Movement-Related Potentials and Event-Related Potentials

Movement-related cortical potentials (MRCPs) are EEG potentials that occur with movement and are recorded using surface scalp electrodes. A technique termed “EEG-EMG back-averaging” is used to obtain MRCPs. The earliest recordable MRCP is the Bereitschaftspotential or readiness potential. Special EEG averaging techniques may also be used to study the cortical processes underlying cognition. Event-related potentials (ERPs) record the cortical activity evoked by a stimulus charged with cognitive significance. The P300 is the most commonly recorded ERP, elicited in an oddball technique of auditory stimulation; the subject is instructed to attend to a rare stimulus presented among a string of frequent stimuli. Only trials triggered by this rare event are averaged. The P300 may be the electrophysiological correlate of selected attention. The N400, another ERP, is assessed during semantic comprehension of language. The chapter discusses normal variants of MRCPs and ERPs, as well as disruptions secondary to neurological disease.

Evaluation of Adrenergic Function

Peripheral adrenergic function is important in the maintenance of postural normotension. It may be impaired in peripheral neuropathies, and this may be manifested as alterations in acral temperature, color, or sweating. Simple, accurate, and reproducible tests of peripheral adrenergic function are now routinely used in clinical autonomic laboratories. For noninvasive evaluation of autonomic function, tests of peripheral adrenergic function can be used to separately evaluate the vagal and adrenergic components of baroreflex sensitivity. The vagal component is derived from the heart period response to blood pressure change and the adrenergic component by the blood pressure recovery time in response to the preceding fall in blood pressure, induced by the Valsalva maneuver.This chapter describes methods used to determine peripheral adrenergic function and their value and shortcomings.

Application of Clinical Neurophysiology

Clinical neurophysiology testing primarily assesses and characterizes neurological disease. Selection of appropriate studies for the problem of an individual patient requires a careful clinical evaluation to determine possible causes of the patient’s symptoms. The approach to testing can be assisted by deciding which structures are likely to be involved. For example, motor and sensory symptoms are best assessed using the different methods of motor and sensory NCS. Deciding which neurophysiological measures to apply in peripheral disorders is sometimes assisted by applying guideline protocols based on the patient’s clinical findings and what is found during testing. Although a clinical neurophysiological assessment rarely provides evidence for a specific diagnosis, it can provide valuable information about the severity, progression, and prognosis of the disease. This chapter reviews the clinical application of neurophysiological tests, particularly nerve conduction studies and needle EMG, in the assessment of patients with a variety of neuromuscular complaints.

Quantitative Electromyography

Semi-quantitative EMG methods are in common use in clinical electromyography laboratories but have a number of drawbacks and limitations, including examiner bias in MUP analysis and challenges distinguishing between MUP categories of normal and neurogenic and normal and myopathic waveforms. An array of formal MUP quantitation methods has been developed in recent decades, which seek to address many of the shortcomings of semiquantitative EMG. The advantages of quantitative EMG (QEMG) include: (1) making measurements of MUP recordings consisting of numerical values derived from precise measurements, (2) generating normative data and allowing comparisons with data from patients with suspected neuromuscular diseases, (3) allowing for reproducible results that can be compared at different times by different examiners and in different labs, and (4) allowing accurate assessment of improvement or deterioration in disease severity over time.

Cranial Reflexes and Related Techniques

Electrophysiological study of the function of cranial nerves, particularly the fifth and seventh cranial nerves, may be useful for assessing cranial neuropathies or facial movement disorders, such as hemifacial spasm or facial synkinesis. Several electrophysiological techniques are available in clinical neurophysiology laboratories to study these nerves and cranial reflexes. These techniques can also provide useful information in some cases of peripheral neuropathy, polyradiculoneuropathy, and brain stem lesions. This chapter reviews the concepts, methods, and applications of cranial reflexes, including the blink reflex, the jaw jerk (or masseter reflex), and the masseter inhibitory reflex (MIR). Two additional techniques—one to assess a sensory nerve in the head that is not a cranial nerve of branchial arch origin, the great auricular sensory nerve, and the other to interrogate trigeminal sensory pathways from the sensory receptor level to the parietal cortex, contact heat evoked potentials—are also discussed.

Repetitive Nerve Stimulation Studies

Repetitive stimulation is a technique that evaluates the function of the neuromuscular junction. It is important not only in the detection, clarification, and follow-up of neuromuscular junction diseases, but also in excluding these disorders in patients with symptoms of fatigue, vague weakness, diplopia, ptosis, and malaise, or with objective weakness of uncertain origin. The technique requires knowledge of the physiology and pathophysiology of neuromuscular transmission and the basic techniques of nerve conduction studies. This chapter includes a brief review of the anatomy and physiology of the neuromuscular junction as it applies to repetitive stimulation, a detailed discussion of the technique, the pitfalls that can occur if not carried out correctly, criteria used to classify the results as normal or abnormal, the patterns of abnormalities that can be seen, and the clinical correlation of those abnormalities with the various different disorders of neuromuscular transmission.

Clinical Applications

Many electrophysiological assessment and techniques of clinical neurophysiology can be used in the assessment of patients with suspected disease of the central nervous system. Each of the techniques is applied either to assist clinicians in assessing disease of the central nervous system or, less commonly, to monitor changes in neural function. These techniques can be used to monitor neural function in observing progression of disease, such as the frequency of seizures, or improvement in a patient’s condition with specific treatment. They are also used in the intensive care unit and operating room to identify progressive neural damage. The clinical neurophysiological testing technique that is most appropriate for a patient depends on the clinical problem, and, often, some combination of techniques best provides the necessary data. This chapter focuses on the application of clinical neurophysiological techniques in assessing patients with suspected central nervous system disorders.

EEG Trend Analysis in the ICU

Continuous EEG monitoring can increase the detection of subclinical seizures, and is important in managing nonconvulsive status epilepticus. In the ICU it presents challenges not routinely encountered in the outpatient EEG laboratory or the epilepsy monitoring unit: multiple sources of artifact, and the need for imaging-compatible electrodes and a robust IT support system. Rhythmic and periodic patterns of indeterminate significance are encountered. There is much debate as to the true significance of these patterns, and clinical correlation is always required. Special techniques can be employed in the application and analysis of ICU EEG monitoring. EEG has been useful in monitoring for ischemia, prognosis, and depth of medication-induced suppression. Quantitative EEG can also be utilized to assist in rapid seizure detection, and to monitor for subtle gradual changes in cerebral function and seizure detection. The special environment, however, requires close attention to technical considerations, and thoughtful interpretations of indeterminate patterns.

Epilepsy Surgery Evaluation

Epilepsy can be a devastating illness for those afflicted. Unfortunately, approximately one-third of people diagnosed with epilepsy are not effectively treated with standard medical management. People with medically refractory epilepsy can be treated and possibly cured of their disease utilizing a surgical approach. The electroencephalogram currently remains the “gold standard” for characterizing and localizing the ictal onset zone. Standard surface and sometimes intracranial EEG, when appropriate, are typically utilized in the evaluation process. The epilepsy surgical evaluation is sometimes enhanced with the utilization of SPECT/SISCOM imaging to further help confirm the seizure focus. Data gathered during the evaluation process guide the surgical resection, with improved remission rates correlating with precise localization of the ictal onset zone. This chapter describes the current presurgical epilepsy evaluation using the EEG and SPECT scanning.

Audiogram, Acoustic Reflexes, and Evoked Otoacoustic Emissions

Pure-tone air-conduction and bone-conduction evaluations separate hearing loss into conductive, sensorineural, or mixed categories, and also indicate the degree of hearing loss and attendant communication difficulties. The inclusion of specific types of speech tests assess the ability of the patient to hear and understand speech. Acoustic reflex threshold and reflex decay evaluations evaluate a complex neural network, including afferent pathways to and through the lower brainstem, decussating brainstem pathways, and efferent innervation of CN VII to the stapedius muscle in the middle ear. Evoked otoacoustic emissions provide objective measurement of the peripheral auditory system coursing from the external canal to the cochlear outer hair cells. They are implemented widely in screening tests for hearing in infants, for patients suspected of auditory neuropathy spectrum disorder, and for patients suspected of pseudohypacusis; that is, feigning or exaggerated hearing loss.