Original - April 6, 1990
Revised - March 14, 1991
Whereas blunt trauma such as direct manipulation or traction on the facial nerve will usually evoke a facial response, our clinical and experimental investigations demonstrate that sharp dissection may evoke little or no response even with complete transection of the nerve. In order to help circumvent this problem, a custom set of insulated stimulating dissection instruments has been designed1. These instruments allow simultaneous monopolar stimulation and nerve mapping during sharp surgical dissection (Figure 1).
Unlike blunt dissection, in which a mechanically evoked potential is obtained after some degree of nerve manipulation or trauma, "Stimulus Dissectors" can alert the surgeon to the proximity of the nerve before a cutting maneuver is performed.
Unlike dedicated facial nerve stimulators, however, Stimulus Dissectors cannot be insulated flush to their tip. In order to allow the instrument an adequate cutting edge, a variable degree of the instrument must remain uninsulated. The area of non-insulation is dependent upon the instrument. For example, a needle dissector may be almost entirely insulated, whereas a pair of microscissors will require a greater degree of noninsulation in order to allow adequate freedom of movements for its parts.
Any stimulator used in the cerebellopontine angle is at risk for false-negative responses due to current shunting. Current may be lost into the adjacent cerebrospinal fluid or blood resulting in inadequate current at the electrode-nerve interface particularly with low levels of electrical stimulation. Prass and Luders2 have demonstrated that insulation of the stimulus probe flush to the tip reduces current shunting.
A stimulus current of 0.05 mA usually elicits a brisk facial response when the stimulator is placed in direct contact with the nerve at the cerebellopontine angle. If there is intervening tissue or if the nerve has become fatigued because of prolonged dissection, the current intensity may be increased up to 1.0 mA. Although brief, intermittent stimulation up to 2 mA is unlikely to result in any neural injury, it is rarely needed and may result in an ambiguous response because of significant current spread through the surrounding tissues.
Therefore, the surgeon using Stimulus Dissectors must be aware of the inherent possibility of current shunting. Fortunately, the near-total insulation of most dissectors precludes shunting under most circumstances. In order to further reduce the possibility of false-negative stimulation particularly during pooling of spinal fluid or blood, the stimulus intensity should be increased (eg from 0.05mA to 0.10mA when working immediately adjacent to nerve; from 0.20mA to 0.30mA or more when working at greater distances from the nerve or when the location of the nerve remains uncertain). These principles hold true for middle ear and mastoid surgery - the stimulus intensity should be increased when the nerve is covered by bone, sheath, granulation tissue or cholesteatoma, and then progressively decreased as dissection proceeds closer to the nerve. When in doubt, a dedicated stimulator insulated flush to the tip should be used. Table I lists other possibilities which should also be excluded when an expected response does not occur (also, see NIM-2 Michigan Ear Institute Application Note for further details).
In order to minimize current shunting, some instruments (Stimulating Round Knives) have had their ordinarily large surface area reduced by creating a hole in the center. The Stimulating Cutting Ring, for example, is primarily intended to debulk the center of an acoustic neuroma while mapping the position of the facial nerve along tumor capsule. Its small surface area minimizes current shunting. When I am dissecting relatively distant to the nerve, I use it at a setting of 0.4mA and aspirate the debulked contents with a 7gauge suction tip. Stimulus intensity and suction tip size are reduced as dissection proceeds closer to the nerve. For middle ear and mastoid surgery, I prefer using the Stimulating Round Knives and Curved Needle.
As always, it should be kept clearly in mind that intraoperative monitoring is an adjunct and not a replacement for experience in surgical decision making.
The presence or absence of facial evoked responses must be interpreted within the context of the surgical procedure.
Because there are specific benefits to the use of monopolar and bipolar probes as well as Stimulus Dissectors, an optional Stimulus Probe Adaptor has been designed to facilitate efficient and convenient switching between up to three different stimulators. Whichever stimulator is used, stimulation is most effective when the probe is attached to the cathode (black "-" sign) plug. The stimulator switching box may be draped in a clear plastic sterile drape and then placed near the operative field for convenient access by the surgeon or scrub nurse. The adaptor plugs into the stimulator outputs on the Patient Interface box. There are receptacles for both monopolar and bipolar stimulator types so they may be plugged in simultaneously. The switch on the box allows easy selection of the active stimulator (Figure 2). When switching between different probes, make certain that it is active by checking 1) the switch box setting and 2) the "CURRENT MEASured" on the NIM screen.
Custom insulated self-retaining retractors have also been designed to avoid the static discharge that two metal instruments may create when they come in contact. Such discharges may be a source of confusion since they can manifest themselves as an artifact which the NIM-2 may erroneously signal as an EMG event. These instruments are simply insulated and require no attachments as the stimulators do.
1. Kartush, J.M. Electroneurography and Intraoperative Facial Monitoring in Contemporary Neurotology. Otolaryngology - Head and Neck Surgery 1989; 101:496-503.
2. Prass R. and Luders H. Constant Current/Constant Voltage Stimulation. J.
Neurosurgery 1985; 62:622-623.