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Neurology India, Vol. 51, No. 2, April-June, 2003, pp. 235-236 Relevance of computerized electroencephalographic topography (Brain Mapping) in ischemic stroke L. Pinheiro, A. K. Roy, G. R. K. Sarma, A. Kumar Department of Neurology, St. John's Medical College and Hospital, Bangalore-560034, India. L. Pinheiro Accepted on 26.05.2001. Code Number: ni03071 ABSTRACT Background and Aims: Computerized electroencephalographic topography (CET) (brain mapping) is a non-invasive, spatially oriented procedure for depicting amplitude and frequency patterns by two-dimensional color-coded frequency and amplitude display of the electroencephalograph (EEG). Key Words: Ischemic stroke, EEG, Brain mapping. Computerized electroencephalographic topography (CET) is a non-invasive, spatially oriented procedure for depicting amplitude and frequency patterns in association with conventional EEG, to produce color-coded maps of spectrally- analyzed EEGs. The results of conventional EEG are objective but not quantitative. Further, there is a chance of inter-observer variation. The procedure of CET is now advanced compared to the initial efforts of Lehmann in 1971.1 Cerebral infarction causes attenuation of electrical activity, which may extend over the whole hemisphere, with or without low-amplitude slow waves within a few hours of the ictus. Within a few days the amplitude of the focal discharge may decrease, probably due to perilesional edema and vasospasm. Reduction in the cerebral blood flow showed a good correlation with the ipsilateral depression in background activity in the EEG in the absence of CT lesions.2 We prospectively studied the changes in cerebral electrical activity in patients with ischemic stroke using computerized electroencephalographic topography (CET) (brain mapping) and correlated the changes to the clinical deficits and the size of the CT lesion. MATERIAL AND METHODS Forty consecutive patients admitted to the wards with ischemic stroke were studied within one week of ictus. Patients with hemorrhagic infarct, severe metabolic complications, life-threatening large hemispherical infarct with mass effect, posterior circulation stroke, and also patients with previous ischemic and hemorrhagic stroke were excluded from the study. Data collected included demographic data, risk factors for stroke, neurologic findings at admission and at the time of EEG studies and brain mapping. All the patients had neurologic deficits for more than one week. Conventional EEG (18-channel) and brain mapping was performed within 24 hours after the CT scan. The abnormalities on brain mapping but not the EEG were compared with the degree of motor weakness. Brain mapping equipment consisted of a computer with software Brain View LG 2100 from Recorders and Medicare Systems. Amplitude brain mapping was shown on the hard copy by a two-dimensional color-coded display depicted by green (0.0 micro volts) to brown (24 micro volts), yellow being at the center (14.5 micro volts). Frequency intervals of 0-4, 4-8, 8-12 and 12-16 Hz were used in the frequency maps. All 18 channels of the conventional EEG were analyzed simultaneously for brain mapping. A neurologist who was `blind' to the clinical data read the EEG and brain mappings. This data was correlated to the CT scan findings. Statistical analysis was done using chi-square test. The level of significance was taken as p < 0.05. RESULTS The mean age of the study population was 56 years (range 25-75 years). CET was abnormal in 95% of the patients whereas EEG was abnormal in only 40% of the patients (p < 0.0001). EEG abnormalities observed included focal slowing ipsilateral to the side of the lesion, bi-hemispherical diffuse slow waves (theta), and attenuation in the alpha activity ipsilateral to the side of the lesion. None of the patients had delta activity. Brain map abnormalities included low-frequency high-amplitude discharges that were depicted in the two-dimensional color-coded frequency and amplitude hard copy. Brain map abnormalities were ipsilateral to the side of the CT lesion in 20 (52.63%) patients. Brain map abnormalities were much wider than would be expected from the CT lesion in 12 (31.58%) patients. Brain mapping abnormalities were ipsilateral to the side of the CT lesion when the lesion was < 2 cm in diameter and they were bi-hemispherical when the CT lesion was > 2 cm in diameter. DISCUSSION Our findings were similar to the earlier studies.3,4 Like in our study bi-hemispherical EEG abnormalities were reported in patients with one hemispherical CT lesion.5 The significance of bi-hemispherical EEG abnormalities with uni-hemispherical lesion is not known. Earlier studies showed good correlation between findings on computed EEG mapping, location of CT lesion, and cerebral blood flow.6 Lack of cerebral angiographic studies in our patients precludes exact comparison with this study. There may be an unfortunate tendency to rely solely on the topographic map as a basis for diagnosis. Significance probability mapping (SBM), in which a single subject's topographic image can be compared with that of a control or reference data set, may improve accuracy. Brain maps may also detect artifacts caused by eye blink, muscle contractions, electrode placement or electrode impedance problems. There may be a possible role for the important procedure of brain mapping during therapeutic interventions for salvaging the ischemic brain and in determining the outcome. ACKNOWLEDGEMENTS The authors would like to thank Elizabeth, EEG Technician in the Department of Neurology, and Dr. Radhika Devi, Consultant Radiologist, and Mr. AS Mohammed, Asst. Professor (Statistics) for their help in carrying out this study. REFERENCES 1. Lehmann D. Multichannel topography of human alpha fields. Electroenceph Clin Neurophysiol 1971;31:439-49.
Copyright 2003 - Neurology India. Also available online at http://www.neurologyindia.com |
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