Suboccipital Craniotomy for Tumor Removal

Monitoring the 5th, 6th 7th, 8th, 10th and 12th cranial nerves bilaterally was carried out as part of a sub-occipital craniotomy for tumor removal. The 44-year-old male presents with a two week history of diplopia which gradually transitioned into difficulties with focus on downward gaze. MRIs reveal a lesion in the superior aspect of the fourth ventricle. He has no other neurological deficits. For electromyographic (EMG) recordings, intramuscular needle electrodes were inserted in masseter, lateral rectus, orbicularis oculi, orbicularis oris and tongue muscles for bilateral trigeminal, abducens, facial and hypoglossal nerve nuclei monitoring. An endotracheal tube with electrode contacts was inserted by anesthesia for monitoring the 10th CN (vagus) nuclei bilaterally. These cranial nerves were monitored for possible mechanical displacement, stretching and/or heating due to electrocautery of during procedure. Evoked EMG activity could also be employed to identify the nuclei of the preceding nerves and used to determine function throughout the case. Specifically, a 200-microsecond square wave pulse (0.01-2.0 mAmps, 5.1stim/sec) was used to evoke EMG activity from muscles innervated by motor cranial nuclei. The stimulus was delivered using a hand-held stimulating probe with the return inserted into tissue near the wound retractor.  Auditory brain stem responses (ABRs) were also recorded to monitor the status of both 8th cranial nerves.  Each ear was tested separately using an electrode montage in which subdermal needle electrodes (inverting leads) were placed, respectively, within the ear canal ipsilateral to the ear that received stimulation (A1/A2).  The non-inverting lead was also a subdermal electrode placed at vertex (Cz). One-hundred microsecond (100 us) square wave pulses produced alternating rarefaction and condensation clicks of 100 dB nHL and were presented at a rate 11.3 stim/sec. Stimuli were presented alternately to each ear every 45 milliseconds. Averages were collected to at least 5000 individual trials. A subdermal needle electrode place on the right shoulder served as ground. ABRs/EMGs were amplified (gain=105) using differential amplifiers (Cadwell-Cascade), monitored and averages collected on a computer (Toshiba).  Listed below is a summary of intraoperative activities pertinent to monitoring procedures.

Incision.
10:15 - Dura open.
10:47 - Begin splitting the cerebellar vermis and entering the IV ventricle.
11:17 - Begin IV ventricle exploration.
11:26 - Tumor identified, begin resection.
11:30 - Frozen section resected for pathology.
11:48 - Pathology reports choroid plexus carcinoma, continue resection.
12:25 - Sub-total resection complete, tumor 80% removed.
12:30 - Begin closing dura.  All channels quiet, no change in ABRs/tcMEPs.
12:50 - Dura closed. ​