How do you differentiate cortical, subcortical, and cortical-subcortical myoclonus based on clinical signs?

Differentiation of Cortical, Subcortical, and Cortical-Subcortical Myoclonus Based on Clinical Signs

The most reliable way to differentiate types of myoclonus is through their distinct clinical presentations, with cortical myoclonus characterized by brief, focal jerks with EEG correlates, subcortical myoclonus presenting without EEG correlates, and cortical-subcortical myoclonus showing features of both with generalized spike-wave discharges. 1, 2

Cortical Myoclonus

• Presents as brief, lightning-like jerks that are typically focal or multifocal in distribution 2
• Often stimulus-sensitive, triggered by action, touch, or sensory stimuli 3
• Consciousness is typically preserved during the myoclonic episodes 3
• Associated with specific EEG findings:
  • Enlarged somatosensory evoked potentials (SEPs) 4, 5
  • EEG discharges time-locked to individual myoclonic jerks 4
  • Back-averaged cortical potentials preceding the jerks 5
  • Enhanced long-latency reflexes 4, 5
• Short-duration electromyographic (EMG) bursts (typically <50 ms) 4
• May demonstrate both positive and negative myoclonus (sudden brief muscle inhibition) 4
• Often shows a cranial-caudal progression of jerks 4
• Commonly occurs in patients with continuous cortical background activity on EEG 6

Subcortical Myoclonus

• Develops without an EEG correlate (key distinguishing feature) 6
• May not require aggressive treatment with antiseizure medications if not interfering with mechanical ventilation 6
• Typically more generalized or proximal in distribution compared to cortical myoclonus 2
• Often rhythmic or semi-rhythmic in nature 2
• Less commonly stimulus-sensitive compared to cortical myoclonus 3
• Normal somatosensory evoked potentials 5
• Absence of time-locked cortical potentials on back-averaging 5
• Longer duration EMG bursts (>50 ms) compared to cortical myoclonus 2
• May be associated with certain postures or specific movements 3

Cortical-Subcortical Myoclonus

• Characterized by features of both cortical and subcortical myoclonus 1
• Associated with generalized spike and wave discharges on EEG, analogous to absence seizures 5
• Commonly seen in primary epileptic myoclonus syndromes (e.g., juvenile myoclonic epilepsy) 1
• Often presents with bilateral, synchronous jerks 7
• Typically occurs in association with or shortly after loss of consciousness 7
• May occur in "lockstep" with epileptiform abnormalities such as burst suppression with identical bursts 6
• Can be distinguished from syncope-related movements, which typically occur after loss of consciousness 7

Clinical Pearls and Pitfalls

• EEG is crucial for distinguishing between different types of myoclonus, particularly for differentiating cortical from subcortical forms 6
• Continuous EEG monitoring may be necessary to detect the episodic and unpredictable nature of epileptiform patterns associated with cortical myoclonus 6
• Status myoclonus starting within 48 hours after cardiac arrest is associated with poor neurological outcomes, though some patients can recover 3
• Myoclonus may be confused with other movement disorders such as tics, which can be temporarily suppressed, unlike true myoclonus 3
• The presence of status myoclonus during the first 72 hours after cardiac arrest, in combination with other diagnostic tests, can help predict poor neurologic outcomes 6
• Treatment approach should be guided by the neurophysiological classification of myoclonus, with levetiracetam suggested as first-line for cortical myoclonus, valproic acid for cortical-subcortical myoclonus, and clonazepam for subcortical myoclonus 1

Understanding these distinctive clinical and electrophysiological features is essential for accurate diagnosis and appropriate management of different types of myoclonus, ultimately affecting patient morbidity, mortality, and quality of life.