Nerve Conduction Study

Original Editor - Mohamed A.Fekri

Top Contributors - Mohamed Abdelraof Mohamed  

Introduction

A nerve conduction velocity (NCV) test — also called a nerve conduction study (NCS) — measures how fast an electrical impulse moves through your nerve. NCV can identify nerve damage.

Factors affecting Nerve Conduction Study

there are some factors you should know about when you diagnose patients with entrapment nerve or any cases related to that by NCS because it can give you false results leading to false diagnosis

physiological Factors

Temperature

• Temperature is the most important of all the physiologic factors.
• Normal physiologic range of limb temperature (approximately 21–34°C).
• Physiologically, cooler temperatures result in the delayed opening of sodium channel sand and prolong the depolarization (Prolonged distal latency), resulting in slowed conduction velocities for the nerve being studied.
• In addition, longer channel opening time results in a larger influx of sodium. Subsequently, each nerve fiber depolarization is larger and longer resulting in a higher amplitude and longer duration for both compound muscle action potentials(CMAPs) and sensory nerve action potentials (SNAPs)
• For motor and sensory conduction velocities, conduction velocity slows between 1.5 and 2.5 m/s for every 1°C drop in temperature, and distal latency prolongs by approximately 0.2 milliseconds per degree.

• There may be significant variation in limb temperature among individuals, moreover, there is a marked variation of temperature for a given nerve, with a trend toward cooler temperatures as the nerve travels distally and superficially within the respective limb. Furthermore, skin surface temperature typically is 1 to 2°C warmer in a warm limb compared to the near nerve temperature.

Maintenance of Temperature

• Distal limb temperatures should be routinely recorded and monitored in all patients and ideally maintained between 32 and 34°C.
• Limbs can be heated with heating lamps, warming packs, or hydrocollators.
• Remember there may be a heating delay between when the skin and the underlying nerve reach the desired temperature.
• For profoundly cool limbs, it may require 20 to 40 minutes for the underlying nerve temperature to equilibrate.

Age

• Conduction velocities decrease slightly with age in adults, most likely as a consequence of the normal loss of motor and sensory neurons that occur with aging.
• This is more prominent for individuals older than 60 years, in whom conduction velocity decreases by approximately 0.5 to 4.0 m/s/decade. The effect is slightly more pronounced for sensory than for motor fibers.
• Age also affects CMAP and SNAP amplitudes. SNAP amplitudes are known to decrease substantially with advanced age.

Height

• Taller individuals commonly have slower conduction velocities than shorter individuals.
• Normal conduction velocities are slower in the lower extremities, where the limbs are longer than in the upper extremities. Because conduction velocity is directly proportional to nerve diameter, the more distally tapered nerves in taller individuals conduct more slowly. The limbs are cooler distally than proximally, and the legs are generally cooler than the arms. Thus, conduction velocity slowing due to cooling is usually more prominent in the legs than in the arms.

Non-physiologic Factors.

1. Electrode impedance mismatch and 60 Hz interference
2. Stimulus artifact
3. Cathode position: reversing stimulator polarity
4. Supramaximal stimulation
5. Co-stimulation of adjacent nerves
6. Electrode placement for motor studies
7. Distance between recording electrodes and nerve
8. Distance between active and reference recording electrodes
9. Limb position and distance measurements

Resources

1. Armstrong TJ, Chaffin DB: Some biomechanical aspects of the carpal tunnel. J Biomech 1979; 12: 567–570.numbered list
2. Armstrong TJ, Silverstein BA: Upper-extremity pain in the workplace—role of usage in causality, in Clinical Concepts in Regional Musculoskeletal Illness. Grune & Stratton, 1987.
3. Bolton CF, Carter KM: Human sensory nerve compound action potential amplitude: variation with sex and finger circumference. J Neurol Neurosurg Psychiatry 1980; 43: 925–928.
4. Buchthal F, Rosenfalck A: Evoked action potentials and conduction velocity in human sensory nerves. Brain Res 1966; 3: 1–22.
5. Buchthal F, Rosefalck A: Sensory conduction from digit to palm and from palm to wrist in the carpal tunnel syndrome. J Neurol Neurosurg Psychatry 1971; 34: 243–252.
6. Buchthal F, Rosenfalck A, Trojaborg W: Electrophysiological findings in entrapment of the median nerve at wrist and elbow. J Neurol Neurosurg Psychiatry 1974; 37: 340–360.
7. Campbell WW, Ward LC, Swift TR: Nerve conduction velocity varies inversely with height. Muscle Nerve 1981; 4: 520–523.
8. Cannon LJ, Bernacki EJ, Walter SD: Personal and occupational factors associated with carpal tunnel syndrome. J Occup Med 1981; 23: 255–258.
9. Daube JR: Nerve conduction studies, in MJ Aminoff (ed): Electrodiagnosis in Clinical Neurology. New York, Churchill Livingstone, 1980, pp 229–264.