Nerve Conduction Study (NCS)• NCS is a test commonly used to evaluate the function of the motor and sensory nerves of the human body.• Nerve conduction velocity (NCV) is a common measurement made during this test.• The term NCV often is used to mean the actual test, but this may be misleading since, velocity is only one measurement in the test suite.
Uses• Nerve conduction studies are used mainly for evaluation of paresthesias (numbness, tingling, burning) and/or weakness of the arms and legs.• The type of study required is dependent in part, by the symptoms presented.• Some indications of nerve conduction studies are: – Symptoms indicative of nerve damage as numbness, weakness. – Differentiation between local or diffuse disease process (mononeuropathy or polyneuropathy). – Get prognostic information on the type and extent of nerve injury.
Common disorders diagnosed by NCS Peripheral neuropathy • Mononeuropathy (ex: carpal tunnel syndrome) • Mononeuritis multiplex (ex: vasculitides, rheumatoid arthritis, lupus erythematosus [SLE], sarcoidosis, leprosy, Lyme disease, amyloidosis) • Polyneuropathy (ex: diabetic neuropathy,) Myopathy • Muscular dystrophies (ex: Facioscapulohumeral muscular dystrophy) • Myotonia • Congenital myopathies • Metabolic myopathies Radiculopathy (problem in which one or more nerves are affected with emphasis on the nerve root; Radix = "root") • Nerve damage from herniated discs Diseases of neuromuscular junction • Myasthenia gravis
Description of the procedure Electrodes • Skin will be cleaned • electrodes will be taped to the skin along the nerves that are being studied Stimulus • Small stimulus is applied (electric current) that activate nerves Current • The electrodes will measure the current that travels down the nerve pathway
Description of the procedure (continued..) If damaged? • If the nerve damaged, the current will be slower and weaker Time • The procedure takes about 30-90 minutes Complications • No reported complication from the procedure • expect feeling discomfort from electrical current, but not painful
Important points about NCS• The test is not invasive.• No contraindication to the procedure, but if there is an artificial pacemaker, appropriate precautions should be taken.• Anesthesia is not used for this procedure.• No special post procedure precautions.• The test is sometimes combined with Electromyography (EMG).
Components of NCS• The NCS consists of the following components: – Compound Motor Action Potential (CMAP); also called Motor nerve conduction study – Sensory Nerve Action Potential (SNAP); also called Sensory nerve conduction study – F-wave study – H-reflex study – A-(Axon) wave study will not be – Blink Reflex study discussed… – Direct Facial Nerve Study
Motor nerve conduction study• This NCS represents the conduction of an impulse along peripheral motor nerve fibers.• It is recorded as a compound evoked potential from a motor point within the muscle.• The time it takes for electrical impulse to travel from the stimulation to the recording site is measured.• This value called latency and measured in milliseconds (ms).• The size of the response called the amplitude and measured in millivolts (mv).• By stimulating in two or more different locations along the same nerve, NCV across different segments can be measured.
Motor nerve conduction study (cont..)• It corresponds to the integrity of the motor unit but cannot distinguish between pre- and postganglionic lesions because the cell body is located in the spinal cord.• It can be abnormal with normal SNAPs if the lesion is proximal to the DRG or affecting a purely motor nerve.• The active and reference pickup should not be too close together. If this occurs, similar waveforms are recorded at both sites and rejected, dropping the amplitude of the waveform effect on the amplitude of varying the inter-electrode separation. I: Normal. Compound Motor Action Potential II: Pickups are too close.
Motor nerve conduction study – sites Median nerves (R & L) at; • Wrist Abductor Pollicis Brevis • Elbow Ulnar nerves (R & L) at; • Wrist First Dorsal Interosseous (FDI) • Elbow Abductor Digiti Minimi (ADM) Peroneal nerves (R & L) at; • Ankle Extensor Digitorum Brevis • Head of fibula Tibialis Anterior Tibial nerves(R & L) at; • Ankle Abductor Hallucis Abductor Digiti Quinti Pedis
Sensory nerve conduction study• This NCS represents the conduction of an impulse along the sensory nerve fibers.• 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.• The recording electrode is placed proximal to the stimulating electrode. (antidromic nerve impulse is recorded)• Like the motor studies, sensory latencies are on the scale of milliseconds (ms).• Sensory amplitudes are much smaller than the motor amplitudes, usually in the microvolt (μV) range.• The sensory NCV is calculated based upon the latency and the distance between the stimulating and recording electrode.
Sensory nerve conduction study (cont..) • It can also be useful in localizing a lesion in relation to the dorsal root ganglion (DRG). • The DRG is located in the neural foramen and contains the sensory cell body. Lesions proximal to it (root, spinal cord) preserve the SNAP despite clinical sensory abnormalities. • This is because axonal transport from the cell body to the axon continues to remain intact. • SNAPs are typically considered more sensitive than CMAPs in the detection of an incomplete peripheral nerve injury. • Antidromic Studies; – Are easier to record a response than orthodromic studies – Are less uncomfortable when orthodromic studies secondary to less stimulation required – Have larger amplitudes due to the nerve being more superficial at the distal recording sites
Sensory nerve conduction study – sites Median nerves (R & L) at; • index finger • thumb Ulnar nerves (R & L) at; • little finger • ring finger Sural nerves (R & L) at; • behind the Lateral Malleolus Saphenous nerves(R & L) at; • anterior to the Medial Malleolus
F-wave study• This NCS evokes a small late response from a short duration supramaximal stimulation.• It initiates an antidromic motor response to the spinal cord followed by an orthodromic motor response to the recording electrode.• It is approximately 5% of the compound motor action potential (CMAP) height.• The configuration and latency change with each stimulation.• This is due to a polysynaptic response in the spinal cord, where Renshaw cells (R) inhibit impulses from traveling the same path each time.
F-wave study (continued..)• This is not a reflex, because action potentials travels from the site of the stimulating electrode in a limb to the spinal cord and back to the limb in the same nerve that was stimulated.• The F- waves latency can be used to derive the conduction velocity of nerves between the limb and spinal cord, whereas the motor and sensory nerve conduction study in the same segment of the limb.• Conduction velocity is derived by measuring the limb length in millimeters from the stimulation site to the corresponding spinal segment (ex: C7 spinous process to wrist crease for median nerve).• This is multiplied by 2 as it goes to the cord and returns to the muscle.• Limitation: This evaluates a long neural pathway, which can dilute focal lesions and hinder specificity of injury location. It only accesses the motor fibers.
H- reflex study (continued..)• This NCS creates a late response that is an electrically evoked analogue to a monosynaptic reflex.• It is initiated with a submaximal stimulus at a long duration (0.5–1.0 milliseconds).• This preferentially activates the IA afferent nerve fibers, causing an orthodromic sensory response to the spinal cord, and then an orthodromic motor response back to the recording electrode.• The morphology and latency remains constant with each stimulation at the appropriate intensity.
H- reflex study• This NCS creates a late response that is an electrically evoked analogue to a monosynaptic reflex.• It is initiated with a submaximal stimulus at a long duration (0.5–1.0 milliseconds).• This preferentially activates the IA afferent nerve fibers, causing an orthodromic sensory response to the spinal cord, and then an orthodromic motor response back to the recording electrode.• The morphology of wave pattern and latency remains constant with each stimulation at the appropriate intensity.
Interpretation of nerve conductions• The speed of nerve conduction is related to – the diameter of the nerve and, – the degree of myelination (a myelin sheath is a type of "insulation" around the nerve).• A normally functioning nerve will transmit a stronger and faster signal than a damaged nerve.• In general, the range of normal conduction velocity will be approximately 50 to 60 meters per second. However, the normal conduction velocity may vary from one individual to another and from one nerve to another.• The Interpretation of nerve conductions is complex, but in general, different pathological processes result in: – changes in the latencies – changes in the amplitudes – slowing of the conduction velocity
Interpretation of nerve conductions (continued..)• Examples; – slowing of the NCS usually indicates there is damage to myelin. – slowing across the wrist for the motor and sensory latencies of the median nerve indicates focal compression of the median nerve at the wrist, called carpal tunnel syndrome. – slowing of all nerve conductions in more than one limb indicates generalized peripheral neuropathy (eg. in diabetes mellitus).
References• National Center for Biotechnology Information (NCBI) web site (26.06.2011) http://www.ncbi.nlm.nih.gov/books/NBK2 7199/#A7198• Wikipedia, the free encyclopaedia (26.06.2011) http://en.wikipedia.org/wiki/Nerve_condu ction_study
Special Thanks!• Dr. Sudath Gunasekera, Consultant Clinical Neurophysiologist, NHSL.
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