What is the meaning of decreased compound muscle action potential (CMAP) amplitude with increased duration on electromyography (EMG)?

From the Guidelines

Decreased compound muscle action potential (CMAP) amplitude with increased duration on electromyography (EMG) is most indicative of an axonal neuropathy, as suggested by the highest quality and most recent study available 1. This pattern suggests that there is damage to the nerve axons themselves rather than to the myelin sheath surrounding the nerves. When axons are damaged or lost, fewer muscle fibers can be activated, resulting in lower amplitude responses. The increased duration occurs because the remaining nerve fibers conduct at varying speeds due to partial damage, causing a more dispersed electrical signal.

Key Points to Consider

• The severity of amplitude reduction often correlates with clinical weakness, while the increased duration reflects the chronic nature of the process and attempted reinnervation.
• This pattern is commonly seen in conditions such as diabetic neuropathy, toxic neuropathies (from medications like chemotherapy agents), vasculitic neuropathies, and certain hereditary neuropathies.
• It differs from demyelinating neuropathies, which typically show normal or mildly reduced amplitudes with significantly prolonged durations and conduction velocities, as noted in the earlier study 1.
• Treatment depends on addressing the underlying cause, such as improving glycemic control in diabetes or removing toxic exposures, along with symptomatic management of neuropathic pain if present.

Clinical Implications

• The identification of fibrillation potentials and positive sharp waves, which are associated with denervation, can be crucial in the diagnosis of axonal neuropathy, as discussed in the study on laryngeal electromyography 1.
• Complex repetitive discharges (CRD) and fasciculations are also important findings that can indicate chronic myopathies and neuropathies or motor neuron diseases.
• The presence of polyphasic MUAPs with multiple baseline crossings can indicate reinnervation attempts by intact neighboring axons, highlighting the dynamic nature of neuropathic damage and recovery.

From the Research

Decreased CMAP Amplitude with Increased Duration EMG Meaning

• A decreased compound muscle action potential (CMAP) amplitude with increased duration on an electromyography (EMG) can indicate various neuromuscular conditions, including critical illness neuromyopathy (CINM) 2 and demyelinating neuropathies 3.
• In CINM, prolonged CMAP durations and severely reduced CMAP amplitudes are common findings, which can be highly specific for the diagnosis of CINM if encountered in more than one nerve 2.
• Demyelinating neuropathies, such as chronic inflammatory demyelinating polyneuropathy (CIDP), can also present with prolonged distal CMAP durations, which can be a useful index for detecting distal demyelination 3.
• The CMAP is a summated muscle fiber action potential recorded from a surface electrode overlying the studied muscle, and its amplitude reflects integrated function of the motor axons, neuromuscular junction, and striated muscle 4, 5.
• Needle EMG findings, such as short-duration motor unit potentials and fibrillation potentials, can also help confirm myopathy and indicate specific pathologic changes on muscle biopsy 6.
• The combination of CMAP and EMG findings can provide valuable information for diagnosing and managing various neuromuscular disorders, including axonal and demyelinating neuropathies, myopathies, and motor neuron diseases 4, 5.