Posterior Fossa Craniotomy

Monitoring of somatosensory evoked potentials (SEPs) to median and posterior tibial nerve stimulation, monitoring of upper and lower limb electromyographic (EMG) activity to transcranial motor evoked potentials (tcMEPs) and monitoring of free running and evoked cranial nerve electromyographic (EMG) activity was carried out as part of a bilateral posterior fossa, upper cervical exploration for tumor removal. The 35-year-old female patient presents with a several year history of motor and sensory deficits in her left foot.  More recently she complains of dizziness and numbness/tingling in her right arm and hand. MRIs reveal an intramedullary tumor mass near the obex with a large cystic component extending to the upper cervical spinal cord. For SEP monitoring, activity was recorded from subdermal needle electrodes placed on the head at Fpz (non-inverting lead) and Cz’/C3’/C4’ (inverting leads).  Stimulation rates were 5.5 pulses/second; current levels were 20.0 mAmps for both left and right wrist/ankle stimulation. EMG activity for tcMEPs was obtained using subdermal needle electrodes placed in the biceps/triceps, thenar/hypothenar, tibialis anterior/gastrocnemius and foot flexor/extensor musculature bilaterally.  Stimulation was carried out with Cadwell Cascade TCS-1 double train stimulator (pulse width-50 microseconds, two trains totally nine pulses, 1.7 ms inter-stimulus interval, 10.0 ms inter-train stimulus) constant voltage stimulation ranged from 100-800 volts.  Transcranial stimulation was achieved using subdermal needle electrodes inserted at C1/C2.  Anodal stimulation applied to C1 produced muscle twitches in right-sided musculature whereas anodal stimulation applied to C2 produced muscle twitches in left-sided musculature. For free running cranial nerve electromyographic (EMG) activity monitoring, subdermal needle electrodes were placed in the trapezius muscle bilaterally for accessory nerve monitoring (11th) and anesthesia placed endotracheal tube with electrode contacts on the vocal chords for monitoring of the vagus nerve (10th) bilaterally. Evoked EMG activity could also be employed to identify the location of the above nerves and used to determine their function throughout the case.  Specifically, a 200-microsecond square wave pulse (0.01-2.0 mAmps; 5.1 stim/sec) was used to evoke EMG activity from muscles innervated by motor cranial nerves.  The stimulus was delivered to the nerve or tissue in question using a hand-held stimulating probe with the return inserted into tissue near the wound retractor.  Response threshold was used primarily to predict possible neuropraxia. SEPs/tcMEPs/EMGs were amplified (gain=105) using differential amplifiers (Cadwell Cascade), averaged and monitored on a computer (Toshiba).  Listed below is a summary of intraoperative activities pertinent to monitoring procedures.