Definition and Overview
Electromyography (EMG) is a test that records and analyzes muscle electrical activity at rest and at various contraction strengths by inserting a disposable concentric needle electrode into the muscle.
It is performed together with nerve conduction testing (NCS) and constitutes an 'electrodiagnostic study', and plays a key role in the differential diagnosis of peripheral nervous system and muscle diseases.
inspection process
Insertional and spontaneous activity
Electrical activity is observed during needle insertion and when the muscle is fully relaxed.
- insertional activity: Electrical activity that occurs when a needle is inserted or moved. Increase or decrease has pathological significance.
- Normal: no electrical activity at rest (electrical silence)
- Abnormal spontaneous potentials: a finding indicative of denervation
The types of abnormal spontaneous potentials are as follows.
- Fibrillation potential: Voluntary contraction of individual denervated muscle fibers. Regular and biphasic waveforms
- Positive sharp wave: Same meaning as fibrillation potential. Characteristic initial positive deflection
- myotonic discharge: “diving bomb” sound. Characteristic in muscular rigidity
- Fasciculation potential: Voluntary contraction of an entire motor unit. Significance in motor neuron disease
Voluntary motor unit analysis
The motor unit potential (MUP) is analyzed as the patient gradually contracts the muscles.
The measurement indicators are as follows. - MUP amplitude: reflects the number of muscle fibers within the motor unit - MUP duration: reflects the area of the motor unit - MUP polyphasic rate: increases during reinnervation or myopathy. - Recruitment pattern: number of motor units activated and firing frequency
Interpretation of results
Neurogenic pattern
When motor nerves are damaged, spontaneous potentials appear in denervated muscle fibers, and surviving motor nerves expand motor units through reinnervation.
- At rest: Fibrillation potential, positive sharp wave (+)
- MUP changes: increased amplitude, increased duration, increased polyphasic (giant MUP)
- Recruitment pattern: reduced recruitment — fewer motor units are activated, and the firing frequency of individual motor units increases.
- Representative diseases: radiculopathy, peripheral nerve damage, motor neuron disease (ALS)
Myopathic pattern
When the muscle fiber itself is damaged, the size of the motor unit is reduced.
- At rest: Normal in most cases (spontaneous displacement possible in some inflammatory myopathies)
- MUP changes: decreased amplitude, decreased duration, increased polyphasic (small polyphasic MUP)
- Recruitment pattern: early recruitment—large numbers of small motor units are recruited during weak contractions
- Representative diseases: polymyositis, dermatomyositis, muscular dystrophy, metabolic myopathy
Neuromuscular junction lesions
It is evaluated using repetitive nerve stimulation (RNS) and single fiber electromyography (SFEMG). In myasthenia gravis, a decrease in CMAP amplitude is characteristic during low-frequency (2-3 Hz) repetitive stimulation.
clinical application
Radiculopathy
Electromyography plays a complementary role to imaging (MRI) in diagnosing radiculopathy.
- Denervation is confirmed in the distribution of muscles controlled by the corresponding nerve root.
- The finding of denervation of the paraspinal muscles suggests that the lesion is proximal to the intervertebral foramen (nerve root level).
- It is useful in distinguishing clinically significant nerve root compression from anatomical abnormalities seen on MRI.
- The timing of the test is important because denervation is fully evident 3 to 4 weeks after symptom onset.
Motor neuron disease (ALS)
Extensive denervation and reinnervation are found in various parts of the body (brainstem, cervical spine, thoracic spine, and lumbosacral spine), and sensory nerve conduction tests are normal. Fasciculation is commonly observed. In the Awaji diagnostic criteria, EMG findings are recognized as objective evidence of clinical involvement.
myopathy
In inflammatory myopathies such as polymyositis and dermatomyositis, spontaneous potentials and myopathy patterns are observed during the active phase, and EMG is also used to select muscle biopsy sites.
traumatic nerve injury
Tracking the extent of denervation and progress of reinnervation after nerve injury helps determine the timing of surgical intervention.
Limitations of the test
- Invasive testing (needle insertion) causes discomfort to the patient.
- The experience and skill of the examiner (neurologist) affects the interpretation of the results.
- In the acute phase (within 1 to 2 weeks after symptom onset), denervation may not yet appear.
- Examination may be limited for deep muscles or muscles that are difficult to access.
- Because the patient's cooperation (voluntary muscle contraction) is required, it is limited in patients with impaired consciousness or severe pain.
Precautions before inspection
- No special fasting needed
- Can be performed in most cases even while taking anticoagulants (caution when testing deep muscle)
- Do not apply moisturizer or lotion to the test area.
- People with pacemakers must be notified before testing.