Excerpt from EEG in Dementia and Encephalopathy

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EEG has been employed clinically for some time as a measure of brain function in the hope of determining and differentiating certain functional conditions of the brain. It is used in patients with cognitive dysfunction, either a general decline of overall brain function or a localized or lateralized deficit. This article primarily addresses the clinical use of EEG in evaluation of dementias and encephalopathies. In addition, aspects of digital EEG and other newer developments are discussed briefly at the end of the article.

Definition of dementia

Criteria from Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) should be used in the diagnosis of dementia. Clinical dementia is a fairly broad-based decline of brain function; most definitions center on the patient's intellectual decline and memory dysfunction. This is, however, a fairly simplistic approach; dementia is much more than these fundamental deficits. Some of the dementias have distinguishing features. The process that constitutes normal aging is still an ongoing debate. As our understanding and testing procedures develop, more people are being classified as suffering from some type of dementia.

In 1998, Widagdo et al performed a quantitative EEG (QEEG) study of age-related changes during cognitive tasks. This study revealed no conclusive differences between the young and the elderly. Cognitive decline, unlike normal aging, is associated with alterations in the temporospatial characteristics of EEG. The diagnosis of the initial stages of dementia is based mainly on neuropsychological testing and clinical suspicion. The EEG findings are nonspecific (see Image 1).

EEG findings in dementia

In early dementia, the resting alpha frequency declines. Most authors agree that the lower limit of normal alpha frequency is 8.5 cycles per second. Medications can slow the posterior dominant rhythm; therefore, medication effect should always be excluded. In assessing the frequency of the alpha rhythm, alerting maneuvers are essential in order to ensure that the patient is in the best awake state and not drowsy. Computerized methods, such as EEG spectral analysis, coherence, and complexity (ie, correlation dimension), have been demonstrated to correspond to cognitive function.

Stevens et al recorded EEGs during 2 resting conditions (eyes closed and eyes opened) and 2 tasks (mental arithmetic and a lexical decision). The goal of the study was to evaluate which temporal and spatial EEG descriptors change with cognitive decline and normal aging. The EEGs were analyzed by using EEG microstates. The primary findings were a significant increase in the number of ultrashort EEG microstates and a reduction in the average duration of EEG microstates in cognitively impaired and demented patients. Cognitive impairment was associated with a reduction or loss of EEG reactivity. In contrast, no alterations in temporal or spatial EEG descriptors were found in normal aging. Cognitive tasks did not add to the information already obtained during the resting states. The reduction in EEG microstate duration correlated with loss of cognitive function.

Therefore, temporospatial analysis of the EEG record is a useful indicator of cortical dysfunction in dementia and correlates with degree of cognitive impairment. Apparently, temporospatial analysis may be useful in distinguishing patients with dementia from those experiencing normal aging. These data are largely preliminary; whether they contribute additional information to the clinical data in evaluating dementia is unclear.

Definition of encephalopathy

Encephalopathy represents a brain state in which normal functioning of the brain is disturbed temporarily or permanently. Encephalopathy encompasses a number of conditions that lead to cognitive dysfunction. Some of these conditions are multifactorial and some have an established cause, such as hepatic or uremic encephalopathy. Because the EEG patterns in most dementias and encephalopathies demonstrate few specific features, they are discussed together. Some notable exceptions include Creutzfeldt-Jakob disease (CJD) and subacute sclerosing panencephalitis (SSPE); however, no specific patterns exist for most dementias and encephalopathies. Other conditions, such as hepatic and renal encephalopathies, carry distinguishing features; nevertheless, similar patterns may be seen in a fairly wide range of illnesses under certain conditions.

EEG findings in encephalopathy

In general, the most prominent feature of the EEG record in encephalopathies (if there is a change) is slowing of the normal background frequency. A gradual and progressive decline over the course of the disease may be noted if serial EEGs are performed. Disorganization of the record may develop gradually. Reactivity to photic or other type of external stimulation may be altered. If a QEEG is done, it may show a frequency shift or decreased interhemispheric coherence of background frequencies. Some conditions are associated with an increase in seizure frequency, and in such cases, epileptic activity may be recorded.

In a given context, the EEG can play a clinically useful role, especially since functional MRI, positron emission tomography (PET), and single-photon emission computed tomography (SPECT) are either still in an experimental stage or require special settings not widely available.

Use of digital EEG data

Although in the following sections digital EEG data are cited frequently, these data represent primarily digital analysis of clinical EEG recording. The referenced data are presumed to be based on an EEG recording that is read by a clinician; presently, it is recorded by using computerized technology for ease and also for availability for further analysis. A variety of mathematical transforms are available after the initial clinical interpretation—for example, coherence, Fourier transform, wavelets, and microstates (see Digital EEG). These allow for further comparisons with norms and control groups but should be interpreted in conjunction with the primary EEG reading.

For excellent patient education resources, visit eMedicine's Brain and Nervous System Center, Procedures Center and Dementia Center. Also, see eMedicine's patient education articles Mad Cow Disease and Variant Creutzfeldt-Jakob Disease, Electroencephalography (EEG), and Stroke-Related Dementia.

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