Neurophysiological diagnostics

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
  • Epilepsy diagnostics: for first seizures with unclear cause, insufficient classification, or treatment-refractory seizures (see “Epileptic syndromes” and “Seizure disorders”)
    • The absence of characteristic epilepsy findings cannot, however, rule out epilepsy.
  • To determine the level of consciousness (bispectral index to determine the depth of anesthesia; diagnosis of brain death)
  • As a part of polysomnography (to identify the different phases of sleep)
• 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.

EEG patterns

• 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.

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.
• Measured parameters
  • 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
    • Indirectly represents the area under the curve of the CMAP, which represents the summation of all axons of the motor nerve that was stimulated.
    • ↓ CMAP amplitude → axonopathy or temporal dispersion due to axonal injury
  • 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.
• Indications
  • Differentiate neuropathic from myopathic muscle weakness
    • Neuropathy: e.g., carpal tunnel syndrome, Guillain-Barré syndrome
    • Myopathy: e.g., polymyositis, muscular dystrophy
  • 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
• Procedure:
  1. Needle electrodes are inserted into a muscle.
  2. The electrical activity of the muscle is measured during rest (tonic activity) and during movements (phasic muscle activity).
• Interpretation