Nerve conduction study is mainly used for the evaluation of paresthesia (numbness, tingling, or burning sensation), weakness of the arms and legs. This type of study is dependent on the part of limbs presented the symptoms. A physical examination and thorough history also help to direct the investigation. Some of the common disorders which we can diagnose by the NCS are the following.
·         Peripheral neuropathy (Median nerve, Ulnar nerve, Radial nerve, Peroneal nerve, Shoral nerve etc)
·         Carpal tunnel syndrome (Median nerve compression)
·         Guillain Barre syndrome (disease of peripheral nerves having numbness and weakness in limbs)
·         Fascio – Scapulo – Humeral muscular dystrophy
·         Spinal disc herniation

Components of NCS:

NCS has the following components.
  1. Motor NCS
  2. Sensory NCS
  3. F – wave
  4. H – reflex

A.   Motor NCS:

Motor NCS are performed by electrical stimulation of peripheral nerve and recording from muscle supplied by that nerve. The time it takes for electrical impulse to travel from the stimulation (electrode) of the nerve to the recording electrode is called latency (ms). The size of the response of the stimulation is called amplitude which is measured in millivolt (mv). The nerve conduction velocity is determined from the differences of the latencies on the two different locations and the distance between the electrodes.

B.   Sensory NCS:

Sensory nerve conduction study are performed by the electrical stimulation of the peripheral nerve and recording a purely sensory portion of the nerve such as on finger i.e. the most distal portion of the limb. Recording electrode will be proximal of the two electrodes (stimulatory electrode is distal). Like the motor nerve conduction study, latency is measured in millisecond (ms) while the amplitude is too low that can not be measured in millivolts (mv) so it can be measured in microvolt (┬Áv). The nerve conduction velocity is calculated from the latency and the distance between the electrodes i.e. nerve conduction velocity is measured in m/s. This is called sensory nerve conduction study.

C.   F – Wave study:

It is the measured of time required for action potential of the motor neuron elicited by applying a supramaximal stimulus (above the threshold value) to the peripheral nerve that is to be transmitted to the Anterior Horn Cells and return as a recurrent discharge along the same nerve to activate the muscle that will be recorded by the recording electrode.
The latency of the F – wave response is approximately 22 – 34 ms in the upper limb and 40 – 58 ms in the lower limb when they are stimulated at the wrist and ankle respectively.
It is the useful supplement to the NC and electromyography and is most helpful in the diagnosis of condition where the most proximal portion of the nerve is damaged like Guillain Barre Syndrome and Thoracic outlet syndrome.

D.   H – Reflex:

It was first suggested by Hoffman and it is useful measurement for rediculopathy and peripheral nerve pathy. It is the testing of both the integrity of sensory and motor monosynaptic pathway of S1 nerve root to some extent for C and C7. When a submaximal stimulus (below the threshold value) is applied to the peripheral nerve, the action potential travel along afferent neuron (I a) and synapse with the AHCs in the spinal cord. AHCs send information along the motor neuron causing contraction of the muscle.
The H – reflex latency is the function of age and leg length.
H – Reflex latency = 0.46 (leg length in cm) + 9.14 + 0.1 (age in years)
Note: Stimulatory electrode should be placed on muscle belly and recording electrode should be placed on muscle origin.