- Sensory NCS: It is similar to Motor NCS and is performed by electrical stimulation of a peripheral nerve and but here the recording is done from a truly sensory portion of the nerve. Sensory latencies are measured milliseconds but sensory amplitudes are much smaller than the motor amplitudes (microvolt). Sensory NCV is calculated in the same way as motor NCVs.
- F-wave study: The motor and sensory segments are concerned about nerve conduction velocities in sections/segments of limb whereas the F-wave latency is used to derive the conduction velocity of nerve between the limb and spine.In a typical F wave study, a strong electrical stimulus is applied above the distal portion of a nerve. The resulting the impulse travels both in both directions: one towards the muscle fibre and the other back to the motor neurons of the spinal cord. These directions are referred to as orthodromic and antidromic, respectively. The orthodromic stimulus on reaching the muscle fibre elicits a strong M-response indicative of muscle contraction. Meanwhile theantidromic stimulus on reaching the motor neuron cell bodies excites a small portion of the motor neurons causing them to backfire resulting in orthodromic wave which travels back down the nerve towards the muscle. This reflected stimulus evokes a smaller response of the muscle fibres resultingin a second CMAP called the F wave.
- H-reflex study: This studyis similar to F-wave study and evaluates conduction between the limb and the spinal cord. Although a subtle difference exists in that here the impulses going toward the spinal cord are in sensory nerves while the impulses coming from the spinal cord are in motor nerves.
The limb length, D(in millimetres)is taken into account for calculations of Conduction velocity.
The interpretation of nerve conduction studies is a complex affair and expert medical practitioners such as neurologists, physiatrists or clinical neurophysiologists are routinely involved. NCS have proven to be very helpful in diagnosis of many diseases related to the nerves. The process is non-invasive, albeit sometimes it can be painful due to minor electrical shocks. Although the low amount of electrical current is considered safe, patients with a harbouring electrical devices such as permanent pacemaker are advised to avoid this kind of test or tell the examiner prior to the test.