NCS&EMG Clinical Correlates

7/27/2019 NCS&EMG Clinical Correlates

1/4

Clinical Correlate 6. Nerve Conduction Studies and Electromyography


Clinical Correlate 6. Nerve Conduction Studies andElectromyography

C. Chalk

Click here for other notes on Nerve Conduction Studies and Electromyography.

Introduction

The electrical properties of the nervous system have been recognized at least since the time of Galvani's studies of animal electricity in the 18th century. However, it has only been since the1940s that developments in electronics have allowed physicians to record and measure theelectrical function of the nervous system in patients. The purpose of today's session is todemonstrate some techniques for measuring the electrical function of the peripheral nervoussystem. These techniques, known as nerve conduction studies and electromyography, are used byneurologists in the evaluation of patients with diseases of the peripheral nerves and muscles.

Nerve Conduction Studies

If a brief electrical shock is applied to a peripheral nerve, the subject will experience two things.First, of course, an electrical sensation will be felt. Second, if the shock is large enough, themuscles innervated by the nerve will make a brief, forceful contraction (a twitch). The effect of the shock is to depolarize some or all of the immediately subjacent axons, causing them togenerate action potentials. Unlike action potentials generated normally in the nervous system,which occur rather randomly and which propagate in one direction only, those resulting from theelectrical shock travel as a synchronous wave, both up the nerve (towards the brain, experiencedas the sensation of a shock) and distally (towards the muscle, causing the twitch).

Motor nerve conduction is evaluated by recording the wave of action potentials travelling to themuscle. The muscle twitch is associated with a brief electrical signal, the compound muscle action

potential (CMAP). The CMAP is the sum of the action potentials occurring in all the contractingmuscle fibres in the muscle. The CMAP is recorded from a pair of electrodes taped to the skinoverlying the muscle; because the CMAP is small (millivolts) and brief (milliseconds), it must beamplified electronically and displayed on an oscilloscope.


2/4

Clinical Correlate 6. Nerve Conduction Studies and Elect romyography


The electrical shock is delivered from a hand held stimulator at sites where the nerve of interest isclose to the surface (to allow as small a shock as possible to be used). With the electrodes in place,and amplifier and oscilloscope turned on, a single shock is given. On the oscilloscope one sees theshock, followed in a few milliseconds by the CMAP. The time between the shock and the

beginning of the CMAP (the latency, in ms) and the size of the CMAP (CMAP amplitude, in mV)are measured from the oscilloscope. The procedure is then repeated, stimulating the nerve at asecond site, and again recording the latency and CMAP amplitude. The distance between the twosites of stimulation is measured. The motor conduction velocity (in m/s) between the two sites of stimulation is calculated by:

Conduction velocity = DistanceLatency A - LatencyB

Sensory nerve conduction is evaluated in a slightly different way. Instead of recording theresponse of a muscle, the sensory nerve action potential (SNAP) is recorded directly from thenerve. The SNAP is the sum of all the action potentials generated in sensory nerve fibres by the

electrical shock. The SNAP is about 1000 times smaller than the CMAP, making sensory nerveconduction technically more difficult to measure. In order to ensure that one is recording action potentials from sensory nerve fibres only, the recording electrodes must be placed along a branchof the nerve which contains only sensory fibres.

As with motor conduction studies, the SNAP amplitude (in microvolts) and latency (in ms) between the shock and the SNAP are recorded using two sites of stimulation. The sensoryconduction velocity (in m/s) is calculated using the equation above.

INTERPRETATION OF NERVE CONDUCTION STUDIES: The techniques described abovecan be applied to almost any peripheral nerve. With the nerve conduction data, the neurologist candetermine two main things: 1) Is the nerve normal? 2) If abnormal, does the problem lie in the

nerve axon or its myelin sheath?

Judging whether a given set of nerve conduction data is normal requires comparing them to valuesobtained from that nerve in normal subjects. Normal values are well established for motor andsensory conduction in most major peripheral nerves. Of course, the neurologist must select thecorrect nerve or nerves to test, depending on each patient's clinical problem. If the nerveconduction studies are abnormal, one can also get some idea of how severely a nerve has beendamaged or injured.

Nerve conduction studies may also allow one to judge whether the main site of damage is theaxon or the myelin sheath. Knowing whether a patient has a disease affecting the nerve axons (an"axonal neuropathy") or the myelin sheaths (a "demyelinating neuropathy") helps to guide the

physician to a diagnosis among the many causes of peripheral neuropathy.

Axonal neuropathies produce a decrease in CMAP and SNAP amplitudes but have relatively littleeffect on conduction velocities. By contrast, in a demyelinating neuropathy, conduction velocitiesare significantly slowed, while the CMAP and SNAP amplitudes are little changed.


3/4



Electromyography

Nerve conduction studies are an extremely useful means to study motor and sensory function inthe peripheral nervous system. However, nerve conduction studies have some limitations,

particularly for examination of motor function. First, motor nerve conduction studies are rather

insensitive to small degrees of abnormality. Since the procedure studies the performance of awhole population of axons and muscle fibres, malfunction of small numbers of axons or musclefibres will produce insignificant changes in parameters like the CMAP amplitude, and may thusgo undetected. Second, many motor axons and their muscles are technically impossible to study,mostly because they are too deep to be stimulated or recorded with electrodes on the skin. Third,abnormalities of CMAP amplitude (the most common abnormality which is found in motor nerveconduction studies) may be due to disease of the motor axons or of the muscle fibres. From themotor nerve conduction studies alone, it is often impossible to make the clinically importantdistinction between these two possibilities.

Fortunately, neurologists can also study muscle and nerve function by recording electrical activitywith an electrode inserted into a muscle, a procedure known as electromyography. A fine needleelectrode is used, and the electrical signals "seen" by the tip of the needle are amplified anddisplayed visually (on an oscilloscope) and audibly (via a loudspeaker). Unlike motor nerveconduction studies, which evaluate a whole population of motor axons and muscle fibres,electromyography allows one to evaluate individual motor units. (Recall that a motor unit consistsof an anterior horn cell, its axon and branches, and all the muscle fibres which it innervates. Alarge limb muscle like the quadriceps contains several hundred motor units, whereas smallmuscles like the extraocular muscles contain only a few dozen.)

The needle electrode is placed in the muscle, and electrical activity is assessed both with themuscle in a relaxed state and with the patient voluntarily contracting the muscle. In normal muscleinnervated by normal nerve, there is no electrical activity at rest. In diseases in which the normalconnection between a muscle fibre and its motor axon is broken, the muscle fibre membrane willspontaneously and regularly depolarize and repolarize. This results in tiny electrical signals called"fibrillation potentials", which can be detected by the needle electrode. The detection of fibrillation potentials is a very sensitive means of determining that there are problems in the

peripheral motor system.

Voluntary contraction of a muscle is accompanied by electrical activity in all the contractingmuscle fibres. Each time a motor unit is activated and its constituent muscle fibres contract, acomplex electrical signal, the "motor unit potential", can be detected by a needle electrode placedin the midst of the contracting muscle fibres. By assessing the size, duration, and firing pattern of the motor unit potentials in a muscle, the neurologist can evaluate whether the motor units in thatmuscle are normal or abnormal. If the motor unit potentials are abnormal, the specific pattern

allows one to judge whether the problem lies in the anterior horn cell, the motor axon, or themuscle itself. In general, in diseases of muscle fibres, motor unit potentials become small,whereas disease of the anterior horn cell or the motor axons result in enlargement of motor unit

potentials.


4/4



WORKSHEET FOR NERVE CONDUCTION STUDIES

MEDIAN NERVE MOTOR CONDUCTIONStimulate Record Latency CMAP amplitude Conduction velocity

MEDIAN NERVE MOTOR CONDUCTIONStimulate Record Latency SNAP amplitude Conduction velocity

NCS&EMG Clinical Correlates

Documents

Transcript of NCS&EMG Clinical Correlates