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Neurophysiological diagnostics

Last updated: August 27, 2021

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Neurophysiological tests complement neurological examination and conventional imaging (e.g., CT, MRI) when assessing nerve, muscle, and/or brain function. The most commonly used neurophysiological test is electroencephalography (EEG) which measures fluctuation of electric potential at different regions of the cortex. EEG is often used to assess epilepsy and sleep disorders. Evoked potentials (EP) are microvolt fluctuations of the CNS in response to stimulation of sensory organs or a peripheral nerve. These fluctuations can be recorded by an EEG and may be used to detect demyelination of white matter (e.g., multiple sclerosis). Nerve conduction studies (NCS) assess the conduction of nerve impulses through peripheral nerves and are used to specify the type of nerve damage (demyelination, compression or transection of a nerve). Electromyography (EMG) measures the electrical activity of muscles at rest and during contraction. EMG is helpful in differentiating neuropathies from myopathies.

  • Definition: a recording that shows the fluctuation of net electrical potential at different points on the cerebral cortex
  • Principle: Every neuron generates an electrical potential irrespective of the degree of neuronal firing. EEG electrodes add up these electrical potentials at different points, which allows visualization of neuronal activity of different areas of the cortex. The entire EEG recording is then examined for physiological or pathological patterns of electrical activity.
  • Procedure
    1. 6–19 electrodes are symmetrically placed on the scalp and the electrical activity is measured
    2. The potential difference between two electrodes is measured and represented as line tracing.
  • Indications
  • Interpretation: The following steps should be followed when interpreting an EEG:
    1. Define the background electrical activity of the brain. This baseline EEG serves as a control.
    2. Look for physiological or pathological patterns of electrical activity (see “EEG patterns”).
    3. Localize the pathology by identifying the EEG tracing associated with abnormal electrical activity.
Physiological EEG waveforms
Wave type Frequency Stage of onset
Alpha waves 8–12 Hz
  • Awake but eyes are closed
  • Berger effect: a physiological phenomenon in which alpha wave activity decreases when a person opens his/her eyes or concentrates (transition into beta waves)
Beta waves 13–30 Hz
  • Awake and attentive with open eyes
  • REM sleep
Gamma waves > 30 Hz
  • Wide-awake and concentrating
Delta waves (EEG) 0.1–4 Hz
  • Stage IV of NREM sleep
Theta waves 4–8 Hz

EEG patterns

Physiological EEG patterns
Patterns Characteristics Occurrence
Vertex waves (V waves)
  • Rapidly-rising, triphasic, positive, low-amplitude waveforms
  • Mostly bilateral and symmetric; best seen at the vertex
Sleep spindles
  • Pattern of low-amplitude waves that occur in rapid succession with a frequency of approximately 13 Hz

K complex

POSTS (positive occipital sharp transients of sleep)
  • Sharp, occipital, positive waves (may be isolated or grouped together with a frequency of 4–5 Hz)

Pathological EEG patterns

Patterns Characteristics Occurrence
Slow activity
  • Background (baseline) activity with a frequency of < 8 Hz in an awake adult.
  • A nonspecific sign of disturbed brain function
Paroxysmal discharge
  • A general term used to describe any pattern of electrical activity which begins suddenly and stands out from the background electrical activity of the brain (e.g., spikes, sharp waves, etc.)

  • An epileptiform discharge
Sharp wave
  • Rapidly rising wave between 70 and 200 ms
Spike (EEG)
  • A steeply sloped peak < 70 ms
Spike-and-wave activity
  • A spike followed by a slow wave
  • Frequency of 3/s
  • Lennox-Gastaut syndrome
Polyspike and polyspike-wave complex
Periodic sharp wave complex
  • Waveform with a frequency of 1–2 Hz
Hypsarrhythmia
  • Generalized, irregular, slow waveforms interspersed with multifocal polymorphic spikes

  • Definition: the electrical response of the CNS (as measured by an EEG) to stimulation of sensory organs or a peripheral nerve
  • Interpretation: Neurological diseases increase the latency and/or decrease the amplitude of the EP.
Types of evoked potential Stimulus Indication
Visual evoked potential (VEP)
  • Light impulse
  • Checkered patterns
Brainstem auditory evoked potential (AEP)
  • Auditory stimuli (delivered via headphones)
Somatosensory evoked potential (SEP)
Motor evoked potential (MEP)

A direct electrical stimulus is applied to a motor nerve (motor nerve conduction study) and/or sensory nerve (sensory nerve conduction study) via surface electrodes at two or more points, and various parameters related to compound action potentials (CMAPs) are measured.

Sensory nerve conduction study

  • Definition: recording of a purely sensory portion of a nerve in response to electrical nerve stimulation
  • Indication: to determine whether sensory symptoms arise proximal or distal to the dorsal root ganglia

Motor nerve conduction studies

Findings Neuropathy Myopathy
Electrical activity at rest Increased activity with needle insertion and pathological spontaneous discharge with fibrillations and fasciculations Little or no spontaneous electrical activity
Motor unit potential Large-amplitude, polyphasic, and prolonged motor unit potential Low-amplitude, polyphasic, and shortened motor unit potentials
Interference pattern Reduced interference pattern: muscle discharges have a low frequency but a large amplitude Full interference pattern: muscle discharges have a high frequency but a small amplitude