What is myoclonus?

What is Myoclonus

Myoclonus is a sudden, brief, involuntary muscle jerk caused by abrupt muscle contraction (positive myoclonus) or by sudden cessation of ongoing muscle activity (negative myoclonus). 1, 2, 3

Core Clinical Characteristics

Myoclonus presents as lightning-like jerks that can affect a single muscle, a group of muscles, or the entire body. 2 The movements are:

• Sudden and brief in duration 2, 3
• Shock-like in quality 4, 5
• Involuntary and cannot be suppressed 3
• Occur with preserved consciousness, which distinguishes myoclonus from epileptic seizures 2

Distinguishing Myoclonus from Similar Movements

In neurological terminology, an individual abrupt involuntary movement is called myoclonus, while the term "convulsion" refers to violent contractions of multiple muscles. 1 This distinction is critical because:

• Myoclonus in syncope occurs when the EEG is slow but not flat, and the presence of myoclonus alone is not sufficient evidence for an epileptic seizure 1
• In syncope, there are few myoclonic movements, whereas generalized seizures produce many (20-100) movements that are more synchronous and rhythmic 1
• Myoclonus differs from tics, which have a more complex pattern and can be temporarily suppressed 2
• Hyperekplexia can mimic myoclonus but is specifically triggered by sudden noise or touch and includes an excessive startle response present from birth 1, 2

Physiological Classification

The most clinically useful classification is based on the anatomical origin of the myoclonus: 3, 4

• Cortical myoclonus: Originates from the motor cortex, commonly occurs with continuous cortical background activity on EEG, and presents as brief focal jerks with EEG correlates 2, 4
• Subcortical myoclonus: Develops without an EEG correlate, which is a key distinguishing feature 2
• Spinal myoclonus: Originates from spinal cord structures 3, 4
• Peripheral myoclonus: Arises from peripheral nervous system 4

Common Triggers and Contexts

Myoclonus may be triggered by: 2

• Excitement or frustration 2
• Postural changes 2
• Sensory stimuli 2
• Specific movements (in kinesigenic forms) 2

Clinical Presentations

Myoclonus is divided into four main clinical categories: 4

• Physiological myoclonus: Normal muscle jerks during drowsiness, falling asleep, or hiccoughs 6
• Essential myoclonus: Myoclonus as the only neurologic abnormality 6
• Epileptic myoclonus: Associated with epileptic disorders 4
• Symptomatic myoclonus: Most common, caused by underlying conditions including post-hypoxia, toxic-metabolic disorders, drug reactions, storage diseases, and neurodegenerative disorders 4, 5

Diagnostic Approach

EEG recording during episodes is crucial to identify awareness, reactivity, and potential epileptic correlates, and to distinguish between cortical and subcortical forms. 2 Key diagnostic steps include:

• Characterize the distribution (focal, segmental, or generalized) and timing (at rest, with posture, or during action) 3
• Assess stimulus sensitivity 3
• Perform electrophysiological testing to determine whether myoclonus is cortical, subcortical, or spinal 3, 7
• Neuroimaging (MRI) is recommended to identify structural lesions 2

Prognostic Significance

Status myoclonus within 72 hours after cardiac arrest predicts poor neurological outcome with 0% false positive rate, whereas isolated myoclonus has an unacceptable 5-11% false positive rate and should not be used alone for prognostication. 2, 8 However, some patients with early-onset myoclonus may evolve into Lance-Adams syndrome with chronic action myoclonus and can have good neurological recovery. 8