An Electromyography (EMG) study is a diagnostic procedure designed to evaluate the health of muscles and the motor neurons, which are the nerve cells that control them. This test provides a functional assessment of the body’s neuromuscular system. The resulting report contains specialized terminology and numerical data describing the electrical signals traveling through nerves and muscles. Understanding these technical elements allows a patient to better interpret findings regarding potential nerve or muscle dysfunction.
Baseline: Key Parameters of a Normal EMG Report
A typical EMG report measures several fundamental electrical characteristics, establishing a baseline for healthy nerve and muscle function. Latency represents the time delay, measured in milliseconds, between stimulating a nerve and recording the resulting electrical response in the muscle or nerve itself. A short latency indicates rapid signal transmission.
The amplitude of the response, measured in microvolts or millivolts, reflects the strength or size of the electrical signal. This value represents the number of nerve fibers or muscle fibers successfully activated. A robust signal with high amplitude suggests many healthy fibers are participating in the response.
Conduction velocity is the calculated speed, in meters per second, at which the electrical impulse travels along the nerve fiber between two points of stimulation. This measurement is derived from the distance between the stimulation points and the difference in the latencies recorded at those points. These numerical values are compared against established reference ranges.
The final baseline measurement involves the Motor Unit Potential (MUP), the electrical signal generated by a single motor neuron and all the muscle fibers it innervates. During a normal, slight muscle contraction, the MUP waveform should exhibit a specific size and shape. A healthy muscle at complete rest should display no MUP activity or spontaneous electrical signals.
Decoding Nerve Conduction Study (NCS) Results
The nerve conduction study (NCS) portion of the report uses surface electrodes to assess the function of the peripheral nerves by measuring the speed and strength of their signals. Abnormalities in the measured parameters provide distinct clues about the type of damage affecting the nerve fiber, distinguishing between damage to the nerve’s insulating layer and damage to the nerve fiber itself.
Demyelination is damage to the protective myelin sheath, which significantly impairs the speed of the electrical impulse. This damage is reflected by a marked increase in latency and a substantial decrease in conduction velocity.
In contrast, axonal loss refers to the destruction of the nerve fiber, or axon. Remaining healthy axons still conduct signals at a relatively normal speed, so conduction velocity remains preserved or only mildly slowed. However, because fewer nerve fibers are available, the recorded amplitude of the response drops significantly.
The NCS also includes specialized measures, such as the F-wave and H-reflex, which assess nerve segments closer to the spinal cord. For example, a prolonged F-wave latency can indicate generalized slowing of conduction in the proximal nerve roots. This comprehensive assessment helps localize the injury along the entire length of the nerve pathway.
Decoding Needle Electromyography (EMG) Results
The needle EMG involves inserting a fine needle electrode directly into various muscles to record their electrical behavior at rest and during contraction. This provides a direct assessment of muscle fiber health and the integrity of the nerve supply.
The muscle is first evaluated for spontaneous activity, which is the presence of electrical signals when the muscle is completely relaxed. Abnormal resting activity, such as fibrillations or positive sharp waves, indicates muscle denervation or fiber instability. These signals represent individual muscle fibers firing spontaneously after losing their nerve connection. Denervation findings typically appear ten to twenty-one days after an acute nerve injury.
During minimal voluntary contraction, the examiner analyzes the size and shape of the Motor Unit Potentials (MUPs). If the problem is neurogenic (originating from the nerve), remaining healthy motor neurons re-innervate orphaned muscle fibers. This process creates MUPs that are characteristically large in amplitude and long in duration.
Conversely, if the problem is myopathic (originating from the muscle itself), the muscle fibers within the motor unit are damaged or lost. This results in MUPs that are small in amplitude and short in duration.
The final component is the recruitment pattern at maximal contraction. A reduced number of MUPs firing rapidly suggests nerve damage, while many small MUPs firing quickly suggests muscle disease.
Identifying Patterns of Neuromuscular Disease
Interpreting an EMG report requires synthesizing findings from both the NCS and the needle EMG to categorize the underlying disorder.
Peripheral Neuropathy
This pattern typically shows generalized abnormalities in the NCS, such as reduced amplitudes (axonal damage) or slowed conduction velocities (demyelination). The needle EMG shows denervation and reinnervation changes, usually most pronounced in muscles furthest from the spinal cord.
Radiculopathy
Radiculopathy, or nerve root compression near the spine, often spares sensory nerves where they exit the spinal cord. The report may show normal sensory NCS results. The needle EMG will demonstrate spontaneous activity and neurogenic MUP changes in the muscles supplied by the affected nerve root. Motor NCS results may be normal or show reduced amplitude if motor axons are significantly damaged.
Myopathy
A Myopathy, or primary muscle disease, is characterized by largely normal NCS results, as the nerves are generally unaffected. The needle EMG shows signs of muscle fiber breakdown, including small, short-duration MUPs with early and rapid recruitment. Spontaneous activity, such as fibrillation potentials, supports the diagnosis of an active muscle disorder.