Case 3: Clenched Jaw Signals Swamp EEG
During a rather complex neurosurgical procedure to remove an egg-sized tumor from the right side of a patient’s brain, a problem was encountered with an electroencephalograph (EEG) data-acquisition system. Following a craniotomy, this device is used to map the functional regions on the surface of the brain to the patient’s various motor and sensory areas. In order to do this, the patient must remain conscious. The EEG operator provides a sequence of electrical, optical, acoustic, and mechanical stimuli in order to identify a surgical approach to the tumor that will cause the least amount of damage to the patient’s neurological function. In addition to the EEG device, other medical devices were involved, including an ultrasound imaging system, radiofrequency position generator, bipolar coagulator, ultrasonic tissue ablation, stimulators, and irrigators. The EEG operator, who was quite experienced and had done dozens of similar procedures, was unable to record acceptable signals because of an intermittent high-frequency noise on all eight EEG channels. It was initially thought that this noise originated from one of the many other instruments in the room, but this could not be confirmed. Filtering of the signals did not work, as the noise appeared to be in the same frequency domain as the EEG signals. To complicate matters, the patient was able to hear fragments of the conversation between the surgeon and the EEG operator discussing the reason for the delay in the surgery. He readily expressed his anxiety. At that point, we were called to assist. Every instrument, in turn, had been turned off to determine whether it was the cause of the EEG interference. Indeed, after a few moments of observation, the extraneous “noise” would almost entirely disappear and then, suddenly, completely mask the desired EEG signal. A closer look at the electrode site was made. The common reference electrode was positioned in the scalp fascia near the craniotomy. The location of the reference electrode is usually not an issue. In this instance, however, the needle electrode appeared to have penetrated some muscle tissue as well. We observed that when the patient was questioned or stimulated, the noise would reappear and sometimes subside as well. We hypothesized that the noise was an electromyographic (EMG) signal caused by tension in the scalp musculature when the patient was aroused or when he felt anxious. A reference electrode site that avoided muscle tissue was carefully chosen. The problem was resolved.