What causes myoclonic jerking?

Causes of Myoclonic Jerking

Myoclonic jerking is primarily caused by sudden, brief, involuntary muscle contractions or inhibitions originating from various areas of the central nervous system, including cortical, subcortical, brainstem, and spinal regions. 1, 2

Primary Etiological Categories

Infectious Causes

• Viral encephalitis, particularly measles virus, can cause myoclonic jerks, which may be associated with a characteristic EEG showing periodic complexes with a 1:1 relationship with the jerks 3
• Nipah virus infection can lead to myoclonus along with dystonia, areflexia, and hypotonia 3
• JC virus causing Progressive Multifocal Leukoencephalopathy (PML) in immunocompromised patients may present with myoclonic movements 3

Epileptic Myoclonus

• Occurs in various epilepsy syndromes including idiopathic generalized epilepsies and severe epileptic encephalopathies 4
• Characterized by movements that begin at or shortly after the onset of loss of consciousness 1
• Can be distinguished from syncope-related movements, which typically occur after loss of consciousness and fall 3, 1
• May be symmetrical and synchronous, though asynchronous jerks can also occur in epileptic myoclonus 1

Post-Hypoxic Myoclonus

• Status myoclonus starting within 48 hours after cardiac arrest is associated with poor neurological outcomes 1, 5
• Requires evaluation off sedation whenever possible and EEG recording to identify awareness and reactivity 5

Toxic-Metabolic Causes

• Drug-induced myoclonus can occur with various medications, particularly:
  • Tramadol has been reported to cause myoclonus in the muscles of the trunk 6
  • Certain medications may aggravate existing myoclonic seizures (e.g., carbamazepine, phenytoin) 4

Neurodegenerative Disorders

• Various storage diseases and neurodegenerative conditions can present with myoclonus 2
• These typically present as symptomatic myoclonus rather than as isolated phenomena 2

Other Causes

• Paroxysmal kinesigenic dyskinesia (movement triggered by sudden motion) can manifest as myoclonic jerks 5
• Essential myoclonus can occur without an identifiable underlying cause 2
• Physiological myoclonus includes normal phenomena such as hypnic jerks during sleep onset 2

Clinical Presentation Patterns

Based on Timing and Triggers

• Rest myoclonus: occurs when the affected body part is relaxed 7
• Action myoclonus: triggered by voluntary movement 7
• Postural myoclonus: appears when maintaining a specific posture 7
• Stimulus-sensitive myoclonus: triggered by various external stimuli including touch, sound, or light 7

Based on Distribution

• Focal: affecting a specific body part 7
• Segmental: involving adjacent body regions 7
• Multifocal: occurring in multiple, non-contiguous body parts 7
• Generalized: affecting the entire body 7

Diagnostic Approach

• Electrophysiological tests are crucial to determine whether myoclonus is cortical, subcortical, or spinal in origin 7, 8
• EEG can help identify epileptiform discharges associated with myoclonic movements 8
• Clinical history should focus on age of onset, character of myoclonus, precipitating factors, family history, and associated symptoms 7
• Physical examination should assess whether myoclonus appears at rest, during posture maintenance, or during action 7

Treatment Considerations

• Valproate, levetiracetam, and benzodiazepines (particularly clonazepam) are effective for many forms of myoclonus 5, 4
• Clonazepam is recommended as the most effective first-line medication for most forms of myoclonus 5
• Focal and segmental myoclonus may be treated with botulinum toxin injections 7
• Treatment should target the underlying cause when possible 2