In Western Europe ischemic stroke is one of the most frequent causes of long-term deficiencies or even severe disability. The development of therapies for acute stroke patients over the last decade has given increasing importance to efficient and clear-cut diagnostic methods. For the patient to receive optimal treatment it is mandatory to obtain reliable diagnostic information on the extent of a brain lesion allowing to distinguish between different kinds of ischemic attacks as early as possible. Any delay between the first occurrence of symptoms and the onset of therapy seems to be critical for its success. The solely clinical evaluation of the functional state of the patient’s brain is extremely difficult and time-consuming, even for the expert neurologist, and computer tomography as imaging technique is not sufficiently sensitive, until possibly irreversible damage is done to the brain.
There is no doubt that somehow the electroencephalogram (EEG) reflects the overall functional state of the human brain. However, a conventional EEG-analysis comprises 20 to 64 channels and thus faces two major difficulties: the measuring device is complicated to administer, and expert knowledge – usually represented by a certified neurologist – is required for the analysis.
... and its Solution!
The Stroke Monitor has been designed to overcome these problems: It requires no more than four electrodes placed on the patient’s head, and approximately five to ten minutes recording time. The analysis of the two-channel EEG-signal is performed online, and the data are at the same time stored on hard disk for re-examination and documentation. Its basic concept is simple: If in one hemisphere a major part of the cortex is damaged the patient’s EEG lacks certain characteristic patterns that are usually present in a healthy subject’s EEG. In order to examine the ”similarity” between the signals from the two cortical hemispheres, the crosscorrelation function of the two EEG-signals is calculated and subsequently analyzed. It exhibits a pronounced dynamic variability, but thorough research has unearthed specific spectral patterns connected with the brain’s -activity that are automatically searched for with the online analysis in progress. A time-averaging of five to ten minutes is sufficient for the method to arrive at the result – displayed in an easily interpretable pictogram: either side of the symbolic head is painted in the color representing its respective degree of synchronous activity: red for low, green for highly synchronous -activity. Quantitatively the Brain Function Index (BFI) is represented by a number between 0 and 100, with 0 at the pathological end, and 100 denoting an exceptionally well correlating EEG.
The Stroke Monitor can clearly distinguish a (usually one-sided) cortical stroke from a small subcortical one or a transitory ischemic attack. Moreover the complete time course of the BFI can be displayed during the recording. Thus the Stroke Monitor allows long-term monitoring of the brain function for the detection of trends in the patient’s brain dysfunction development and possibly even the documentation of a therapy effect. In combination with a printer the result can be easily documented and added to the patient’s record sheet.
The Stroke Monitor’s Advantages
are obvious. It is a non-invasive measuring device that is easily applicable by a non-encephalographer, e.g. a nurse, and even under adverse circumstances. It has no side-effects, and allows an objective quantitative measurement of the degree of synchronicity in brain function independent of the observer within a short period of time (5 to 10 minutes). A deterioration in neurological function is determined, before possibly irreversible damage is done to the brain, thus opening new perspectives in the emergency situation of acute stroke patients by yielding relevant information for impending therapy decisions. The Stroke Monitor has been tested with over 4,000 patients on the stroke unit of Heinrich-Heine-University Düsseldorf.
For more details please see also the neuro-monitoring section!
The Stroke Monitor has been patented in Germany under DE 197 25 214. The European Patent is EP 0990211, with nationalizations obtained in Germany, France, Great Britain, Finland and Iceland. The United States patent has been granted under No. US 09/464,274.
The typical voltage of EEG-signals lies in the range of 10 uV. A specific bipolar 4-channel DC/AC amplifier transforms these to the range of 1 V. The amplifier also performs a ”soft” initial filtering with cutoff frequencies of 1.6 Hz (lower) and 40 Hz (upper). An automatic impedance check for all electrodes is periodically performed and the impedance – usually 2-5 KW – is displayed on screen. The signals are then passed on to an A/D-board with automatic setup of appropriate digital anti-aliasing filters and a pre-selected overlap of 50%. The signals are sampled at a rate of 256 Hz and automatically stored on hard disk, typically for 5 to 10 minutes.
The computerized analysis of the signal makes use of the signal’s Fast Fourier Transform (FFT) with a time window of 4 seconds and a frequency resolution of 0.25 Hz. The spectra of both channels are simultaneously displayed either as an averaged or instantaneous graph.
Last modified 05/18/07 11:48 Upward