Myoclonus: Definition, Classification, and Management
Myoclonus is defined as sudden, brief, shock-like, involuntary movements caused by muscular contractions or inhibitions. 1 It represents a distinct movement disorder that can occur in various clinical contexts and with different underlying pathophysiologies.
Types of Myoclonus
Myoclonus can be classified in several ways:
Based on Clinical Presentation:
- Physiological myoclonus: Normal phenomenon (e.g., sleep jerks)
- Essential myoclonus: Occurs without other neurological abnormalities
- Epileptic myoclonus: Associated with epilepsy
- Symptomatic myoclonus: Secondary to an underlying disorder 2
Based on Neurophysiological Origin:
- Cortical myoclonus: Originates from the motor cortex (most common)
- Subcortical myoclonus: Originates from structures below the cortex
- Brainstem myoclonus: Originates from the brainstem
- Spinal myoclonus: Originates from the spinal cord
- Peripheral myoclonus: Originates from the peripheral nervous system 2
Based on Distribution:
- Focal: Affects a specific body part
- Segmental: Affects adjacent body parts
- Multifocal: Affects multiple non-contiguous body parts
- Generalized: Affects the whole body 2
Clinical Characteristics
The key features of myoclonus include:
- Sudden, brief, shock-like movements
- Can be positive (muscle contraction) or negative (brief cessation of muscle activity)
- May occur at rest, during maintained posture, or with voluntary action
- Can be spontaneous or stimulus-sensitive (triggered by touch, sound, light) 1
Special Types of Myoclonus
Post-Anoxic Myoclonus
Two important types:
Status myoclonus: Continuous myoclonic jerks lasting >30 minutes occurring within 72 hours after cardiac arrest, associated with poor neurological prognosis (99.8-100% specificity for poor outcome) 1, 2
Lance-Adams Syndrome: Action or intention myoclonus that develops days to months after hypoxic cerebral injury in patients who have regained consciousness. It typically disappears at rest and has a more favorable prognosis than early post-anoxic myoclonus 2
Diagnostic Approach
Diagnosis of myoclonus requires:
- Clinical evaluation: Determine distribution, triggers, timing, and associated symptoms
- EEG: Critical for differentiating epileptic from non-epileptic myoclonus 2
- Somatosensory evoked potentials: Help identify cortical myoclonus 2
- Screening for common causes: Hypoxia, toxic-metabolic disorders, drug reactions, neurodegenerative diseases 2
Treatment
Treatment approach depends on the underlying cause and neurophysiological classification:
For Cortical Myoclonus:
- First-line: Valproate, levetiracetam, and piracetam 2
- Avoid: Phenytoin and carbamazepine (can worsen cortical myoclonus) 2
For Subcortical/Brainstem Myoclonus:
- First-line: Clonazepam 2
For Post-Anoxic Myoclonus:
For Focal/Segmental Myoclonus:
- Consider: Botulinum toxin injections 2
Important Clinical Considerations
- Status myoclonus within 72 hours post-cardiac arrest strongly predicts poor neurological outcome 1, 2
- Lance-Adams syndrome has a more favorable prognosis, with 85% of patients showing improvement over time 2
- Medications like opiates can induce or worsen myoclonus 2
- Valproate should be avoided in women of childbearing age due to risk of fetal malformations 2
- Continuous EEG monitoring should be considered in patients with status epilepticus 2
Treatment Challenges
- Single agents rarely provide complete control; polytherapy is often needed 2
- Treatment response varies based on the underlying neurophysiological mechanism
- Deep brain stimulation targeting the globus pallidus may be considered in refractory cases 2
Proper classification and identification of the underlying cause are essential for effective management of myoclonus, as treatment approaches differ significantly based on the neurophysiological origin.