Nerve Conduction Studies (NCS), often called Nerve Conduction Velocity (NCV) tests, are a common tool used by neurologists to evaluate the health of the body’s peripheral nerves. This diagnostic procedure measures how quickly electrical signals travel through the motor and sensory nerves outside of the brain and spinal cord. By stimulating a nerve with a mild electrical impulse and recording the response, the test assesses nerve function to locate and characterize nerve damage. The objective data from an NCV test is used for diagnosing conditions that cause numbness, tingling, or weakness.
The Physiology Measured by Nerve Conduction Tests
Nerve conduction tests translate the speed and strength of an electrical signal into three primary measurements that reflect nerve health.
Conduction Velocity
Conduction Velocity is the speed at which the electrical signal travels along the nerve fiber. A slowing of this velocity suggests damage to the myelin sheath, the fatty covering that insulates the nerve axon and speeds up signal transmission.
Latency
Latency is the time delay between the electrical stimulation and the resulting nerve or muscle response. Prolonged latency indicates the signal is taking too long to reach its destination, often pointing to nerve compression or demyelination near the stimulation site.
Amplitude
Amplitude represents the strength of the electrical signal recorded and is directly proportional to the number of functional nerve fibers (axons) conducting the impulse. A reduction in amplitude signifies axonal injury, where the core conducting wire of the nerve is damaged. Analyzing these three metrics allows the physician to distinguish between damage to the nerve’s insulation (demyelination) and damage to the nerve fiber itself (axonal loss).
Factors That Influence Test Reliability
The reliability of a nerve conduction test can be affected by several physiological and technical factors. Limb temperature is the most significant physiological variable, as colder temperatures naturally slow down nerve conduction velocity. A drop in skin temperature by just a few degrees Celsius can prolong latency and slow conduction velocity, potentially leading to a false interpretation of nerve damage. For this reason, specific protocols require the limb to be warmed to a standardized temperature, typically between 32°C and 34°C, before testing begins.
Technical factors, such as the operator’s skill, also play a role in reliability. Precise placement of the stimulating and recording electrodes is necessary to measure the nerve segment accurately. Inconsistent electrode placement or improper measurement of the distance between electrodes can introduce errors in the calculated velocity and latency values. The accuracy of the equipment, including proper calibration, is also necessary to ensure that the subtle electrical signals are recorded faithfully.
Understanding Diagnostic Precision
Diagnostic precision is measured using the concepts of sensitivity and specificity. Sensitivity refers to the test’s ability to correctly identify a condition when it is present, minimizing false-negative results. Specificity describes the test’s ability to correctly rule out a condition when it is absent, minimizing false-positive results.
For a focal nerve entrapment, such as Carpal Tunnel Syndrome (CTS), the accuracy of NCV testing is very high and is often considered the gold standard for confirmation. Studies on CTS often report high specificity, sometimes exceeding 90%, meaning a positive result is highly likely to indicate the condition. However, the sensitivity can be lower, sometimes around 73% for certain measurements, indicating that some people with mild CTS may still have a negative test result.
NCV testing for generalized conditions like Diabetic Polyneuropathy (DPN) presents a different scenario. NCV is considered the gold standard for diagnosing large fiber neuropathy, but its precision can vary depending on the specific nerve and measurement used. For instance, certain amplitude measurements in the sural nerve have shown sensitivities in the range of 86% to 92% and specificities from 43% to 82% in studies comparing results to clinical consensus criteria.
NCV Testing as Part of a Comprehensive Diagnosis
Nerve conduction testing rarely stands alone, serving instead as a foundational component within a broader electrodiagnostic evaluation. It is almost always performed alongside a Needle Electromyography (EMG), which involves inserting a fine needle electrode directly into the muscle.
The NCV assesses the peripheral nerve’s ability to transmit a signal, measuring the function of the larger, heavily myelinated nerve fibers. The EMG assesses the electrical activity of the muscles at rest and during contraction, providing information about the health of the muscle and the smaller, unmyelinated nerve fibers. Combining data from both tests allows the physician to differentiate whether symptoms stem from a primary muscle disorder, a nerve problem, or a disorder at the nerve-muscle junction. The final diagnosis relies not only on these objective test results but also on a thorough review of the patient’s clinical history and a physical examination.