A nerve conduction study (NCS) is a specialized test designed to evaluate the functional capacity of your peripheral nerves. This procedure involves applying small electrical impulses to a nerve and measuring how quickly and how strongly the nerve transmits the signal. The primary purpose of this test is to identify damage or dysfunction within the nerve fibers, which may be causing symptoms like tingling, numbness, or muscle weakness. The resulting report provides objective evidence that helps medical professionals determine the nature and severity of any nerve impairment.
Understanding the Report’s Structure
A typical nerve conduction report begins with administrative details and a list of the specific nerves tested. A neurologist selects the nerves, such as the Median, Ulnar, or Peroneal nerves, based on the location of your symptoms. The body of the report is systematically organized into two main categories: Motor Studies and Sensory Studies.
Motor studies focus on nerve fibers that transmit signals to muscles, measuring the electrical response known as the Compound Muscle Action Potential (CMAP). Sensory studies evaluate the sensory nerve fibers that relay touch and pain information, recording the Sensory Nerve Action Potential (SNAP). This separation is important because some conditions affect motor nerves differently than sensory nerves, providing a diagnostic clue. Both study types include a column labeled “Normal Values” or “Reference Range,” which is the benchmark for comparison.
Decoding the Key Numerical Measurements
The core of the NCS report lies in three distinct numerical measurements that describe nerve function. These values are compared against the established “Normal Value” range, often representing the 95th percentile of healthy individuals. Any deviation from this reference range suggests a functional impairment.
Latency is a time-based measurement, in milliseconds, representing the delay from the moment the electrical stimulus is delivered until the nerve’s electrical response begins. A prolonged or slowed latency suggests a problem with the myelin sheath, the fatty covering that insulates the nerve fiber. This often indicates compression or focal demyelination at the site of stimulation.
The Amplitude measurement, expressed in millivolts or microvolts, quantifies the strength or size of the resulting electrical signal. A reduced amplitude indicates that fewer individual nerve fibers are successfully transmitting the signal. This suggests a loss of functioning axons, which are the main cable-like extensions of the nerve cells.
The third measurement, Conduction Velocity (CV), is a direct calculation of how fast the signal travels along a specific nerve segment, measured in meters per second. It is calculated by dividing the distance between two stimulation points by the time difference between the responses. A significantly slowed conduction velocity is a strong indicator of damage to the myelin sheath, as this insulation is necessary for the rapid conduction of the electrical impulse.
Translating Results into Diagnostic Findings
The interpretation of the NCS report involves analyzing the pattern of abnormalities across the three key measurements to classify the type of nerve injury. A pattern showing a significantly reduced Amplitude with relatively normal Conduction Velocity and Latency suggests an Axonal Injury. This indicates a primary loss of the nerve’s inner fiber (axon), which correlates with the severity of the damage and often implies a slower potential for recovery.
Conversely, a pattern characterized by markedly prolonged Latency or severely slowed Conduction Velocity, while Amplitude is relatively preserved, points toward a Demyelinating Injury. This means the damage is primarily to the myelin insulation, causing the signal to travel sluggishly without immediately killing the nerve fiber. This type of injury is often seen in conditions where the nerve is compressed, such as carpal tunnel syndrome.
The extent to which measured values deviate from the normal range determines the reported severity (mild, moderate, or severe). For example, a mild case might show a slight prolongation of latency, while a severe case involves a drastic reduction in amplitude and velocity. The final section contains the neurologist’s written interpretation, which synthesizes these numerical findings into a cohesive diagnostic conclusion, localizing the site of injury and classifying the underlying pathology.