What are the types of electrophysiology studies for diagnosing neuromuscular (NM) weakness?

From the Guidelines

Electrophysiological studies for neuromuscular weakness include repetitive nerve stimulation (RNS), electromyography (EMG), and nerve conduction studies (NCSs), which are essential diagnostic tools for evaluating patients with muscle weakness. These studies can help elucidate the etiology of muscle weakness and distinguish between different neuromuscular diseases, such as botulism, myasthenia gravis, and Guillain-Barré syndrome 1.

Types of Electrophysiological Studies

• Repetitive nerve stimulation (RNS) involves electrically stimulating a motor nerve at either low or high frequency and recording the response in the distal muscle, which can help detect neuromuscular junction disorders like myasthenia gravis by demonstrating decremental responses 1.
• Electromyography (EMG) involves inserting a needle electrode into a muscle and recording the electrical activity at rest and with effort, showing motor unit potentials or motor unit action potentials, and can help identify abnormal spontaneous activity, motor unit potential changes, and recruitment patterns 1.
• Nerve conduction studies (NCSs) involve providing an electrical stimulus to a nerve and recording the electrical response from a sensory nerve or muscle, which can help identify nerve damage, demyelination, or axonal loss 1.

Additional Specialized Tests

• Single-fiber EMG measures the variability in neuromuscular transmission and can be more sensitive than general EMG, but requires specialized training and equipment 1.
• Quantitative EMG provides detailed analysis of motor unit potentials and can help distinguish between neuropathic and myopathic causes of weakness 1.
• Blink reflex studies assess cranial nerve function, while somatosensory evoked potentials evaluate sensory pathways 1. These tests are typically performed by neurologists or physiatrists with specialized training in clinical neurophysiology and are crucial for guiding appropriate treatment strategies 1.

From the Research

Types of Electrophysiology Studies for Neuromuscular Weakness

• Nerve conduction studies (NCS) and electromyography (EMG) are useful adjuncts to clinical examination of the peripheral nervous system and striated skeletal muscle 2
• Repetitive nerve stimulation (RNS) can identify defects of neuromuscular junction (NMJ) transmission, pre- or post-synaptic 2, 3, 4
• Single-fiber electromyography (SFEMG) can be used to evaluate patients with suspected myasthenia gravis or Lambert-Eaton myasthenic syndrome 3, 5
• Needle EMG examination can detect myopathic changes in muscle and signs of denervation 2
• Combinations of these procedures can establish if motor and/or sensory nerve cell bodies or peripheral nerves are damaged, and also indicate if the primary target is the axon or the myelin sheath (i.e. axonal or demyelinating neuropathies) 2

Applications of Electrophysiology Studies

• Diagnosis of myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEMS) 3, 4, 5
• Evaluation of patients with suspected neuromuscular junction (NMJ) disorders 3, 4, 5
• Assessment of nerve damage and its distribution (generalized, multifocal, or focal) 2
• Diagnosis of other neuromuscular diseases, such as motor neuronopathy, sensory ganglionopathy, or neuropathy 2, 6

Characteristics of Electrophysiology Studies

• RNS has a sensitivity and specificity of 83.3% and 96.2%, respectively, for the diagnosis of MG in an inpatient setting 4
• SFEMG can be abnormal in a number of myasthenia mimics, and routine nerve conduction studies and electromyography should always be undertaken 5
• EMG continues to play an important role in the diagnosis and management of patients with neuromuscular diseases, even with the increase in molecular genetic diagnostics 6