Approach to Investigating Myoclonus
The investigation of myoclonus requires a systematic approach that begins with anatomical classification, followed by targeted diagnostic testing including EEG, with particular attention to distinguishing epileptic from non-epileptic myoclonus. 1
Anatomical Classification
First, determine the anatomical origin of myoclonus, which guides both investigation and treatment:
- Cortical myoclonus: Brief, stimulus-sensitive jerks affecting distal limbs
- Subcortical myoclonus: Generalized jerks, often rhythmic or semi-rhythmic
- Spinal myoclonus: Segmental, rhythmic jerks that persist during sleep
- Peripheral myoclonus: Focal jerks in specific muscle groups
Key Diagnostic Steps
1. Clinical Characterization
- Document temporal pattern: at rest, with posture maintenance, or during action 2
- Note distribution of jerks (focal, multifocal, generalized)
- Assess for stimulus sensitivity (light, sound, touch)
- Determine if myoclonus is positive (muscle contraction) or negative (brief lapses in muscle tone) 2
2. Electrophysiological Testing
- EEG: Essential to determine if myoclonus has an epileptiform correlate 1
- Continuous EEG monitoring: Recommended for suspected status myoclonus 1
- EMG: To confirm myoclonus and distinguish from other movement disorders
- EEG-EMG back-averaging: To identify cortical correlates of myoclonus 3
3. Neuroimaging
- MRI brain to identify structural lesions, particularly for cortical myoclonus
- Functional neuroimaging (PET/SPECT) in selected cases
4. Laboratory Testing
- Basic metabolic panel, liver function, renal function
- Drug and toxin screens
- Specific testing based on clinical suspicion:
- Autoimmune panels (paraneoplastic antibodies)
- Metabolic testing (lactate, pyruvate, amino acids)
- Genetic testing for hereditary forms 3
Special Considerations
Post-Cardiac Arrest Myoclonus
- Distinguish between isolated myoclonus and status myoclonus
- Status myoclonus (continuous jerking >30 minutes) within 72 hours post-arrest has high specificity (99.8-100%) for poor neurological outcome 4
- Isolated myoclonus has lower specificity (77.8-100%) for poor outcome 4
- EEG is crucial to detect associated epileptiform activity 4, 1
Genetic Myoclonus
- Consider genetic testing when:
- Positive family history
- Early onset
- Associated features suggesting genetic syndrome
- Next-generation sequencing is increasingly used for diagnosis 3
Treatment Considerations
While not the focus of investigation, treatment planning should be considered:
- Cortical myoclonus: Valproate, levetiracetam, piracetam 2
- Subcortical/brainstem myoclonus: Clonazepam as first-line 1
- Post-anoxic myoclonus: Levetiracetam particularly effective 1
- Status myoclonus: Requires aggressive treatment 1
- Focal/segmental myoclonus: Consider botulinum toxin injections 2
Common Pitfalls to Avoid
- Failing to distinguish myoclonus from other movement disorders
- Not recording EEG during myoclonic episodes
- Overlooking treatable causes of myoclonus
- Using phenytoin or carbamazepine, which may worsen cortical myoclonus 1
- Relying on single agents when polytherapy is usually required 2
By following this systematic approach, clinicians can efficiently diagnose the cause of myoclonus and develop appropriate management strategies.