Muscle Relaxants Increases The Yield Of Bedside EEGs

P. Rajesh, EEG T
Department of Neurology, The Royal Hospital, Muscat, Sultanate of Oman

Performing and interpreting Bedside EEGs in Critical and Intensive Care units is often a challenge to EEG Technologists and interpreters. This is mainly because of the electrically hostile environment created by various monitors, syringe pumps, air-beds, ventilators and other machines (such as dialysis machines) using in the unit(1). Besides these electrical disturbances, people moving around the patient and static electricity in the polyethylene aprons of theatre staffs will also affect the quality of EEG especially if it is recording with amplifier gains at high sensitivity. Myogenic artifacts are not a common cause for a totally un-interpretable EEG in a patient with Glasgow Coma Scale 3. We are reporting one such case encountered in our Adult Intensive Care Unit.

In this case report, the patient is a 65 yr old woman who underwent mandible removal surgery for malignant Carcinoma. Post-operatively, the patient had cardiac arrest and generalized convulsions on the 3rd day and fell to coma stage. There was no seizure history for the patient or the family members. Patient again developed seizures and soon started treatment for status epilepticus. After controlling the seizures, the patient did not regain consciousness and so an EEG had been ordered to rule out a Non-convulsive status epilepticus.

EEG:

A 16 channel digital EEG was done with a portable Electro Encephalograph (Nicolet-Bravo) in this unresponsive and intubated patient. Electrodes placed according to the international 10-20 system of electrode placement with a scalp-electrode impedance of < 10 kOhm. Clinically, the patient was presented with occasional nystagmus of both eyes slightly more on the left side. The EEG was obscured by high frequeny artifact of about 30-40 Hz in almost all channels. Frontal leads showed occasional lateral eye movement artifacts and thus the EEG was totally un-interpretable.

Entire EEG recording system including the reference electrodes and machine ground were rechecked and found to be satisfactory. Routine inspections were performed to identify the source of the artifact. As the artifact resembles myogenic activity and in view of the eye movement artifacts, a low dose of muscle relaxant had been administered to the patient (Rocuronium bromide (Esmerone)25mg). Subsequently, an artifact free and interpretable EEG obtained which showed an anteriorly dominant, diffuse areactive alpha activity (8-9 Hz) consistent with a pattern of complete alpha coma(2). Patient died within 24 hours after this EEG.

When we compare both EEGs, it is clear that the classical alpha coma pattern was disturbed and masked out by myogenic artifacts and was un-interpretable. Usual techniques of filtering or reducing the scalp-electrode impedances by time consuming abrasion (3) were not contributory in this case. The low pass filter of the initial part of the EEG was kept in 10 Hz to suppress the high frequency myogenic artifacts. The application of low pass filter gave an erroneous appearance of cerebrogenic alpha-beta activity in a few channels in figure 1. This is because of the troublesome property of the low pass filter to distort the waveform morphology from a sharpish to a more blunted peak (sharpish myogenic artifacts appeared as more blunted alpha/beta waveforms)(4). Later portion of the EEG (fig 2) displayed with a low pass filter of 70 Hz. The well-formed diffuse alpha waves in this filter settings proved that the artifact was originated from muscles.

This case report emphasizes the role of muscle relaxants to increase the yield of bedside EEGs with suspected myogenic artifacts. Different types of instrumental artifacts in bedside EEGs have been reported earlier. A case, related to myogenic artifacts in a comatose patient has not yet reported. As the muscle relaxants affect the respiratory muscles, the patient should be intubated and that is the limitation of this method. Also the poor prognostic factor (2) of the complete alpha coma pattern is once again confirmed in this case.

Reference:

1. Lininger AW, Volow MR, Gianturco DT. Intravenous infusion motor artifact. Am J EEG Technol. 1981 Dec;21(4): 167-73.
2. Magdalena Berkhoff, Filippo Donati, Claudio Bassetti. Postanoxic alpha (theta) coma: a reappraisal of its prognostic significance.
Clinical Neurophysiology 111 (2000) 297-304.
3. Thomas C.Ferree, Phan Luu, Gerald S. Russell, Don.M. Tucker. Scalp electrode impedance, infection risk, and EEG data quality Clnical Neurophysiology 112 (2001) 536-544.
4. Neidermeyer E, Lopes da Silva F. Electroencephalography Basic principles, clinical applications and Related Fields, 2nd Edition p.69 Urban & Schwarzenberg. Baltimore 1987.

Footnote for figure 1.

EEG before giving muscle relaxant. Sensitivity 5?v/mm HFF 10Hz LFF 0.5Hz Note the lateral eye movement artifacts in F7, F8 leads due to nystagmus.

Footnote for figure 2.

EEG after giving muscle relaxant. Sensitivity 5?v/mm HFF 70Hz LFF 0.5 Hz Note the widespread monotonous alpha activity.