Objective: The facial colliculus is a reliable landmark for a surgical approach via the fourth ventricle. Our aim is to elucidate the most suitable electrophysiological methods for identification of the facial colliculus. We evaluated the usefulness of facial electromyography and antidromic evoked potentials of the facial nerve. The effect of stimulation on cardiorespiratory function is also studied.
Methods: We localized the facial colliculus by facial electromyography and antidromic facial evoked potentials in adult dogs. To determine the most effective stimulus pattern, intensity was varied, and both monopolar and bipolar electrical stimulation were tried. To confirm the cardiorespiratory effect of the stimulation, systemic blood pressure, heart rate, respiratory rate, and thoracic excursion were measured. After administration of atropine sulfate, changes in vital signs were recorded.
Results: A stable facial electromyographic wave form was produced by 0.1-mA monopolar stimulation of a small portion of the fourth ventricular floor (4 mm2). Using 0.1-mA bipolar stimulation, the same wave form was obtained. As saline was gradually added around the electrodes, the amplitude of the response gradually decreased; however, the response with monopolar stimulation was more stable than that with bipolar stimulation. Stimulation of the facial colliculus with greater than 2 mA caused transient hypotension and bradycardia; respiratory arrest occurred with 3 mA stimulation. Administration of atropine sulfate (0.01 mg/kg) decreased these responses. Antidromic facial evoked potentials were recorded only at "hot points" that existed within 2 mm of the facial colliculus.
Conclusion: Our study resulted in three findings. First, the most suitable electrophysiological stimulation of the fourth ventricular floor for identification of the facial colliculus was 0.1-mA monopolar stimulation. Second, significant alteration in cardiorespiratory function appeared with greater than 1-mA stimulation. Third, a recording of an antidromic facial evoked potential can identify the facial colliculus more safely than direct stimulation of the facial colliculus. Both orthodromic and antidromic methods were useful for identification of the facial colliculus in brain stem surgery.