Phase cancellation and temporal dispersion are vital concepts for the electromyographer to understand in order to accurately interpret variations in recorded compound potentials (sensory nerve action potentials, compound muscle action potentials) that are seen with distal and proximal stimulation. This becomes important in determining physiologic from pathologic conditions. Although the concepts are intrinsically linked and represent the same underlying electrophysiological principle, the best way to begin understanding them is by first separating them. Evoked potentials recorded in standard nerve conduction studies are a response summation of electrical fields produced by depolarization along nerve and muscle fibers of varying size. During a motor and sensory nerve conduction study, stimulating a nerve at both proximal and distal sites while recording from a single location produces waveforms with slight differences in morphology. Temporal dispersion is the increase in the difference between the conduction times along the different axons within a nerve.
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Needle electromyography (
EMG) is usually performed after nerve conduction studies have been completed, but can be performed first or by itself depending on the clinical indication. No electrical stimulation is used with EMG; the needle is simply a tool to measure the electrical activity in the muscle. The muscles that will be evaluated are chosen carefully based on the differential diagnosis. The examiner should evaluate only the minimum number of muscles that are necessary to do an adequate study, as examining more muscles may make the test more uncomfortable for the patient. It is important to begin with an environment that is conducive to performing a study. Whenever possible, this includes a comfortable, quiet, interference-free environment, which is stocked with the appropriate supplies to limit interruptions. This chapter discusses basic EMGtechnique. It focuses on reducing anxiety and discomfort of the needle examination. The chapter provides protocol for procedure.
Nerve conduction and needle examination interpretation requires knowledge of normal values and understanding the significance of variations from normal. Abnormalities can help diagnose a focal neuropathy, distinguish a demyelinating neuropathy from axonal neuropathy, and detect a myopathic process. This chapter discusses nerve conduction study interpretation. Nerve conductions can be used to determine if the lesion is distal, proximal, or segmental. Amplitude can be used to determine if axon loss, conduction block, or both are present along a nerve segment. Nerve conductions can help determine if the lesion is neuropathic or myopathic. After nerve injury, motor axons may remain excitable for up to 7 days, and sensory axons may remain excitable for up to 11 days. There are four components of needle electromyography that are used to guide interpretation: insertional activity, motor unit morphology, motor unit recruitment, and pattern of muscle abnormalities.
Cervical radiculopathy results from a disruption of the nerve roots from the cervical spine. It occurs much less frequently than lumbar radiculopathy. Electrodiagnostic studies are a useful tool to supplement history and physical examination in the diagnosis of a cervical radiculopathy. This chapter discusses cervical radiculopathy and its clinical presentation. It also describes the anatomy of the cervical spine. Cervical spinal nerves exit the spinal column above the level of their similarly numbered cervical vertebrae (C8 exit s above T1 vertebra). Comprised of the ventral root (motor fibers) whose cell body is in the anterior horn of the spinal cord, and the dorsal root (sensory fibers) whose cell body is outside the spinal cord in the dorsal root ganglion. The optimal evaluation of a patient with suspected radiculopathy is at least one motor and one sensory nerve conduction study in the involved limb.
The human body constantly generates electrical energy. Specifically, the muscle and nerve cells constantly use electric discharges to communicate among different parts of the body. These electric discharges can be recorded, displayed, measured, and interpreted by using specialized equipment. In the presence of disease or injury, the architecture and normal processes of nerves and muscles are altered. Recognizing these changes can be useful for diagnosis, monitoring disease progression, and assessing treatments. Electrodiagnostic (
EDX) medicine is the process of observing and interpreting neuromuscular electrical discharges for clinical purposes. EDXincludes nerve conduction studies and electromyography. This chapter provides the introduction to electrodiagnostics. It presents the basic logistical details for novice EDXconsultant’s first day of performing electrodiagnostics. The chapter also discusses the basic construction of an electrodiagnostic consult.
This chapter discusses motor unit action potential analysis. A motor unit action potential (
MUAP) is the electromyographic response that results from the discharge of all the muscle fibers that are innervated by a single motor neuron. The size and shape of an MUAPcan be described in terms of amplitude, duration, phasicity, and stability. The size and shape of MUAPsare dependent on the size, number, and functional state of muscle fibers innervated by a single motor neuron. Firing characteristics and recruitment of motor units can also be quantified, enabling identification of different patterns of pathology of the neuromuscular system. The chapter provides a brief description on motor unit morphology and changes in MUAPcharacteristics and morphology in diseases such as myopathies; denervation and reinnervation.
Facial neuropathy is the most common cranial neuropathy. Diabetes and pregnancy are predisposing conditions. Usually presents as paresis or paralysis of upper and lower facial muscles ipsilateral to the facial nerve lesion. In the face, originate from the primary motor cortex and control contralateral lower motor neurons. Common approaches include facial motor studies and the blink reflex. The facial motor study assesses the peripheral efferent motor pathway of cranial nerve (
CN) VII, whereas the blink reflex involves afferent conduction via CNV and efferent conduction along CNVII. The blink reflex is most useful when localization of the lesion is in question. Needle electromyography used to demonstrate motor activity and/or denervation in selected facial muscles. It does not require comparison to a normal contralateral side and thus may be helpful diagnostically when bilateral pathology is present. Muscles commonly sampled include frontalis, orbicularis oris, orbicularis oculi, and mentalis.
Femoral nerve injuries are uncommon, and thus easily missed. The femoral nerve innervates the hip flexors and knee extensors, which are key muscle groups in ambulation. The saphenous nerve is a distal extension of the femoral nerve. The most common causes for femoral nerve injury are iatrogenic from surgery, particularly abdominal and pelvic, related to lithotomy positioning or compression from retractors. History of present illness and physical examination are important components in electrodiagnostic assessment as they help the examiner to localize the lesion. The chapter discusses the anatomy of femur: inguinal ligament; femoral triangle (Scarpa triangle); and Hunter canal. It provides a brief description on electrodiagnostic approach for femoral neuropathy. Nerve conduction study can be quite uncomfortable due to nerve depth.
Radial neuropathy is commonly associated with lead poisoning (wrist drop), penetrating trauma and humeral fracture, sleeping with an arm slung around a chair or partner (Saturday night palsy, honeymooner’s palsy), Wartenberg syndrome (superficial radial neuropathy at the elbow), posterior interosseous nerve (
PIN) syndrome (compression about the elbow, neuralgic amyotrophy), and trauma or compression injury at the wrist (“cheiralgia paresthetica”, handcuff neuropathy). This chapter discusses radial neuropathy. It provides a brief description on its clinical presentation. The chapter describes the anatomy of the radial nerve. It discusses electrodiagnostic approach such as nerve conduction studies and electromyography for evaluating radial neuropathy. The chapter also focuses on recovery in radial neuropathy.
High frequency ultrasound is an excellent complimentary modality to electrodiagnostic techniques. It provides detailed anatomic correlation to the physiologic information obtained with electrodiagnosis. It has many advantages for assessing neuromuscular conditions over other imaging modalities. It can be used to assess pathology in peripheral nerves, mimicking neuromuscular conditions and muscles diseases. It can also be used to safely guide electromyography needle insertion in vulnerable regions and assist with anatomic localization of challenging nerves and muscles. This chapter discusses the uses of ultrasound with electrodiagnosis. It list out the advantages of using ultrasound with electrophysiologic techniques and over other imaging techniques. The chapter provides the principles of imaging peripheral nerves with ultrasound. It also provides imaging strategies to improve visualization of peripheral nerves and principles of imaging muscle for neuromuscular assessment with ultrasound.