Nerve Conduction Studies NCS

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Prudence Alexandrina Stewart
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1 Nerve Conduction Studies NCS Nerve conduction studies are an essential part of an EMG examination. The clinical usefulness of NCS in the diagnosis of diffuse and local neuropathies has been thoroughly validated since the 1940 s The findings reflect the functional state of the myelinated motor nerves, the neuromuscular transmission and the muscle fibers. Two motor units

2 Stimulation A nerve is a Chain of Polarized Cells (Myelin Action). Stimulation in one point generates a depolarization A nerve could be depolarized by: Electrical Chock Mechanical Compression Magnetic Field -

Median - Ulnar Palmer Radial Median Wrist

3 Stimulation Points Extremities Upper Plexus Axillary Median Elbow Ulnar Wrist Median - Ulnar Palmer Radial Median Wrist Ulnar Above Elbow Ulnar Below Elbow Stimulation Points Extremities Lower Stimulation Points Trunk Head & Crural Higher Erb Point Sciatic Trunk Thoracodorsalis Peroneal Peroneal Sural Tibial Popliteal fossa Posterior Tibial Medium Auricular Posterior Lower Spinal XI Phrenic Erb Point Long Thoracic Crural

4 Current Stimulation Depolarization NERVE + - NERVE + - Current Stimulation Supramaximal Stimuli Min. 3 Times Sensory Threshold ANTIDROMIC ORTHODROMIC Once a nerve is depolarized at some point, a wave of depolarization passes in both directions from that point. Propagation - Refractory Period + - NERVE Propagation by Successive Depolarization followed by Repolarization. Time before Repolarization is called the Refractory Period.

5 Motor + - Motor Response NERVE + - S MOTOR LATENCY in ms = Propagation Time from S to M M Motor Response Motor Conduction Median Nerve Peak Amplitudes Onset Area Negative Deflection Stim. 1 Wrist 3.5 ms Peak to Peak Elbow 8.2 ms Stim. 2 Diff.: 4.7 ms Stimulation Artifact Duration Recording: Surface Electrodes Stimulation: Handgrip or Bipolar Distance mm: 240 C.V.: 51 m/s

6 Stimulation & Recording RULE To reduce Stimulation Artifact as much as possible, Ground must be placed between Stim. & STIMULATION RECORDING Anode Cathode Active Reference GROUND Nerve BLACK TO BLACK Stimulation Polarity: NEGATIVE Motor & Sensory MOTOR Remarks General Amplitude 0.5 mv to 5 mv - Direct response Normally biphasic Duration 1 to 3 ms - depends of stimulation site Stimulation Current depends on nerve and site, ma SENSORY Amplitude 5 µv to 35 µv Direct response for high amplitude potentials - otherwise Averager is needed Normally biphasic Duration 1 to 3 ms - depends on stimulation site Stimulation Current depends on nerve and site, 5-15 ma Pathologic Responses Normal Axonal Degeneration Amplitude Duration Nl Nl Nl Nl+/- Conduction Nl+/- Block Severe Demyelination Focal Nl Nl Slowing Myelin Damaged Nl = Normal Carpal Tunnel CV General Application MOTOR Motor Median Nerve Wirst Stim. 1 Elbow Stim. 2 Sensory Median and Ulnar Nerves SENSORY Digit 2 Stim.Digit Digit 3 Ulnaris Medianus Digit ms 6.2 ms 2.2 ms 2.4 ms 1.9 ms

7 M F-Waves Ulnaris Nerve Volontary contralateral motor action to facilitate F M F F-Waves As playing with this game Shorter Distance Better Shoot! Stim. F block M Block Minimum F is normally measured F Latence 30 to 50 ms F Motor Neuron Refractory Period CV General Application Stim. Interval : ms 4.00 Sensory Conduction Ulnaris Stimulation Nerve AVERAGING ms Digit V Distance mm 155 Stim Recording CV m/s ms Digit IV Distance mm 175 CV m/s 56 Double Stimulation Start with 4ms delay Decrease by steps of 0.05ms Refractory period: 1.80 ms 10ms

8 Inching Ulnaris Nerve Palm Sites Electrodes MNCS Settings SNCS All All Surface Surface/Needle Stim. 1 inch dist. between Stim. Sites No Stim. Wrist High FQ (KHz) Low FQ (Hz) Sensitivity mv/div µv/div Input 5 10 AVG 5-10 Increased Other graphics with Amplitude and Duration Sweep Speed Upper 2 2 ms/div Lower 5 2 Stim. Intensity Supramaxima 3x Threshold Duration ms Frequency c/s Maximal recording amplitude Lower % diff. between 2 sites Normal Values Motor NCS Sensory The results of NCS - Motor, Sensory must be compared to some normal values. These must be age-related as well. Infants & young children: Quite slow conduction rate. Adults: Amplitudes and conduction rate decrease with age. The most sensitive normal value for a particular NCS is usually obtained by performing the same NCS on the corresponding nerve in the contralateral limb (assuming it is normal). A decrease of more than 50% in amplitude is considered abnormal. NCS Age Nerve Amp. Lat. C.V. µv mv ms m/s Median >20 <3.4 >50 Ulnar >15 <3.1 >50 Radial >18 <2.7 Median > 6 <4.0 >50 Median (palm) <2.2 Ulnar (palm) <2.2 Sural > 6 <4.5 >40 Peroneal <4.5 Peroneal > 3 <5.5 >40 Tibial (post.) > 8 <6.0 >40 Pitfalls High recording impedance electrode reducing amplitude Recording polarity inversion - incorrect onset latency Stimulation polarity inversion - increasing latency Unbalanced stimulation electrode - artifact Low stimulation intensity - impedance - infra stimulation Painful stimulation - movement artifact External interference (patient is an antenna) Incorrect setup - filters, sweep-speed...

Nerve Conduction Studies NCS

Nerve Conduction Studies NCS Nerve conduction studies are an essential part of an EMG examination. The clinical usefulness of NCS in the diagnosis of diffuse and local neuropathies has been thoroughly