Q 1.33. Electromyography (EMG)

Electromyography (EMG) is a diagnostic procedure used to assess the health of muscles and the nerve cells that control them, known as motor neurons. As medical students, understanding EMG is crucial for evaluating neuromuscular abnormalities.

Here are the key points about EMG:

1. Procedure:

   • During EMG, one or more small needles (also called electrodes) are inserted through the skin into specific muscles.
   • Electrical activity is recorded from these muscles while they are at rest and during contraction.

2. Normal Muscle Activity:

   • Resting muscle is electrically silent.
   • With minimal contraction, action potentials of single motor units appear.
   • As contraction increases, the number of muscle action potentials increases, forming an interference pattern.

3. Abnormal Findings:

   • Denervated muscle fibers show increased activity with needle insertion and abnormal spontaneous activity (fibrillations and fasciculations).
   • Fewer motor units are recruited during contraction in denervated muscles, resulting in a reduced interference pattern.
   • Surviving axons may branch to innervate adjacent muscle fibers, leading to giant muscle action potentials.

4. Nerve Conduction Studies (NCS):

   • EMG is often combined with nerve conduction studies.
   • In NCS, peripheral nerves are stimulated with electrical shocks at various points along their course to a muscle.
   • The time an impulse takes to traverse a measured length of nerve determines conduction velocity.
   • Slower conduction occurs when larger myelinated fibers are damaged.
   • In neuropathy, conduction may be slowed, and the response pattern may show dispersion of action potentials due to unequal involvement of myelinated and unmyelinated nerve fibers.

5. Clinical Applications:

   • EMG assesses muscle and nerve function by recording electrical activity in muscles. EMG helps differentiate whether weakness is due to a nerve, muscle, or neuromuscular junction disorder.  Detects abnormal muscle activity (fibrillations, fasciculations) associated with conditions like amyotrophic lateral sclerosis (ALS). Differentiates between myopathic (muscle-related) and neuropathic (nerve-related) disorders. Useful in radiculopathies, plexopathies, and myopathies.
   • NCS evaluates the function of peripheral nerves by measuring their ability to conduct electrical impulses. NCS helps identify nerve damage or disease, such as carpal tunnel syndrome, diabetic neuropathy, or radiculopathies. By assessing specific nerves, it helps pinpoint the site of nerve dysfunction (proximal or distal). NCS provides information on nerve conduction velocities (CV), amplitudes, and latencies. Useful for tracking changes over time during treatment or disease progression.

Remember, EMG provides valuable insights into the functioning of muscles and nerves, allowing clinicians to identify and manage various neuromuscular conditions.

References:

1 msdmanuals.com

2 mayoclinic.org

3 hopkinsmedicine.org

4 springer.com