EEG Voltage Classification
The standard voltage classifications for EEG amplitude are: low voltage (<20 microvolts), medium/normal voltage (20-200 microvolts), and high voltage (>200 microvolts), with clinical significance primarily at the extremes.
Voltage Thresholds
The available evidence provides specific microvolt cutoffs that define these categories:
- Low voltage: <20 microvolts of background activity 1
- Medium/Normal voltage: 20-200 microvolts (typical range for most EEG activity)
- High voltage: >200 microvolts, with pathological spikes ranging up to 350 microvolts 2
Clinical Context and Interpretation
Low Voltage EEG (<20 microvolts)
Low-voltage EEG activity defined as less than 20 microvolts without appearance of normal waveforms carries significant clinical implications. In acute encephalitis, this pattern predicts unfavorable neurological outcomes and intractable epilepsy 1. The 10-microvolt threshold is also used in automated detection algorithms to identify periods of extreme suppression, such as during burst suppression or postictal states 3.
Important Caveats
Low-voltage activity requires careful interpretation:
- Mental activation can produce physiologic low-voltage fast activity (desynchronization)
- Drowsiness also produces low-voltage patterns
- These states can be distinguished by the presence of slow horizontal eye movements (SEM), which indicate drowsiness rather than activation 4
Normal/Medium Voltage Range
The typical EEG operates within 20-200 microvolts 5. Normal posterior alpha rhythms and other physiologic patterns fall within this range. The specific amplitude varies by:
- Brain state (eyes open vs. closed)
- Frequency band (alpha, beta, theta, delta)
- Electrode location
High Voltage Activity
Amplitudes exceeding 200 microvolts are generally considered high voltage. Pathological epileptiform discharges (such as midline spikes) can reach 20-350 microvolts 2, with the upper range representing clearly abnormal, high-amplitude activity.
Practical Application
When interpreting EEG voltage:
- <10 microvolts: Extreme suppression, often pathological (burst suppression, severe encephalopathy)
- 10-20 microvolts: Low voltage, requires clinical correlation
- 20-200 microvolts: Normal operating range
- >200 microvolts: High voltage, often associated with epileptiform activity
The clinical significance depends heavily on the pattern, distribution, and clinical context rather than voltage alone.