The Xomed Monopolar Cranial Nerve Stimulator Electrode: A Surprising Handy Tool for Deep Dissection of Epidermoid Tumors

Abstract

Epidermoid cysts are notorious for their propensity to sneak into deep recesses between cranial nerves in the posterior fossa. Attempts to achieve complete excision using ordinary instruments when tempted by the seeming ease of dissection is known to cause unacceptable deficits. The Xomed monopolar stimulator electrode probe of the nerve integrity electromyography monitor has several advantages when used as the primary dissection tool for deep-seated epidermoid cysts. Cerebellopontine angle epidermoid is the classical prototype of a strategically placed deep-seated epidermoid tumor. The author describes the use of the monopolar stimulator electrode of the nerve integrity electromyography monitor as the primary dissection tool for excising epidermoid cysts of the cerebellopontine angle. Thin profile, rounded nontraumatic tip, and springiness of the body of the monopolar electrode aid the dissection. The monopolar electrode was used to tease and scoop out the flakes aided by a fine suction. An initial internal decompression allowed the capsule to be folded away and separated from neurovascular structures using the dissector. The thin profile of both the instruments allowed good visualization and delicate control over the dissection in depths of the resection cavity without undue traction or impacts on superficially dissected cranial nerves. This novel use of the monopolar electrode was employed in the surgical excision of epidermoid cysts of the cerebellopontine angle in nine patients. Total or near total excision was possible in eight of the nine patients who underwent excision using this technique. In three patients, mild deficits primarily of the fifth nerve sensory function were noted. The Xomed monopolar stimulating electrode of the nerve integrity electromyography monitor is an ideal instrument for deep dissection of epidermoid cysts in areas where neurovascular structures are at risk.

Keywords: cerebellopontine angle; cranial nerve palsy; electromyography; epidermoid cyst; iatrogenic injury; intraoperative neuromonitoring; intraoperative physiological monitoring.

Figure 1. 'The epidermoid dissector'

The monopolar electromyography stimulator electrode probe. Note the blunt point of its slender malleable body. It was used without any modifications. The cranial nerve integrity monitor is seen in the background.

Figure 2. The view through the operating microscope

The view through the operating microscope shows the stimulator-dissector on the right side (arrow). The slender size of the ‘epidermoid dissector’ is obvious in comparison to the size 1 French suction cannula on the left.

Figure 3. Preoperative and postoperative magnetic resonance imaging (MRI)

Heavily T2-weighted MRI with thin slices (constructive interference in steady state) shows the preoperative lesion that had dug itself into the pontine surface (A). The postoperative MRI was taken three months after the surgery and it shows complete excision of the tumor (B). A flow related hypo-intensity is seen in the cerebrospinal fluid collection close to the fifth nerve (arrow). Removing the part of the tumor that was insinuating into the pontine surface caused mild and transient numbness in the trigeminal nerve distribution in the immediate postoperative period.