WHAT IS NCS ? A nerve conduction study (NCS) : is a test used to evaluate/ detect the function of motor and sensory nerves of the human body. Nerve conduction velocity (NCV): is a common measurement made during this test.
The nerve conduction studies most commonly performed are compound muscle action potentials (CMAPs) for motor nerves, sensory nerve action potentials (SNAPs) for sensory nerves, compound nerve action potentials (CNAPs) for mixed (sensory and motor) nerves and late responses (primarily F- waves and H-reflexes).
NCS Latency Latency – time interval between the Velocity onset of a stimulus and the onset of a amplitude response (can also be referred to as a motor latency or a sensory latency). Amplitude – the maximal height of the action potential. Conduction velocity – how fast the fastest part of the impulse travels (can also be referred to as a motor conduction velocity or a sensory conduction velocity).
NORMAL VALUES LATENCY AMPLITUDE VELOCITYULMOTOR N. < 4.0 ms 5-10 mV 50-60m/sSENSORY N. <3.2-3.4 ms 5-50 µV 50m/sLLMOTOR N. <5.2ms 2-10mV 40-50m/sSENSORY N. <3.5-4.0 ms 5-40µV 35-40m/s
ELECTRODE PLACEMENT ACTIVE ELECTRODE : placed on the motor point of the muscle REFERENCE ELECTRODE : placed nearby tendon or bone GROUND ELECTRODE : in between active and reference electrode
B. DISK ELECTRODEA. GROUND ELECTRODE C. RING ELECTRODE
DIRECTION OF CONDUCTION Orthodromic conduction Antidromic conduction Orthodromic – when the electrical impulse travels in the same direction as normal physiologic conduction (e.g., when a motor nerve electrical impulse is transmitted toward the muscle and away from the spine or a sensory impulse travels toward the spine). Antidromic – when the electrical impulse travels in the opposite direction of normal physiologic conduction (e.g., conduction of a motor nerve electrical impulse away from the muscle and toward the spine).
M-WAVE Orthodromically stimulated motor response known as CMAP Supramaximal stimulus MNCV = distance between 2 stimulus sites difference between 2 latency
USES : When all motor fibers undergo Demyelination long latency Slow NCV Partial demyelination Normal latency and velocity Reduced amplitude Well marked demyelination Absence of conduction Axonal degeneration Reduced amplitude Neuropraxia Distal to the lesion – normal NCV Proximal to the lesion – reduced conduction velocity
F-WAVE A compound muscle action potential evoked by antidromically stimulating a motor nerve from a muscle using maximal electrical stimulus. It represents the time required for a stimulus to travel antidromically toward the spinal cord and return orthodromically to the muscle along a very small percentage of the fibers
F wave latency : median nerve – 22.34 ms ulnar nerve – 23.32ms peroneal nerve – 40.56ms tibial nerve – 38.58ms
USES : A prolonged asymmetric F waves suggest a proximal root lesion. Clinical application best for plexopathy. Quite prolonged in demyelination and mild prolongation in axonal injury. Assesses proximal neuropathology Eg: GBS Thoracic outlet syndrome Charcot’s Mary-tooth disease proximal nerve entrapment
SNCV It is performed by electrical stimulation of a peripheral nerve and recording from a purely sensory portion of the nerve, such as on a finger. 14 cm standard distance is used Threshold level stimulus is required sensory latencies are on the scale of milliseconds(ms). sensory amplitude are on microvolt range. 2 TYPES : ORTHODROMIC TESTS ANTIDROMIC TESTS
An Idealized Sensory Waveform S = Stimulus point, T = Takeoff point, P = PeakThe time (latency) from S to T is typically about 3 milliseconds. The amplitude would be measured in microvolts (μV).
H-REFLEX A compound muscle action potential evoked by orthodromically Stimulating sensory fibers, synapsing at the spinal level and returning Orthodromically via motor fibers. The response is thought to be due to a Monosynaptic spinal reflex (hoffmann reflex) found in normal adults in the Gastrocnemius-soleus and flexor carpi radialis muscles.
4. H - REFLEX Assesses continuity and function of sensory and motor monosynaptic pathway of 1st sacral nerve root Submaximal stimulus/LONG LATENCY- 29.8ms Disappears during supra maximal stimulus H-reflex study uses stimulation of a nerve and recording the reflex electrical discharge from a muscle in the limb.
Normal latency : 29.8 millisecond ( got long latency) Implication : used in the diagnosis of S1 and C7 root lesions the study of proximal nerve segments in either peripheral or proximal neuropathies. Its absence or abnormal latency on one side strongly indicates disease
REPETITIVE STIMULATION Repeated electrical stimulus applied to the motor neuron at a rate of 3-5 / second , the amplitude of the recorded muscle response is constant Decrement of more than 10% is abnormal Supramaximal stimulus Used to find NMJ abnormalities Eg: myaesthenia gravis Lambert Eaton syndrome MMEP is measured
REPETITIVE STIMULATION A repetitive nerve stimulation study demonstrating a 61 percent decrement in area and a 54 percent decrement in amplitude from the first to the fourth stimulation.
EVOKED POTENTIALS Assessing the electrical activity in CNS