Electroencephalography in Neuropsychiatry

Special Report: Neuropsychiatry

Electroencephalography (EEG) is a noninvasive, widely available, and relatively inexpensive test that can help exclude or identify structural or functional factors contributing to psychiatric syndromes. This article defines the clinical usefulness of EEG in evaluating neuropsychiatric disorders, emphasizing the complementary nature of the visually inspected standard EEG (SEEG) and the computeranalyzed quantified EEG (QEEG).

The recent evolution of neuropsychiatry/behavioral neurology as a subspecialty linking these formerly disparate fields represents a paradigmatic shift regarding the responsibility of psychiatrists in diagnosing and managing behavioral disorders with concomitant and demonstrable brain pathology such as dementia or head injury. In addition, the biologic underpinnings of many mental illnesses, including bipolar disorder and depression, are now described in terms of their anatomy and physiology. QEEG analysis increases the sensitivity of EEG to physiologic or pathologic changes associated with such disorders.

SEEG refers to the visual analysis of ongoing voltage recordings from multiple scalp locations. Two types of EEG deviations are usually indicative of significant cerebral pathology. The first is paroxysmal activity, including sharp waves, spikes, and episodic slow waves, indicating episodic abnormal neuronal discharges. These can be focal, suggesting structural pathology, or bilateral and more suggestive of functional pathology. The second type of EEG deviation is sustained slowing of normal brain rhythms. Slowing also can be diffuse, indicating more generalized pathology, or focal, indicating a localized pathology.

The most frequent reason for an EEG referral is to exclude a general medical condition, such as delirium, or a specific neurologic problem, such as epilepsy, as a cause of or a contributing factor to the presenting symptoms. Since the use of routine test batteries to exclude medical conditions is costly, clinicians must rely primarily on 2 red flags to trigger organic workups: unusual presentations and atypical age at onset. The yield is consistently low when using neuroevaluative tests to uncover causes such as tumors or aneurysms for syndromes presenting without manifest neurologic disturbances. It is much more likely that an EEG will uncover a factor that may be contributing to, but does not necessarily fully explain, the syndrome. It may also be helpful in revealing a factor that could help guide treatment, such as temporal lobe spiking in panic disorder.¹

After the EEG has been recorded and visually interpreted by the electroencephalographer, it may be analyzed further using quantitative means.² QEEG analysis is always a post hoc procedure done after visual interpretation of the SEEG by a qualified electroencephalographer. It is specifically not recommended for use clinically as a stand-alone procedure. At its most basic level, QEEG provides a method of calling the electroencephalographer's attention to aspects of the original EEG record that may have been overlooked. QEEG's quantitative nature makes it exquisitely sensitive to subtle frequency changes and to abnormalities in the coherence of activity within and among brain regions.

Brain activity varies among healthy people, and normal variability must be distinguished from that outside the normal range. An underlying assumption is that the more unusual the patient's brain activity compared with that of normal persons, the more likely it is that the statistical abnormality represents pathology. The establishment of normal limits is greatly aided by quantitative analysis comparing the patient's QEEG with those derived from large groups of healthy persons. Brain activity also changes with age, and QEEG tracks the moving window of normal limits across the entire life span.

In addition, QEEG can help the physician arrive at a specific diagnosis. Patients who have known neuropsychiatric disorders often show characteristic QEEG profiles, distinguishing them from patients who have similar disorders. When facing a difficult diagnostic question, the physician can compare the patient's QEEG with the profiles characteristic of the different diagnostic possibilities, looking for the best fit. An extensive body of research shows that the accuracy of such computerized diagnostic classifications on the basis of QEEG alone typically exceeds 80%, although in an actual clinical setting the physician always makes the diagnosis, informed by other sources of information in addition to QEEG. Well-replicated studies have demonstrated QEEG classification accuracies high enough to be useful in diagnosing learning and attention disorders in children, and mood disorders (including bipolar disorder) and dementia among adults.³