What are the electroencephalogram (EEG) findings of a myoclonic seizure?

EEG Findings in Myoclonic Seizures

Myoclonic seizures are characterized on EEG by generalized spike-wave or polyspike-wave discharges that average >2.5 Hz for ≥10 seconds (>25 discharges in 10 seconds) or any pattern with definite evolution lasting ≥10 seconds. 1

Key EEG Characteristics

Primary Patterns

• Generalized spike-wave (SW) and polyspike-wave (PSW) discharges that predominate anteriorly 2
• Fast, irregular, generalized polyspike-waves associated with myoclonic jerks 2
• Rhythmic EEG spike potentials at approximately 20Hz during myoclonic jerks 2
• Bilateral-symmetrical time-locked cortical potentials that precede the myoclonic jerk 3

Timing and Distribution

• Discharges are typically diffuse or generalized 2
• Changes are most prominent at sleep onset and upon awakening 2
• Provoked awakenings are more likely to activate interictal paroxysmal abnormalities than spontaneous awakenings 2

Associated Features

• Normal background activity is typically present 2
• Photoparoxysmal response is common (present in approximately 30% of cases) 2
• Sleep deprivation is a valuable activating procedure to elicit these discharges 4

Special Diagnostic Techniques

• Jerk-locked back averaging technique: Particularly useful when standard EEG is normal; can identify bilateral-symmetrical time-locked cortical potentials that precede the myoclonic jerk, confirming cortical origin 3
• Video-EEG monitoring: Helps correlate clinical myoclonic jerks with EEG discharges 5
• Sleep and sleep deprivation: Important activation methods to increase diagnostic yield 2

Variations and Potential Confounding Patterns

• Some patients may show focal changes that can lead to misdiagnosis of partial seizures 6
• In juvenile myoclonic epilepsy, EEG may occasionally show non-specific changes 2
• In myoclonic-astatic epilepsy, the spike-and-wave morphology may be characterized by a positive-negative-deep-positive wave followed by a large negative slow wave 5

Clinical Correlation

• Myoclonic jerks appear to be associated with rhythmic EEG spike potentials at frequencies in the range of movement-related fast sensorimotor cortex physiological rhythms (around 20Hz) 2
• The intensity of atonia in myoclonic-atonic seizures may correspond to the depth of the positive component of the spike-and-wave complexes 5
• Paired transcranial magnetic stimulation (TMS) studies show defective intracortical inhibition due to impaired GABA-A mediated mechanisms 2

Diagnostic Pitfalls

• Normal scalp EEG does not rule out myoclonic epilepsy; additional electrophysiological techniques such as jerk-locked back averaging may be necessary 3
• Focal EEG abnormalities can occur in generalized myoclonic epilepsy and may lead to misdiagnosis 6
• Proper identification requires skilled interpretation of video-EEG recordings 1

Correct categorization of EEG findings in myoclonic seizures is essential for accurate diagnosis and appropriate treatment selection, ultimately affecting patient morbidity and mortality.