Retrosigmoid Tumor
This 33 year old woman is undergoing a L retrosigmoid craniotomy for resection of a L sided acoustic neuroma of approximately 2 cm CP angle component which also fills the medial 2/3 of the IAC. She presented with a one year history of tinnitus; an audiogram showed a mild decrease but with 96% speech discrimination. She is otherwise neurologically intact. Removal of this tumor carries attendant risks of damage to multiple cranial nerves, particularly the VIIth and VIIIth, with possible postoperative deficits in facial motility and hearing. In order to help identify and preserve these nerves, intraoperative monitoring of spontaneous and evoked EMG and of the auditory brainstem response (ABR) was performed at the request of the surgeon.
08:26 - Sterile needle electrodes were placed in the following muscles ipsilateral to the tumor: masseter (Vm), orbicularis oculi and o. oris (VII), and trapezius (XI). No muscle relaxants were used in order not to interfere with EMG recording. Amplified and filtered EMG was displayed on a Cadwell Cascade system and monitored on a loudspeaker. Spontaneous EMG was continuously monitored to detect any activity associated with mechanical stimulation of nerves by traction, dissection, or other intraoperative events. As needed, a flexible tip probe was used to stimulate with constant current (0-1 mA) cathodal pulses (200 µs, 8/s) to elicit evoked EMG responses in order to positively identify and trace the course of nerves in relation to the tumor.
The ABR was elicited by 90 dBSPL, 100 µs rarefaction clicks (8.1/s) delivered through an insert earphone in the ipsilateral ear, and recorded from ipsilateral earlobe (A1) referred to a mid-forehead needle electrode, and to the contralateral earlobe as well. Averaged ABRs (N=1000) were recorded on the Cadwell system, displayed for comparison with baseline responses, and stored on magnetic disk.
09:26 - Initial baseline: all EMG channels quiet, no excessive 60 Hz artifact. ABR shows all waves to be robust, with interpeak latencies of I - III = 4.05 ms, III - V = 1.88 ms. This is a very abnormally delayed wave I-III interpeak latency, typically seen in acoustic neuroma.
09:56 – Some masseter EMG after incision, anesthesia has already given remifentanil to correct.
CPT: 95920 (Intraoperative neurophysiological monitoring)
95867 (EMG, cranial nerve supplied muscles)
92585 (Brainstem evoked response recording)
08:26 - Sterile needle electrodes were placed in the following muscles ipsilateral to the tumor: masseter (Vm), orbicularis oculi and o. oris (VII), and trapezius (XI). No muscle relaxants were used in order not to interfere with EMG recording. Amplified and filtered EMG was displayed on a Cadwell Cascade system and monitored on a loudspeaker. Spontaneous EMG was continuously monitored to detect any activity associated with mechanical stimulation of nerves by traction, dissection, or other intraoperative events. As needed, a flexible tip probe was used to stimulate with constant current (0-1 mA) cathodal pulses (200 µs, 8/s) to elicit evoked EMG responses in order to positively identify and trace the course of nerves in relation to the tumor.
The ABR was elicited by 90 dBSPL, 100 µs rarefaction clicks (8.1/s) delivered through an insert earphone in the ipsilateral ear, and recorded from ipsilateral earlobe (A1) referred to a mid-forehead needle electrode, and to the contralateral earlobe as well. Averaged ABRs (N=1000) were recorded on the Cadwell system, displayed for comparison with baseline responses, and stored on magnetic disk.
09:26 - Initial baseline: all EMG channels quiet, no excessive 60 Hz artifact. ABR shows all waves to be robust, with interpeak latencies of I - III = 4.05 ms, III - V = 1.88 ms. This is a very abnormally delayed wave I-III interpeak latency, typically seen in acoustic neuroma.
09:56 – Some masseter EMG after incision, anesthesia has already given remifentanil to correct.
CPT: 95920 (Intraoperative neurophysiological monitoring)
95867 (EMG, cranial nerve supplied muscles)
92585 (Brainstem evoked response recording)