Patent Publication Number: US-2007123795-A1

Title: Device for determining acoustically evoked brain potentials

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a continuation of U.S. application Ser. No. 10/471,058, filed Feb. 12, 2004, which was a U.S. national phase filing of PCT/EP02/02534, filed 7 Mar. 2002. All priorities are claimed. 
    
    
      The derivation of acoustically evoked electrical brain potentials of a subject is a known audiometric diagnostic method for testing hearing and for evaluating various causes of hearing damage without the active participation of the subject.  
      This method is referred to in the field as ERA (electric response audiometry) or BERA (brainstem electric responses audiometry) or brainstem audiometry. Areas of application for this method including for example the performance of the first hearing tests in newborns, testing the hearing of infants or of unconscious persons such as accident victims for example, and the diagnosis of neurologic diseases, for example neurinomas of the acoustic nerve. Intraoperative hearing tests are also possible with this method.  
      Electrical brain potentials are triggered by acoustic stimulation of the ear with conduction through air or bone. Headphones are usually used for the purpose. The electrical signals that are thus generated by the brainstem are picked up by electrodes applied to the head. Usually three electrodes are used, namely one electrode to determine the reference potential and two active electrodes to derive the acoustically-evoked electrical signals at two different locations on the head. Acoustic stimulation of the ear can take for example the form of click stimuli or, for direct determination of the hearing threshold, of a rapid sequence of clicks with increasing volume. Other types of stimuli are of course also possible. The brainstem generates potential waves at each click, which are averaged after bring picked up and conducted away by the electrodes.  
      In the previously known devices the measuring results rely on a constantly good electrical connection between the head and the electrode, resulting in low impedance. In practice, this is however not always a simple task. The measurements may often be repeated due to poor measuring results, which are not detected during measurement. There is for this reason a need for improvement of the previously known devices of this type.  
      It is therefore an object of the invention to provide a device to permit simpler and easier use and hence achievement of better results while performing brainstem audiometry.  
     SUMMARY OF THE INVENTION  
      This object is achieved by the device which includes a pickup electrode to be applied to one location on a subject&#39;s head, a reference electrode to be applied to another location, means determining for the impedance between the electrodes and the subject&#39;s head, and means for visually indicating the impedance level.  
      By providing a visual indication of the impedance level there is possibility of checking this during measurement and hence provide an immediate correction to the positioning of the electrode so as to achieve the desired low impedance.  
      In a preferred embodiment the means for visually indicating a level of impedance are located on the structure holding the electrodes, or a preamplifier where the electrodes are connected via cables. This allows for a simultaneous visual contact with the structure during a correction operation and the indicator means.  
      Preferably the means for determining the impedance are integrated with the structure or the device. This also improves the operation and visibility of the indicator means.  
      In a preferred embodiment the means for determining the impedance comprises switching arrangements for each electrode enabling the switching of each electrode into an impedance measuring mode and back to normal brain potential measuring mode. This provides for a possibility of obtaining a reliable measurement of the impedance without disturbing the measurement.  
      In one preferred embodiment the means for visually indicating a level of impedance comprises a single light emitter, e.g. diode, indicating either a low or a high impedance level.  
      In another preferred embodiment the means for visually indicating a level of impedance comprises two light emitters, e.g. diodes, where one is indicating a low impedance level and the other is indicating a high impedance level.  
      In a device according to the invention, the electrode unit can also include an electroencephalograph (“EEG”) amplifier as a component, so that a minimum conduction path is provided between the pickup electrodes and the EEG amplifier, and thus the possibility of stray potentials being picked up is minimized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a schematic circuit forming part of the device according to the invention,  
       FIG. 2  shows a top view of a part of an example of a device according to the invention,  
       FIG. 3  shows a bottom view of a part of a device according to the invention, and  
       FIG. 4  shows a side view of a part of a device according to the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The device according to the invention will now be described briefly in terms of its important details, with reference to the embodiments shown in the enclosed drawings.  
      From  FIG. 1 a  schematic diagram appears. The diagram shows three inputs  1 , 2 , 3  each leading to switches  4 , 5 , 6 . The switches switch between a measuring mode and a test mode. In the measuring mode the input signals are transmitted to an EEG amplifier  7  and further to an audiometer used in a measuring process. In the test mode the impedance is measured between the electrode and the skin of the individual on which the electrodes are placed. The test of the impedance is carried out with a predetermined sampling frequency and is controlled by the control electronics  8  adapted to control the switches via connections  9 , 10 , 11 . The result of the impedance measurement is indicated by the diodes  12 , 13 , where one indicates a too high level of the impedance and the other one indicates a satisfactory low level of the impedance.  
      The device according to  FIGS. 2, 3  and  4  consists of a housing  14  with a plurality of arms that have electrodes  15 , 16 , 17  at their ends, and a earphone  18  integrated in the housing. An EEG amplifier (not shown) forms part of the device.  
      A cable (not shown) connects the device with the rest of the audiometer used for brainstem audiometry, the audiometer generating the signals for acoustic stimulation of the ear and processing and evaluating the derived brainstem potentials. In the embodiment, a single cable is shown that can contain both a line to supply electrical click signals and also a line to conduct the preamplified brainstem potentials from EEG amplifier. Of course, separate cables or wireless transmission pathways can also be used.  
      The electrical potentials generated in the brainstem by acoustic stimulation of the ear are picked up by the electrodes on the arms. Unusually three electrodes are used, namely a reference electrode for detecting a reference potential and two pickup electrodes. The reference electrode is brought into contact with the head in front of the ear, and one of the two deriving electrodes is placed behind the ear and the other in the area of crown of the head.  
      In the embodiment according to  FIG. 3 , electrode  17  is the pickup electrode that detects brainstem potentials in the vicinity of the crown of the head and electrode  16  is the reference electrode. The second pickup electrode  15  is applied to the head behind the ear.