engleski

Neurophysiological examinations for dementias

Dementia is a neurochemical disorder characterized by decreased level of acetylcholine resulting in a global cognitive impairment. The cholinergic system, however, influences the activity of EEG and evoked brain activity. The role of EEG in the study of dementias is to help in the differential diagnosis of the multiple causes of this syndrome, being useful in differentiating early on between treatable and as of now untreatable forms of dementia. The hallmark of EEG abnormalities in Alzheimer’ s disease (AD), most common dementia, is slowing of the rhythms and a decrease in coherence among different brain regions. In early dementia, the resting alpha activity (8-13 Hz) declines. Gradually, an increase in theta activity (4-7 Hz) is followed by a decrease in beta activity (above 13 Hz). Delta activity (less than 4 Hz) increases later during the course of the disease. Topographically, an increase in slow activity is prominent in the left temporal area of AD patients. Furthermore, these abnormalities are correlated with the severity of the disease. Computerized methods, such as quantitative EEG (qEEG) analysis, coherence, and complexity (i.e. correlation, dimension), have been demonstrated to correspond to cognitive dysfunction in dementia and correlates with degree of cognitive impairment. Even without EEG abnormality present in MCI (mild cognitive impairment), qEEG can help to identify MCI subgroups that develop to AD. Finally, the EEG is very useful for distinguishing AD patients from patients with senile depression with an accuracy of about 70-85%. The combination of EEG and neuroimaging techniques (PET), however, variables result in approximately 90% of overall correct classification with specificity of 100%. As depression in AD may interfere with neuropsychological testing, event related potentials (ERPs) have been proposed as more objective electrophysiological tools of assessing cognitive processes. ERPs are cerebral responses associated with various psychological events or cognitive functions such as recognition of certain stimuli, and is an objective parameter reflecting cognitive functions. The most extensively studied, P300 (latency of 300 ms) has shown prolonged latencies and smaller amplitudes in patients with AD, as P300 directly reflects currents triggered by cortical postsynaptic potentials and seems to be mainly generated in the temporo-parietal cortex, an area where a pronounced synaptic loss in AD can be found. Additionally, exogenous-evoked potentials like visual, brain stem auditory, and somatosensory-evoked potentials lack the sensitivity required for the detection of early cognitive stimuli. References: 1. Huang C, L.-O. Wahlund, T. Dierks, P. Julin, B. Winblad, V. Jelic. Discrimination of Alzheimer's disease and mild cognitive impairment by equivalent EEG sources: a cross-sectional and longitudinal study. Clin. Neurophysiol ; 2000 ; 111:1961-1967. 2. Murdie P. Event-related potentials identify patients with early-stage AD. Nature Clinical Practice Neurology 2006 ; 2: 235-236.

dementias; neurophysiological examination; stroop test

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