A Nerve Conduction Study (NCS) measures the electrical response of peripheral nerves, which are the communication lines extending from your spinal cord to your limbs. This test involves applying small electrical impulses to a nerve and recording how the nerve and the muscle it controls react. The purpose of an NCS is to objectively assess the function of motor and sensory nerves to diagnose conditions that affect them, such as nerve entrapments or generalized nerve damage. The report provides quantifiable data that helps physicians understand the location, type, and severity of a peripheral nerve disorder.
Navigating the Report Layout and Abbreviations
NCS reports are organized to separate the nerves tested and the types of nerve fibers examined. You will find distinct sections for motor and sensory nerve conduction studies, often listing results side-by-side for comparison with established normal ranges. Report columns detail the Nerve Tested, the Site of stimulation, the distance measured, and the specific numerical results.
Understanding the abbreviations is the first step in decoding the report’s language. A CMAP (Compound Muscle Action Potential) refers to the electrical response recorded from a muscle after stimulating its motor nerve, reflecting the health of the motor fibers. Conversely, a SNAP (Sensory Nerve Action Potential) is the response recorded directly from a sensory nerve, indicating the function of sensory fibers.
The report often includes F-wave and H-reflex studies, which are known as “late responses.” The F-wave assesses the entire length of a motor nerve from the limb to the spinal cord and back, which is useful for evaluating more proximal nerve segments. The H-reflex, most commonly tested in the leg, is a reflex arc that evaluates both sensory and motor pathways, providing insight into nerve root function near the spine. These specialized measurements help localize damage that standard nerve segment testing might miss.
The Three Core Numerical Measurements
Every NCS report centers on three numerical measurements that describe how efficiently the nerve transmits an electrical signal. Latency measures the time (in milliseconds) it takes for the electrical impulse to travel from the point of stimulation to the recording electrode. This time primarily reflects the speed of the fastest conducting nerve fibers. A prolonged latency—a number higher than the normal range—suggests that the signal is traveling too slowly, indicating a conduction issue.
The second measurement is Amplitude, which represents the strength or height of the electrical response. Physiologically, the amplitude is a reflection of the total number of nerve fibers that successfully transmit the electrical signal. A low amplitude reading means that fewer nerve fibers are responding to the stimulus, suggesting a loss of functional axons.
Finally, the Conduction Velocity is the speed at which the impulse travels along a specific segment of the nerve, usually expressed in meters per second ($m/s$). This value is calculated by dividing the measured distance between two stimulation points by the difference in their latencies. Conduction velocity measures the nerve fiber’s ability to conduct a signal rapidly, and a reduced velocity is a sign of impaired signal propagation along the nerve segment.
Translating Abnormal Numbers into Nerve Damage Types
The patterns of abnormality across these three measurements allow the clinician to distinguish between the two main types of nerve damage: demyelination and axonal loss. Demyelination involves damage to the myelin sheath, the fatty insulation surrounding the nerve fiber. Since the insulation is compromised, the primary effect is a slowing of the electrical signal.
In demyelination, the report will show a significantly reduced Conduction Velocity and a prolonged Latency. The Amplitude, however, may remain relatively normal because the underlying nerve fiber itself is still intact and capable of carrying the signal, albeit slowly. This pattern is often characteristic of conditions like Guillain-Barré syndrome.
Axonal loss, in contrast, involves the degeneration or death of the nerve fiber, or axon, itself. When the axon is damaged, it cannot transmit an electrical signal, leading to fewer signals reaching the recording electrode. Consequently, the hallmark of axonal loss is a significant reduction in Amplitude, reflecting the loss of functional nerve fibers. The Conduction Velocity in the remaining healthy axons may be normal or only mildly slowed.
How the Data Leads to a Final Clinical Diagnosis
The final step in interpreting the NCS report involves synthesizing the localized patterns of damage with the patient’s symptoms to arrive at the clinical diagnosis, which is typically summarized in the “Impression” or “Conclusion” section. The physician looks for whether the abnormalities are focal (concentrated in one specific nerve segment) or generalized (widespread across multiple nerves). A focal pattern of slowing, such as a prolonged latency in the median nerve only at the wrist, points clearly to a localized nerve compression.
This specific focal finding of slowed conduction in the median nerve at the wrist is the electrodiagnostic signature of Carpal Tunnel Syndrome. Conversely, if the report shows reduced amplitudes in multiple nerves, especially those tested in the feet, the pattern suggests a Polyneuropathy, a generalized condition affecting many nerves simultaneously. The clinical diagnosis is therefore a correlation between the numerical evidence of damage type, the anatomical distribution of the affected nerves, and the patient’s presenting symptoms. This final assessment confirms the severity, localizes the problem, and determines the most appropriate course of treatment.