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Nerve Conduction Studies CLINICAL Nerve conduction William Huynh Matthew C Kiernan studies This article forms part of our ‘Tests and results’ series for 2011 which aims to provide information In addition to these diagnostic and monitoring about common tests that general practitioners order regularly. It considers areas such as indications, roles, clinical neurophysiology may provide what to tell the patient, what the test can and cannot tell you, and interpretation of results. information about prognosis and can guide management. Keywords: nerve conduction studies; electrodiagnosis; electromyography; peripheral Precautions neuropathy; nerve disorders Cardiac pacemakers or defibrillators are not an absolute contraindication but discussion with the patient’s cardiologist is advisable, particularly if Nerve conduction studies (NCS) and needle the stimulation site is in close proximity to the electromyography (EMG) are collectively chest wall. Studies have demonstrated the safety termed ‘clinical neurophysiology’. They of routine NCS in patients with implanted cardiac enable the clinician to detect signs that devices.1 cannot be confirmed by neurological As EMG involves insertion of fine needles, examination alone and can guide diagnosis care is also required for patients prescribed and treatment. anticoagulation therapy (eg. warfarin) to avoid the development of haematoma. Indications Patient information and Clinical neurophysiology aids diagnosis of disorders preparation of the peripheral nervous system (Table 1). Testing helps to: Nerve conduction studies involve the stimulation • localise the site or level of the lesion; of nerves with small electrical impulses over determining if the pathology involves the several points (usually limbs) and measuring the peripheral nerve, neuromuscular junction, resultant responses. Surface electrodes are used plexus, nerve root or anterior horn cells to both deliver and detect the electrical impulses • identify the pathophysiology, in particular (Figure 1). The test is safe and well tolerated distinguishing axonal loss from demyelination with only minor discomfort and no long term side • diagnose mononeuropathies (eg. common nerve effects. Most patients describe the effects as a entrapments such as carpal tunnel syndrome, ‘tingling’ or ‘tapping’ sensation. ulnar neuropathy at the elbow and peroneal Patients should avoid prior application of topical palsy) creams as these may increase skin resistance • diagnose more diffuse processes (eg. to the applied current, and therefore require generalised peripheral neuropathy due to stronger levels of electrical stimulation. In cold diabetes or inflammatory neuropathy such as environments, the limbs may need warming as cool Guillain-Barré syndrome). peripheries (<32°C) slow the conduction velocity Nerve conduction studies are also used to of nerves. No fasting is required and patients monitor nerve function over time to determine can return to normal activities such as driving disease progression, to assess the complications immediately afterwards. Patients may continue to of treatment (eg. chemotherapy), as well as take all their regularly prescribed medications. identifying the disease course (acute/subacute/ Nerve conduction studies are performed chronic). by clinical neurophysiologists, usually in an Reprinted from AUSTRALIAN FAMILY PHYSICIAN VOL. 40, NO. 9, SEPTEMBER 2011 693 CLINICAL Nerve conduction studies Table 1. Common conditions referred to clinical neurophysiology Level of lesion(s) Common disorder Common presenting symptoms Peripheral nerve Carpal tunnel syndrome Sensory disturbance of hands and fingers with entrapment syndromes nocturnal exacerbation Ulnar neuropathy at elbow Sensory disturbance of digits four/five; weakness and wasting of intrinsic hand muscles Peroneal palsy at fibular neck Foot drop Polyneuropathy • Metabolic (eg. diabetes) Length dependent sensory and/or motor impairment • Toxic (eg. alcohol) • Medications (eg. chemotherapy) • Hereditary Plexus Brachial plexus Unilateral shoulder pain with sensory disturbance and (eg. brachial neuritis) weakness in upper limb Nerve root Cervical radiculopathy Neck and upper limb pain; sensory disturbance and weakness in arm/hand/fingers Lumbosacral radiculopathy Lower back pain; sciatica type symptoms and weakness in lower limb Anterior horn cell Motor neuron disease (amyotrophic Progressive limb weakness; bulbar weakness; muscle (‘neuronopathy’) lateral sclerosis) fasciculations Neuromuscular junction Myasthenia gravis Fatiguable proximal weakness; diplopia; bulbar weakness Muscle Inflammatory myopathies (eg. Proximal muscle weakness and myalgia; elevated serum polymyositis) creatine kinase outpatient rooms setting. Typically, NCS take between 15 minutes and an hour, depending on the number of sites requiring testing and the complexity of the clinical presentation. A Medicare rebate is available for the study and varies depending on the complexity of the study and the number of nerves examined. distance (elbow-wrist) Conduction = Principles of nerve conduction velocity latency (elbow) – latency (wrist) studies 250 mm Sensory, motor or mixed nerves can be studied. Eg. CV = = 55.6 m/s 7.5 ms –3 ms Pairs of electrodes are used – one to initiate the impulse and the other to record the response Peak further along the path of the nerve (distally 5 mV within the innervated muscle for motor nerves 3 ms Amplitude Duration or proximally along sensory nerves). For motor Wrist Onset Figure 1. Motor conduction study of the nerves, a depolarising square wave current is latency applied to the peripheral nerve to produce a median nerve with recording electrodes over the abductor pollicis brevis muscle compound muscle action potential (CMAP) due Elbow and resultant CMAP responses following to summation of the activated muscle fibres. stimulation of the nerve at the wrist and elbow In sensory nerves, a propagated sensory nerve action potential (SNAP) is created in a similar • latency (ms) – from stimulus to onset of conduction, and is reduced in demyelinating manner. evoked response processes). The parameters obtained and used for • duration of response (ms) Late responses interpretation include (Figure 1): • conduction velocity (m/s) – calculated from the • amplitude – from baseline to peak (reflects the distance between stimulation and recording Late responses can be used to assess more number of conducting fibres and is reduced in points, divided by latency (reflects integrity proximal segments of the peripheral nervous axonal loss) of the myelin sheath important for impulse system, such as the plexus and nerve roots. 694 Reprinted from AUSTRALIAN FAMILY PHYSICIAN VOL. 40, NO. 9, SEPTEMBER 2011 Nerve conduction studies CLINICAL a proportion of motor neurons, resulting in a area/amplitude of at least 50% at a proximal 2 mV delayed and smaller muscle response as the compared to a distal site of stimulation). 5 ms impulse travels back down the motor nerve. Conventional NCS may be unremarkable in the Wrist The H reflex, which is elicited by submaximal case of demyelinating diseases involving the more stimuli, is the electrical equivalent of a tendon proximal segments (eg. Gillain-Barré syndrome), jerk (or monosynaptic stretch reflex) and involves in which case, prolongation of the F wave and H stimulation of a sensory axon to produce a reflex latencies may be the only abnormality. delayed motor response. Amplitude is less Electromyography relevant for late responses – useful parameters Elbow of F waves and H reflexes are their presence and Electromyography is typically undertaken in minimal latencies. conjunction with NCS when more specific Figure 2. Median nerve CMAP recorded information is required. Electromyography is over abductor pollis brevis demonstrating Interpretation most commonly used to investigate weakness temporal dispersion when stimulating at the wrist (top) and elbow (bottom) When interpreting NCS data, initial considerations and helps distinguish myopathic from neurogenic are: causes. Fine needles are inserted into muscle • Is the compound potential normal in size and fibres and then the patient is asked to contract These proximal segments are not accessible shape, or reduced? these muscles. Electromyography enables by routine NCS techniques and is not possible • Is the conduction velocity normal? assessment of the morphology of single motor to position the electrodes in close proximity to Table 2 outlines key differences between units (neuron, axon and innervated muscle fibres) the spinal cord or its nearby structures. This is NCS results in axonal loss and demyelinating and the recruitment pattern of these units. important as proximal segments may be the only processes. As a simple rule, axonal degeneration Changes in EMG morphology reflect changes in site of pathology, for example, in radiculopathy leads to a loss of amplitude and demyelination the number and size of muscle fibres innervated due to disc protrusion or in polyradiculopathy leads to prolonged conduction time. by single motor axons. Neurogenic lesions from Guillain-Barré syndrome. Late responses typically demonstrate polyphasic large motor Axonal loss also provide information about longer nerve units with a reduced recruitment pattern, while pathways so may detect some conduction The most common abnormality is reduction in myopathic motor units are small and polyphasic abnormalities that would not be identified on compound amplitude reflecting fewer functioning with early recruitment. Electromyography the shorter NCS range, particularly
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