From the Guidelines
Electrodiagnostic studies can typically detect neuropathy approximately 2-3 weeks after the onset of nerve damage, as the pathophysiological changes take time to develop and become detectable 1. When considering the diagnosis of neuropathy using electrodiagnostic studies, it is essential to understand the timeframe required for detectable changes to occur.
- The process of Wallerian degeneration, where damaged axons break down, generally takes about 7-10 days to begin and continues over several weeks 1.
- For demyelinating neuropathies, conduction slowing may be detected earlier, while for axonal neuropathies, changes in amplitude and fibrillation potentials on EMG may take longer to appear 1.
- In some cases of mild neuropathy, changes might not be detectable until 4-6 weeks after symptom onset, emphasizing the importance of timely and repeated electrodiagnostic studies 1. The sensitivity and specificity of EMG, RNS, and NCSs for diagnosing neuropathy are crucial, and electrodiagnostic findings should always be considered in the context of clinical, epidemiologic, and laboratory data 1.
- Electrodiagnostic studies such as repetitive nerve stimulation (RNS), electromyography (EMG), and nerve conduction studies (NCSs) can help elucidate the etiology of muscle weakness 1.
- Distinctive classical findings of neuropathy include an increment in the compound motor nerve action potential amplitude, with RNS rates of 30–50 Hz; fibrillation; decreased recruitment of muscle units; decreased duration of muscle unit potentials with EMG; and decreased motor-evoked amplitude on an NCS with otherwise normal findings 1. It is vital to note that early in the disease course, electrodiagnostic studies might be normal or almost normal and therefore not helpful 1, and more focused EMG studies, such as single-fiber EMG with measurement jitter, might be more sensitive (although it is nonspecific) than the general EMG 1.
From the Research
Diagnosis of Neuropathy on Electrodiagnostic Studies
- Electrodiagnostic studies, including nerve conduction studies (NCS) and electromyography (EMG), are essential for diagnosing neuropathy 2, 3, 4, 5, 6
- NCS provide information on nerve conduction velocity and the amplitude of sensory and motor action potentials, helping to identify defects in neuromuscular junction transmission and detect myopathic changes in muscle 2
- EMG examination can detect signs of denervation and myopathic changes in muscle, and combinations of these procedures can establish the type and distribution of nerve damage 2, 4
- The pattern of findings on motor and sensory nerve conduction studies and needle EMG can help identify the type of neuropathy, define the underlying pathophysiology, and narrow the list of possible causes 3, 5
Types of Neuropathy
- Electrodiagnostic studies can help distinguish between primary axonal and demyelinating neuropathies 4, 5
- NCS and EMG can help categorize polyneuropathy as primary axonal versus primary demyelinating, which can narrow the differential diagnosis and increase the chances of finding a treatable etiology 5
Importance of Proper Interpretation
- Electrodiagnostic studies should be performed and interpreted by physicians with comprehensive knowledge of neurological and musculoskeletal disorders to assure accurate interpretation and diagnosis 6
- The same physician should directly supervise and interpret the NCSs, including those performed by an EDX technician, and have oversight over the design and performance of the study 6