From the Guidelines
Electrophysiological studies for neuromuscular weakness include nerve conduction studies (NCS), electromyography (EMG), repetitive nerve stimulation, single-fiber EMG, and quantitative EMG, which are essential diagnostic tools for evaluating patients with muscle weakness. These tests help identify nerve damage, demyelination, or axonal loss, and abnormalities in muscle function or nerve supply.
Types of Electrophysiological Studies
- Nerve conduction studies (NCS) measure how quickly and effectively nerves conduct electrical signals by placing electrodes on the skin and recording the speed and amplitude of nerve impulses.
- Electromyography (EMG) involves inserting small needles into muscles to record their electrical activity both at rest and during contraction, revealing abnormalities in muscle function or nerve supply.
- Repetitive nerve stimulation can detect neuromuscular junction disorders like myasthenia gravis by measuring decremental responses.
- Single-fiber EMG examines individual muscle fiber function and is particularly sensitive for neuromuscular junction disorders.
- Quantitative EMG analyzes motor unit potentials mathematically to detect subtle abnormalities, while motor unit number estimation assesses motor neuron loss in conditions like ALS, as discussed in 1.
Selection of Tests
These tests are typically performed by neurologists with specialized training in clinical neurophysiology and are selected based on the patient's clinical presentation, with results interpreted in the context of the patient's symptoms, examination findings, and medical history, as noted in 1.
Interpretation of Results
The results of these tests can help diagnose various neuromuscular disorders, including Guillain-Barré syndrome, myasthenia gravis, and amyotrophic lateral sclerosis (ALS), and guide treatment decisions, as discussed in 1.
From the Research
Types of Electrophysiology Studies for Neuromuscular Weakness
The following types of electrophysiology studies are used to diagnose and evaluate neuromuscular weakness:
- Nerve conduction studies (NCS) to quantify nerve conduction velocity and the amplitude of sensory nerve action potentials (SNAPs) and compound motor action potentials (cMAPs) 2
- Electromyography (EMG) to detect myopathic changes in muscle and signs of denervation 2
- Repetitive nerve stimulation (RNS) to identify defects of neuromuscular junction (NMJ) transmission, pre- or post-synaptic 2, 3, 4, 5
- Single-fiber electromyography (SFEMG) to measure jitter and impulse blocking in the diagnosis of neuromuscular junction disorders 3, 4, 5
- Needle EMG examination to detect myopathic changes in muscle and signs of denervation 2, 6
- Direct muscle stimulation to differentiate between critical illness myopathy (CIM) and critical illness polyneuropathy (CIP) 6
Specific Electrophysiology Studies for Neuromuscular Junction Disorders
For neuromuscular junction disorders such as myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS), the following studies are used:
- RNS to demonstrate a decrement in amplitude from the first to fourth or fifth intravolley waveform while stimulating at 2-5 Hz 3, 4, 5
- SFEMG to measure abnormal jitter or impulse blocking 3, 4, 5
- Concentric needle EMG as an alternative to SFEMG jitter in disorders of neuromuscular transmission 3
Electrophysiology Studies for Critical Illness
In critically ill patients, the following studies are used to evaluate neuromuscular weakness: