EMG and NCS are some of the mainstays of neurology diagnostic procedures. Interpretation and clinical correlation are imperative in order to be a great diagnostician and neurologist. Too, these skills will help you excel on the RITE, board exam, and in your career. For the medical student Shelf exam, it is not necessary to be able to interpret EMG/NCS in detail. In this chapter, you will explore EMG and NCS basic physiology and interpretation, and then test your retention at the end with our question bank! A handful of real EMG recordings have also been incorporated into the chapter. For more, check out our EMG Video Gallery and our EMG/NCS Case Bank.
Author: Brian Hanrahan MD
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Introduction
Nerve conduction studies (NCS) and needle electromyography (EMG) are integral tools for the clinical neurologist and are considered to be an extension of the neurological examination. They can provide invaluable information to allow one to diagnose various peripheral nervous system (PNS) or muscle disorders. They can help localize a lesion (peripheral nerve, neuromuscular junction, muscle), discern the extent of the pathology (focal, multifocal, diffuse), and evaluate whether nerve-related pathology is axonal or demyelinating.
Basics
- Neurons have an intracellular resting membrane potential of -70 mV. When nerves conduct electrical impulses along their axon, the membrane is depolarized by an influx of sodium (Na+) ions via voltage-gated sodium channels (Figure 1).
Figure 1: The (a) resting membrane potential is a result of different concentrations of Na+ and K+ ions inside and outside the cell. A nerve impulse causes Na+ to enter the cell, resulting in (b) depolarization. At the peak action potential, K+ channels open and the cell becomes (c) hyperpolarized.
- This influx causes a voltage change to adjacent regions and changes the resting potential further down the axon thereby propagating the potential down the axon (Figure 2). The axon membrane potential will then stabilize with the inactivation of sodium channels and delayed opening of voltage-gated potassium channels.
Figure 2: A traversing action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes.
- When unmyelinated, fibers can conduct potentials in the range of 1-5 m/sec. However, myelinated motor nerve axons and many sensory nerve axons have conduction velocities up to 150 m/sec. This is due to a process called saltatory conduction.
- Produced by Schwann cells, myelin sheaths are concentrically wrapped around an axon.
- In between myelin sheaths, there are gaps of exposed axon called nodes of Ranvier. It is at these nodes where action potentials occur.
- When an action potential occurs at a node, that current flows passively within the myelinated internode of the axon until the next node is reached.
- Saltatory Conduction: Current flows across the neuronal membrane and jumps from node to node (Figure 3).
Figure 3: Myelinated axon showing saltatory conduction
Reminder:
Demyelination of peripheral nerves will result in a reduction of the conduction velocity.
Clinical Definitions
- Myopathy: Disease in which muscle fibers are not functioning properly.
- Myelopathy: A CNS neurologic deficit related to the spinal cord.
- Neuronopathy: A PNS disorder affecting the cell body. Includes sensory neuronopathy, motor neuronopathy, or ganglionopathy.
- Neuropathy: A PNS disorder affecting any part of the nerve (cell body, axon, myelin).
- Motor neuron disease: A PNS and CNS disorder of the motor neuron itself. It can affect upper or lower motor neurons.
- Neuromuscular junction disorder: A PNS disorder specifically affecting the junction of nerve and muscle.
Nerve Conduction Studies (NCS)
- NCS are used to evaluate large myelinated sensory and motor fibers.
- Small myelinated and unmyelinated fibers carry autonomic information and pain/temperature sensations and can not be recorded with standard nerve conduction techniques.
NCS Terms
Conduction velocity
- Measures of the speed of the fastest conducting motor axons in the stimulated nerve.
- Example: Decreased velocity (and therefore prolonged latency) in demyelinating disorders due to loss/impairment of saltatory conduction.
Amplitude
- Voltage difference from baseline to maximal negative peak with depolarization.
- Reflects the number of muscle fibers that depolarize.
Latency
- Time from stimulus to the initial CMAP deflection from baseline.
- Inversely proportional to the speed of neuronal transmission (conduction velocity).
Duration
- Time from an initial deflection from baseline to the first baseline crossing.
- Measures action potential synchrony.
- Some motor fibers conduct slower than others, causing increased overall duration, i.e. in demyelinating diseases.
Pathological findings
Axonal damage
- Will have a drop in amplitude with largely normal conduction velocity/latency.
Demyelination
- Will have prolonged latency and decreased conduction velocity, with relatively normal amplitudes.
- With severe demyelination, secondary axonal involvement can occur, thus affecting the amplitudes.
- With severe focal demyelination, a conduction block can occur.
Electromyography (EMG)
- EMG studies are used to evaluate the communication between nerve and muscle to differentiate between specific nerve, root, plexus, and myopathic pathologies.
- It predominantly evaluates the motor unit action potentials of type 1 muscle fibers.
- Patients with type 2 fiber myopathies, such as steroid-induced myopathy, will therefore have normal EMG testing!
- Three types of activity can be measured to evaluate for any type of neuropathy/myopathy: Insertional activity, spontaneous activity, and exertional (voluntary) activity.
Fasciculations
- Irregular spontaneous firing of a motor neuron at rest.
- Audibly, it can be compared to popcorn popping in the microwave.
- Fasciculations can be normal and benign if rare, but when they are frequent, they can represent acute muscle denervation.
- Widespread fasciculations involving many muscles can be appreciated with motor neuron disease (ALS)
Complex repetitive discharges (CRDs)
- Bursts of recurrently-firing complex MUAPs that repeat at a regular frequency.
- They typically have an abrupt beginning and end.
- They are non-specific but can be a sign of either neuropathic or myopathic processes.
- Audibly, they can be compared to a running motor.
Myotonic discharges
- Chronically depolarizing muscle membranes with a characteristic waxing and waning frequency.
- Often described as having a “dive bomber” sound.
- Can be due to a wide range of myopathic disorders including myotonic dystrophy, myotonia congenita, periodic paralysis, Pompe disease, Isaac syndrome, and toxic myopathy.
Myokymia
- A rhythmic firing of grouped motor units (doublets, triplets, multiplets).
- Audibly, can be compared to marching soldiers.
- This is seen in post-radiation neuropathy/plexopathy.
What you need to know
Video identification of these discharges can be seen on Board examinations but not on in-service or shelf examinations. Image identification of abnormal spontaneous activity can be asked on in-service examinations.
Exertional (voluntary) activity
- Assesses the motor unit action potentials (MUAPs) during muscle contraction for morphology, activity, recruitment, firing rate.
- Recruitment: the number of firing motor units firing per the force applied during voluntary contraction.
- Increased/early/rapid recruitment is seen in myopathy
- Decreased recruitment is seen in disorders involving axon loss/damage.
Neurogenic findings (chronic)
- Collateral spouting by surviving neurons leads to an increase in the number of muscle fibers innervated by a single motor unit.
- Chronic neuropathic lesions lead to high amplitude, polyphasic MUAPs with increased duration, decreased recruitment (low number of firing motor units per applied force), and increased firing rate.
- Reinnervation occurs 3 months after the onset of the injury.
Myopathic findings
- Small amplitude, decreased duration, and increased/early recruitment (high number of firing motor units per applied force).
Tip
Acute neuronal injuries have abnormal spontaneous activity. Chronic neuronal injuries have abnormal exertional activity.
The H-Reflex
- Equivalent to an S1-ankle reflex.
- This is a true reflex with a sensory afferent, a synapse, and a motor efferent segment.
- H reflexes are recorded from the leg by stimulating the sensory fibers of the tibial nerve and recording the reflex at the gastrocnemius muscle belly.
- The waveform is optimal with submaximal stimulation and has relatively constant latencies.
- It is helpful to assess proximal lesions.
What you need to know:
The H-reflex is useful in the diagnosis of S1 root lesions as well as the study of proximal nerve segments in either peripheral or proximal neuropathies.
F-waves
- F-waves are used to assess the entire length of the motor axon and good for evaluating proximal neuronal function.
- An electrical stimulus is placed above a motor nerve which then travels antidromically to the motor neuron cell bodies and then in an orthodromic fashion back down towards the muscle.
- F waves are assessed with supramaximal stimulation and have varying latencies and morphology.
- Prolonged F waves are often the earliest sign of Guillain-Barre syndrome/AIDP and other demyelinating illnesses.
References
- Mesrati, F., and M.f. Vecchierini. “F-Waves: Neurophysiology and Clinical Value.” Neurophysiologie Clinique/Clinical Neurophysiology, vol. 34, no. 5, 2004, pp. 217–243., doi:10.1016/j.neucli.2004.09.005.
- Mills KR. The basics of electromyography. J Neurol Neurosurg Psychiatry. 2005;76 Suppl 2(Suppl 2):ii32–ii35. doi:10.1136/jnnp.2005.069211
- OpenStax College, Anatomy and Physiology. OpenStax CNX. Nov 1, 2018 http://cnx.org/contents/[email protected].
- Paganoni S, Amato A. Electrodiagnostic evaluation of myopathies. Phys Med Rehabil Clin N Am. 2013;24(1):193–207. doi:10.1016/j.pmr.2012.08.017
- Purves, D., Augustine, G. J., Fitzpatrick, D., Hall, W. C., LaMantia, A.-S., Mooney, R. D., Platt, M. L., … White, L. E. (2018). Neuroscience.
- Cornblath, D. R., Asbury, A. K., Albers, J. W., & Feasby, T. E. (1991). Research criteria for diagnosis of chronic inflammatory demyelinating polyneuropathy (CIDP). Neurology.
- Weiss, Jay, et al. Easy EMG: a Guide to Performing Nerve Conduction Studies and Electromyography. Butterworth-Heinemann, 2016.
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