders

• Idiopathic REM Sleep Behavior Disorder (iRBD): Fragmentary myoclonus occurs in 75.9% of iRBD patients, representing an early stage of neurodegeneration with brainstem involvement, compared to only 34.5% in controls 2
• Parkinson's disease and Lewy body dementia: These conditions share pathophysiology with iRBD and commonly present with fragmentary myoclonus 3, 2
• Creutzfeldt-Jakob disease: Presents with myoclonus in the context of rapidly progressive dementia and focal neurological signs 3
• Alzheimer's disease: Can present with significant myoclonus, creating diagnostic confusion with CJD 3

Epileptic Myoclonus (Key Distinction)

• Cortical myoclonus: Brief, focal jerks with EEG correlates showing continuous cortical background activity—consciousness is preserved during episodes 1, 4, 5
• Familial Adult Myoclonic Epilepsy (FAME): Characterized by cortical myoclonus, generalized tonic-clonic seizures, and pentanucleotide repeat expansions; often misdiagnosed as juvenile myoclonic epilepsy or essential tremor 6
• Juvenile myoclonic epilepsy: Presents with myoclonic and generalized tonic-clonic seizures but typically has earlier onset than FAME 6
• Progressive myoclonic epilepsies: Including Unverricht-Lundborg disease (younger individuals) and late-onset variants like sialidoses in adults 3, 6

Movement Disorders

• Essential tremor: Cortical myoclonus may mimic essential tremor, requiring neurophysiological studies to distinguish 6
• Drug-induced tremor: Must be differentiated from cortical myoclonus through clinical history and EEG 6
• Tics: Brief jerks with more complex patterns that can be temporarily suppressed, unlike myoclonus 1

Metabolic and Toxic Causes

• Metabolic encephalopathy: Myoclonus is frequent in metabolic or hydroelectrolytic disorders 3
• Post-anoxic myoclonus: Distinguishing acute post-anoxic phase myoclonus from Lance-Adams syndrome (chronic action myoclonus occurring as sequelae) is critical 7, 3
• Hashimoto's encephalopathy: Presents with myoclonus and tremor, either acute or insidious onset 3
• Toxic exposures: Less frequent cause but must be considered 3

Structural and Infectious Causes

• Thalamic lesions: Can cause unilateral asterixis or unilateral myoclonus superimposed on dystonic posture 3
• Spinal cord disorders: Segmental spinal myoclonus or propriospinal myoclonus associated with various spinal pathologies 3
• Viral encephalitis: Particularly measles virus, Nipah virus, and JC virus (PML) can cause myoclonic jerks 8
• Progressive myoclonic ataxia (Ramsay-Hunt syndrome): Myoclonus with progressive ataxia suggests celiac disease, mitochondriopathies, spinocerebellar degenerations, or late metabolic disorders 3

Critical Diagnostic Approach

EEG Findings Are Essential

• EEG recording during episodes helps identify awareness, reactivity, and potential epileptic correlates to distinguish epileptic from non-epileptic myoclonus 1, 7
• Absence of EEG correlate suggests subcortical myoclonus, which may not require aggressive antiseizure treatment 1
• Cortical-subcortical myoclonus may occur "in lockstep" with epileptiform abnormalities such as burst suppression 1

Key Clinical Features to Assess

• Timing relative to consciousness: Epileptic myoclonic movements begin at or shortly after loss of consciousness, while syncope-related movements occur after consciousness is lost 8
• Symmetry and synchrony: Asymmetrical and asynchronous movements are more common in syncope but can occur in epilepsy 8
• Triggers: Excitement, frustration, postural changes, sensory stimuli, or specific movements (kinesigenic forms) 1
• Associated symptoms: Dementia, ataxia, renal failure, opsoclonus, or palatal tremor narrow the differential significantly 3

Common Pitfall

Do not assume all myoclonus requires aggressive antiseizure treatment—subcortical myoclonus without EEG correlates may not need such intervention unless interfering with mechanical ventilation 1. Conversely, fragmentary myoclonus in the context of iRBD signals potential neurodegenerative progression and warrants longitudinal monitoring 2.