What causes myoclonus?

Causes of Myoclonus

Myoclonus is caused by multiple conditions affecting different levels of the nervous system, with the most common etiologies being cortical, subcortical, spinal, or peripheral in origin. 1, 2

Classification of Myoclonus

Myoclonus can be organized into several categories based on clinical presentation:

Physiological Myoclonus

• Sleep jerks (hypnic jerks)
• Hiccups
• Exercise-induced myoclonus

Epileptic Myoclonus

• Occurs as part of seizure disorders
• May present in epileptic syndromes with myoclonic features 3
• Characterized by EEG abnormalities correlating with jerks

Essential Myoclonus

• Occurs without other neurological abnormalities
• May be hereditary or sporadic

Symptomatic Myoclonus (Most Common)

Post-hypoxic Myoclonus

• Occurs following cerebral hypoxia/anoxia
• Can present as:
  • Acute post-anoxic myoclonus in the immediate post-arrest period 1
  • Lance-Adams syndrome (chronic post-hypoxic myoclonus) 1
  • Status myoclonus (continuous, generalized myoclonus within 48 hours of cardiac arrest) - associated with poor outcome 1

Toxic-Metabolic Causes

• Renal failure
• Hepatic failure
• Electrolyte disturbances (especially sodium, calcium, magnesium)
• Hypoglycemia
• Nonketotic hyperglycemia 4

Drug-Induced Myoclonus

• Levodopa (Level II evidence)
• Cyclic antidepressants (Level II evidence)
• Bismuth salts (Level II evidence)
• Serotonergic drugs (SSRIs, MAOIs) - can cause serotonin syndrome with myoclonus 1, 5
• Opioids
• Antipsychotics
• Antibiotics
• Anticonvulsants (paradoxical effect) 5

Infectious Causes

• Creutzfeldt-Jakob disease (CJD) - characterized by rapidly progressive dementia with myoclonus 4
• Viral encephalitis
• Subacute sclerosing panencephalitis
• HIV-related opportunistic infections

Neurodegenerative Disorders

• Alzheimer's disease
• Lewy body dementia
• Multiple system atrophy
• Corticobasal degeneration
• Progressive myoclonic epilepsies 4

Storage Diseases

• Lafora body disease
• Unverricht-Lundborg disease
• Neuronal ceroid lipofuscinosis
• Sialidosis

Autoimmune/Inflammatory

• Opsoclonus-myoclonus syndrome (idiopathic or paraneoplastic in adults) 4
• Hashimoto's encephalopathy (characterized by myoclonus and tremor) 4
• Multiple sclerosis (rare)

Structural Lesions

• Thalamic lesions (can cause unilateral myoclonus) 4
• Brainstem lesions
• Spinal cord lesions (causing segmental or propriospinal myoclonus) 4

Neurophysiological Classification

Based on the anatomical origin of the myoclonus:

Cortical Myoclonus

• Brief duration (10-50 ms)
• Often stimulus-sensitive
• May have EEG correlate preceding the jerk 6
• Common in post-hypoxic states, metabolic encephalopathies, and neurodegenerative diseases

Subcortical Myoclonus

• Longer duration (>100 ms)
• Often generalized
• No EEG correlate
• Seen in reticular reflex myoclonus and some drug-induced states

Spinal Myoclonus

• Rhythmic or semi-rhythmic
• Affects segments corresponding to spinal levels
• No EEG correlate
• Persists during sleep
• Associated with spinal cord pathology 4

Peripheral Myoclonus

• Rare
• Associated with peripheral nerve damage
• Controversial whether true myoclonus 4

Clinical Approach to Diagnosis

When evaluating myoclonus:

1. Determine distribution (focal, multifocal, segmental, generalized)
2. Identify activating factors (rest, action, stimulus-sensitive)
3. Look for associated neurological signs (dementia, ataxia, neuropathy)
4. Consider timing of onset (acute, subacute, chronic)
5. Review medication history thoroughly
6. Evaluate for metabolic derangements

Important Clinical Pearls

• Myoclonus in syncope is common and does not necessarily indicate epilepsy - in syncope, there are typically few myoclonic movements compared to many (20-100) in generalized seizures 1
• Status myoclonus starting within 48 hours of cardiac arrest is strongly associated with poor neurological outcome 1
• The presence of myoclonus with rapidly evolving dementia should raise suspicion for CJD, though myoclonus can also occur in Alzheimer's disease and Lewy body dementia 4
• Drug-induced myoclonus typically resolves with withdrawal of the offending agent 5
• Serotonin syndrome presents with the clinical triad of mental status changes, autonomic hyperactivity, and neuromuscular abnormalities (including myoclonus in 57% of cases) 1

Understanding the cause of myoclonus is essential for appropriate management, as treatment approaches differ based on the underlying etiology and neurophysiological classification.