Nerve Conduction Study

Original Editor - Mohamed A.Fekri

Top Contributors - Mohamed Abdelraof Mohamed  

Introduction

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

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 delayed opening of sodium channel sand subsequently prolong the time of 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 velocityslowsbetween1.5 and 2.5 m/s for every 1°C drop in temperature, and distal latency prolongs by approximately 0.2 mellisecon per degree.

• There may be significant variation in limb temperature among individuals, moreover, there is marked variation of temperature over the course of a given nerve, with a trend toward cooler temperatures as the nerve travels distally and superficially within the respective limb. Furthermore, in a warm limb, skin surface temperature typically is 1 to 2°C warmer 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 delay when heating between when the skin and the underlying nerve reach the desired temperature.
• For limbs that are profoundly cool, it may require 20 to 40minutesfor 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 approximately 0.5 to 4.0 m/s/decade. The effect is slightly more pronounced for sensory than for motor fibers.
• Age also has an effect on CMAP and SNAP amplitudes. SNAP amplitudes are known to decrease substantially with advanced age.

Height

• Taller individuals commonly have slower conduction velocities than do 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. limbs are cooler distally than proximally and the legs generally are cooler than the arms. Thus, conduction velocity slowing due to cooling usually is 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

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