Electromyography (EMG) is a diagnostic procedure that measures the electrical activity of muscles in response to nerve stimulation. It evaluates the health of muscles and their controlling nerve cells, called motor neurons. The test’s purpose is to identify neuromuscular abnormalities, which are conditions affecting nerves and muscles.
What an EMG Test Measures
An EMG study typically involves two main parts: a nerve conduction study (NCS) and needle electromyography. The nerve conduction study uses surface electrodes placed on the skin over specific nerves to measure how quickly and strongly electrical signals travel along a nerve. This part of the test assesses nerve function.
Needle EMG involves inserting a thin needle electrode directly into various muscles. This records the electrical activity of muscle fibers, both at rest and during voluntary contraction. The test helps identify issues with nerve dysfunction, muscle dysfunction, or problems in the communication pathway between nerves and muscles.
Understanding Normal EMG Findings
In a normal nerve conduction study, electrical signals travel along nerves at expected velocities and with sufficient strength, known as amplitude. Signal speed varies by nerve but falls within established normal ranges. Amplitude reflects the number of nerve fibers conducting the signal.
With needle EMG, a healthy muscle typically shows no electrical activity at rest, appearing “silent.” During a slight voluntary contraction, individual motor units fire, producing characteristic electrical signals called motor unit action potentials (MUAPs). As contraction increases, more motor units are recruited, and their electrical activity becomes dense and overlapping, indicating normal muscle function and nerve supply.
Interpreting Abnormal EMG Readings
Deviations from normal findings signal issues affecting nerves, muscles, or their junction. In nerve problems (neuropathies), nerve conduction studies may show slowed velocities, indicating myelin sheath damage (demyelination). Reduced signal amplitudes suggest nerve fiber loss (axonal loss). Prolonged latencies, measuring signal travel time from stimulation to muscle response, also point to nerve damage. Conditions like carpal tunnel syndrome often show slowed conduction and prolonged latencies, and peripheral neuropathy can similarly show these changes across multiple nerves.
Muscle problems (myopathies) are typically identified through needle EMG. Abnormal muscle electrical activity at rest, such as spontaneous firing of individual muscle fibers (fibrillations) or positive sharp waves, indicates muscle fiber instability or damage. During voluntary contraction, motor unit action potentials may appear small and polyphasic, meaning they have an unusual shape with multiple phases, reflecting unsynchronized firing due to muscle disease. Muscular dystrophies and inflammatory myopathies are common conditions causing such activity.
Problems at the neuromuscular junction, the specialized synapse where nerve and muscle communicate, can also be detected. These issues may manifest as variability in the size and shape of motor unit action potentials with repetitive stimulation, indicating inconsistent signal transmission. Myasthenia Gravis, an autoimmune disorder affecting this junction, is an example where such findings are observed. Interpreting these abnormal readings requires a comprehensive understanding of the entire EMG study, along with the patient’s clinical history and symptoms.
Factors Influencing EMG Results
Several factors can influence EMG readings, which clinicians consider during interpretation. Technical elements like precise electrode placement and equipment calibration are important. Skin temperature significantly affects nerve conduction velocity; colder temperatures slow nerve signals, so warming the limb might be necessary before testing.
Patient-specific factors also play a role. Age can introduce normal variations in nerve conduction parameters, with slight velocity decreases in older individuals. Certain medications can impact nerve and muscle function, potentially altering electrical signals. Pre-existing conditions, such as diabetes, can affect nerve health and lead to abnormal findings that must be differentiated from the symptoms being investigated.