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.
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