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.
- 6–19 electrodes are symmetrically placed on the scalp and the electrical activity is measured
- The potential difference between two electrodes is measured and represented as line tracing.
- Epilepsy diagnostics: for first seizures with unclear cause, insufficient classification, or treatment-refractory seizures (see “” and “”)
- To determine the level of consciousness (bispectral index to determine the depth of anesthesia; diagnosis of brain death)
- As a part of (to identify the different )
- Interpretation: The following steps should be followed when interpreting an EEG:
|Physiological EEG waveforms|
|Wave type||Frequency||Stage of onset|
|Alpha waves||8–12 Hz|
|Beta waves||13–30 Hz|| |
|Gamma waves||> 30 Hz|| |
|Delta waves (EEG)||0.1–4 Hz|| |
|Theta waves||4–8 Hz|| |
|Physiological EEG patterns|
|Vertex waves (V waves)|| || |
|Sleep spindles|| || |
| || |
|POSTS (positive occipital sharp transients of sleep)|| || |
Pathological EEG patterns
|Slow activity|| || |
|Paroxysmal discharge|| || |
|Sharp wave|| || |
|Spike (EEG)|| || |
|Spike-and-wave activity|| || |
| || |
|Polyspike and polyspike-wave complex|
|Periodic sharp wave complex|| || |
- 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)|| |
|Brainstem auditory evoked potential (AEP)|| |
|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
- Definition: Recording of a muscle's compound action potentials (CMAP) after stimulation of its innervating motor nerve.
- Nerve conduction velocity: ↓ nerve conduction velocity indicates nerve demyelination and/or nerve compression (e.g., carpal tunnel syndrome - median nerve compressed in flexor retinaculum)
- Amplitude of the CMAPs
- Distal motor latency: time period from stimulation of the motor nerve to action potential
- F-wave latency: a second action potential generated by the antidromic (proximal) conduction of the nerve that was stimulated
- Definition: A diagnostic test that measures the activity of muscles in response to neural stimulation, in order to detect dysfunction at the level of the neuromuscular junction, the muscle, or the corresponding nerve.
- Differentiate neuropathic from myopathic muscle weakness
- Determine the prognosis after nerve damage
- Estimate the age of a nerve lesion
- Diagnose sub-clinical myopathies
Localize neurological injury
- Motor neurons in the brain or spinal cord: e.g., ALS, poliomyelitis
- Nerve root: e.g., herniated disk
- Peripheral nerves: e.g., carpal tunnel syndrome, Guillain-Barré syndrome, other polyneuropathies
- Neuromuscular junction: e.g., myasthenia gravis, Lambert-Eaton myasthenic syndrome, botulism
- Muscle: e.g., inflammatory myopathies, myotonic syndromes, muscular dystrophies
- Needle electrodes are inserted into a muscle.
- The electrical activity of the muscle is measured during rest (tonic activity) and during movements (phasic muscle activity).
|EMG findings of neuropathy vs. 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|