How do you differentiate between cortical and subcortical myoclonus?

Differentiating Cortical from Subcortical Myoclonus

The key distinction is that cortical myoclonus has an EEG correlate with epileptiform activity, while subcortical myoclonus develops without an EEG correlate. 1

Primary Diagnostic Tool: EEG Recording

EEG is crucial for distinguishing between cortical and subcortical forms of myoclonus. 1 The presence or absence of epileptiform activity on EEG during myoclonic episodes is the definitive differentiating feature. 1

Cortical Myoclonus Characteristics:

• Brief, focal jerks with EEG correlates showing epileptiform discharges 1
• Enlarged somatosensory evoked potentials (SEPs) on electrophysiological testing 2
• Back-averaged potentials demonstrating cortical origin 2
• Enhanced long latency reflexes on neurophysiological studies 2
• Commonly occurs in patients with continuous cortical background activity on EEG 1

Subcortical Myoclonus Characteristics:

• Absence of EEG correlate - this is the key distinguishing feature 1
• Absence of enlarged SEPs, back-averaged potentials, and enhanced long latency reflexes 2
• Does not require aggressive treatment with antiseizure medications if not interfering with mechanical ventilation 1

Clinical Features That Suggest Cortical Origin

While EEG is definitive, certain clinical patterns suggest cortical myoclonus:

• Early age of onset 3
• Facial involvement 3
• Provocation by action (action myoclonus) 3
• Brief, lightning-like jerks with preserved consciousness 1

Cortical-Subcortical Myoclonus

A third category exists where cortical-subcortical myoclonus occurs in "lockstep" with epileptiform abnormalities such as burst suppression with identical bursts. 1 This represents myoclonic seizures with generalized spike-and-wave discharges on EEG, analogous to absence seizures. 2

Practical Diagnostic Approach

1. Perform EEG recording during myoclonic episodes to detect epileptiform activity 1, 4
2. Conduct electromyography (EMG) with back-averaging to identify cortical potentials preceding jerks 2
3. Measure somatosensory evoked potentials - enlargement indicates cortical origin 2
4. Assess long latency reflexes - enhancement suggests cortical involvement 2

Clinical Pitfalls

• Clinical diagnosis alone is accurate in only 78% of cases for myoclonus subtype 3, making electrophysiological testing essential for verification
• The distinction between cortical and subcortical forms lies on a spectrum, and some cases may have mixed features 2
• Functional jerks account for 47% of cases in tertiary centers and should be considered in the differential 3