Our nervous system controls every function in our bodies. The neurons at every level of the nervous system—cortex, brainstem, spinal cord, cranial and peripheral nerves, neuromuscular junctions, and muscles—are ultimately involved in every type of voluntary and involuntary activity that occurs within our bodies to allow us to function as human beings. Without our nervous systems, we cannot function appropriately or even survive. In conditions resulting in a focal loss of neurons, one may experience symptoms localized to one part of the body, such as paralysis, involuntary shaking, or loss of feeling in an arm or a leg; in other situations, abnormally functioning neurons in a more diffuse distribution may result in generalized symptoms, such as a loss of consciousness or diffuse weakness. The electrical properties of the neurons and their communications with each other and other organs help to maintain the multitude of functions that allow us to exist. Clinical neurophysiology is the study of the electrical properties of these cells. If these electrical signals “go awry,” resulting in either an increase in excitation or loss of excitability, neurologic symptoms develop. For example, hyperexcitability of cortical or subcortical neurons may result in seizures or movement disorders, whereas loss of function of central or peripheral neurons may result in paralysis, orthostatic hypotension, or peripheral neuropathy. In clinical neurophysiology, neural function is assessed by measuring the electric potentials generated by neural tissue and the changes in these potentials produced by disease.
These potentials can be studied in every system—consciousness, motor, sensory, autonomic, and movement. They can be studied in awake or asleep patients in the outpatient setting, in patients in a state of unconsciousness in the intensive care unit (ICU), or in patients undergoing surgery in the operative setting. The various tests used to assess nerve function have been routinely used in clinical practice for decades. While the basics of the tests and the underlying concepts of basic neurophysiology have not dramatically changed in the past several years, advances in the ability to study the intricacies of the systems, technologic improvements in equipment, and advances in the methods to detect subtle changes in the nerve function continue. The fifth edition of Clinical Neurophysiology focuses on neurophysiologic techniques and applications that are used in clinical practice to assist physicians in the evaluation of a variety of neurologic symptoms and diseases.
An understanding of the basic concepts of neurophysiology is critical to understanding the meaning and implications of each type of test performed in clinical practice. Furthermore, understanding the generator sites and waveforms produced from the electrical signals is important, as they form the basis of interpretation of the studies. The book begins with three chapters that review of the concepts of basic neurophysiology (Chapter 1), neurophysiology generators (Chapter 2), and basic waveforms (chapter 3); these chapters provide an important foundation for understanding the rationale and responses obtained with the different types of testing detailed in subsequent sections.
Clinical electroencephalography (EEG) records the continuous electrical signals arising from cerebral cortex using electrodes applied to the scalp. The patterns of the signals can provide important clues to the underlying function of the cortex and the presence of diseases that affect the brain. The techniques of EEG are used primarily to assess disorders that affect the cerebral cortex, including seizures, spells, and disorders of consciousness, and are used to monitor the function of the cerebral cortex during surgeries that place the cortex at risk of injury. The EEG techniques and patterns detailed in Chapters 4–9 reflect the normal and abnormal EEG findings and the alterations in disease processes that directly involve the cerebral cortex in the adult and pediatric populations.
Expansion of EEG beyond that performed in the outpatient EEG laboratory setting has proven necessary in order to more effectively study epilepsy and related conditions. For example, longer recordings may be needed to document infrequent episodes or sporadic interictal activity and to provide clinical correlation. Long-term, computer-assisted ambulatory EEG recordings can be used to provide a longer duration EEG recording in a patient’s home environment in cases where symptoms are suspicious for seizures yet the routine EEG is negative (Chapter 10). In patients who are unable to utilize ambulatory EEG or those in whom more in-depth assessment of epilepsy to identify a precise seizure focus often require prolonged video-EEG monitoring in an inpatient setting.
Video-EEG monitoring in an epilepsy monitoring unit (EMU) allows correlation of a patient’s clinical activity as viewable on video with the EEG, which helps in determining whether a patient’s clinical events are seizures, syncope, or due to other causes (Chapter 11). Long-term EEG analysis is also being increasingly utilized in patients in the ICU to established trends that indicate cerebral function or seizures. The EEG is an important tool in the ICU setting for the diagnosis and management of status epilepticus. It is also useful in this setting for the purposes of monitoring cerebral activity in certain neurologic critical care disorders, such as for the detection of vasospasm in subarachnoid hemorrhage, changes in function in traumatic brain injury and stroke, and the detection of nonconvulsive seizure activity that may impact neurologic function (Chapter 12). Finally, patients being considered for epilepsy surgery require highly specialized recordings, utilizing intracranial electrodes and advanced methods of analysis, including new correlations with magnetic resonance imaging (Chapter 13 and 14). While each of these techniques is discussed in detail in their respective chapters in this section, Chapters 15 and 16 provide a practical overview of the clinical applications of neurophysiologic testing when assessing patients with spells or seizures in the outpatient setting and during evaluation for epilepsy surgery.