Needle electromyography (EMG)
This is an invasive procedure in which a special needle electrode is inserted directly into the muscle belly. The device records the activity of the motor unit (MU), which consists of a spinal motor neuron, its axon, and the group of muscle fibers it innervates.
The signal on the monitor is formed by the spatiotemporal summation of electrical discharges from individual muscle cells, producing the motor unit action potential (MUAP). Normally, a relaxed muscle is electrically silent.
In pathology, the pattern changes:
- Spontaneous activity at rest: the appearance of fibrillations, positive sharp waves, or fasciculations indicates denervation.
- Neurogenic lesion (motor neuron involvement): due to the death of some nerve cells, the surviving ones take over the innervation of the "orphaned" fibers (sprouting). This leads to the formation of high-amplitude, wide "giant" MUAPs.
- Myopathic lesion (primary muscle disease): the number of functional fibers within the MU decreases, so EMG reveals numerous small potentials of reduced duration and amplitude.
Nerve conduction electromyography
The method is based on percutaneous electrical stimulation of a nerve followed by recording the response in the innervated muscle (M-response) or directly on the nerve. It measures nerve conduction velocity (NCV) and assesses response amplitude.
Normal values for motor nerves:
- Upper extremity NCV: above 50 m/s.
- Lower extremity NCV: above 40 m/s.
- M-response amplitude: above 3.5 mV (for the ulnar nerve — from 6 mV).
Differential diagnosis of lesions:
- Axonopathy: a sharp drop in M-response amplitude (due to axonal loss), while NCV remains normal.
- Myelinopathy: a marked decrease in conduction velocity due to destruction of the insulation (myelin), with normal amplitude.
- Conduction block: a drop in amplitude by more than 25% between two stimulation sites along the same nerve (commonly seen in entrapment syndromes).
A distinct technique is repetitive nerve stimulation (decrement test) used to evaluate the neuromuscular junction. Normally, delivering 5 stimuli at a frequency of 3 Hz causes no change in response amplitude. In myasthenia gravis, junctional exhaustion occurs, and the amplitude of the final response drops by more than 10% (positive decrement test).
Transcranial magnetic stimulation (TMS)
A device with a magnetic coil generates a varying magnetic field over the motor cortex projection. This method painlessly activates central motor neurons (in the frontal and parietal lobes), the descending corticospinal tract, and peripheral motor neurons. The response is recorded from the limb muscles.
The primary goal of TMS is to detect latent or overt upper motor neuron involvement. This is particularly relevant when suspecting multiple sclerosis or amyotrophic lateral sclerosis. Indicators of pyramidal tract pathology include prolonged latency, decreased response amplitude, increased response threshold, and potential dispersion.
Electroencephalography (EEG)
EEG records the wave-like bioelectrical activity of brain neurons (from 0.5 to 30 Hz) using electrodes placed on the scalp. The study is performed in a soundproof room while the patient is in a relaxed, awake state with eyes closed.
Main normal rhythms:
- Alpha rhythm: 8–12 Hz, amplitude up to 100 µV. Dominates in normal states, best seen in occipital leads. Disappears upon eye opening.
- Beta rhythm: 14–40 Hz, up to 25 µV. Dominates in anterior (frontal and central) leads.
Slow waves (theta 4–7 Hz and delta 0.5–3 Hz) in the waking state with an amplitude exceeding 40 µV are considered pathological.
Functional tests are used to reveal hidden abnormalities: eye opening/closing, photic stimulation, and hyperventilation. In generalized absence seizures, the EEG shows a classic epileptic pattern — synchronous spike-and-slow-wave discharges (at about 3 Hz and up to 200 µV in amplitude).
Briefly: answers to topical questions
What central nervous system disorders can be diagnosed using TMS?
Transcranial magnetic stimulation diagnoses lesions of the corticospinal (pyramidal) pathway of various origins. The method is especially valuable when the clinical picture is ambiguous in the following conditions:
- multiple sclerosis;
- amyotrophic lateral sclerosis;
- degenerative and dysmetabolic brain and spinal cord diseases;
- vascular central nervous system lesions;
- oncological central nervous system lesions.
How is conduction velocity in sensory nerve fibers evaluated?
Sensory nerve conduction velocity is assessed by percutaneous nerve stimulation and recording the resulting sensory nerve action potentials. Stimulating electrodes are placed on the skin over the nerve's course.
There are two recording methods:
- Orthodromic method — recording is performed proximally relative to the stimulation sites.
- Antidromic method — recording is performed distally relative to the stimulation sites.
Normally, conduction velocity for the ulnar and median nerves exceeds 50 m/s, and for the sural nerve exceeds 40 m/s. Reduced velocity indicates myelin sheath damage.
Which EEG rhythms are recorded in healthy individuals during different sleep stages?
During various sleep stages, specific rhythms and patterns of bioelectrical activity are recorded on the electroencephalogram.
- Stage II of slow-wave sleep — characterized by the appearance of sleep spindles.
- Stages III–IV of slow-wave sleep — high-amplitude delta rhythm is recorded (delta sleep).
- Rapid eye movement (REM) sleep — the trace flattens, low-amplitude rhythmic activity and fast rhythms appear.
The paradoxical EEG phase is a necessary preparation for normal active waking.
For nerve conduction EMG interpretation, remember: "Myelin is speed, axon is strength (amplitude)." If NCV drops, the myelin sheath is affected; if the M-response amplitude decreases, the axon is damaged.
Class and exam questions
What is the main limitation of electrophysiological methods?
In most cases, they lack nosological specificity. That is, the studies indicate the level, nature (axon/myelin), and degree of the lesion, but do not provide a precise diagnosis.
What EMG changes are characteristic of anterior horn cell lesions of the spinal cord?
Needle EMG at rest reveals spontaneous activity (fibrillations, fasciculations), while voluntary contraction shows giant high-amplitude motor unit potentials.
What are the normal values for motor nerve conduction velocity?
For upper limb nerves, the normal value is above 50 m/s; for lower limb nerves, it is above 40 m/s.
What else to study on this topic
- Topical diagnosis of PNS lesions: differentiating root, plexus, and peripheral nerve levels
- Muscle reinnervation mechanisms and the formation of giant MUAPs
- Orthodromic and antidromic methods for assessing sensory nerve fibers
- Analysis of functional tests on EEG and their role in detecting epileptiform activity
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