Nerve Stimulation Monitoring
Nerve stimulation during thyroidectomy has been described in the 1940s, where palpation of the larynx during recurrent laryngeal nerve stimulation was used as a confirmatory measure. In the 1980’s, palpation of the posterior cricoarytenoid muscle, with concurrent nerve stimulation, was described as an intraoperative measure.
These measures are grouped into the category of intermittent intraoperative nerve monitoring. In the early 2000’s, continuous intraoperative nerve monitoring gained popularity as a means to monitor nerve function throughout an entire surgery. Continuous monitoring while in close proximity to nerves can help achieve complete resection, while keeping a safe distance from nerve structures.
Typically, a modified Blaire incision is made in front of the ear, within a skin crease. A skin flap is elevated anteriorly to expose the fascia overlying the parotid gland. The facial nerve or one of its branches is found via one or more established methods. During superficial parotidectomy, the superficial portion of the gland is excised directly overlying the facial nerve. It is important to include the entirety of the adenoma, in order to prevent pseudopods from developing into recurrent disease. For this reason, it is vital to identify the facial nerve trunk early in the case, in order to plan the rest of the dissection in a safe manner. The branching pattern of the facial nerve can be highly variable, so a strong knowledge of anatomic variants combined with intraoperative nerve monitoring will aid in safe surgery.
Pleomorphic Adenoma/Parotid Cancer
Pleomorphic adenoma is the most common benign salivary gland tumor. Anatomically, the parotid gland is the most commonly affected of the three major salivary glands, comprising about 80 percent of the cases. In the parotid, 75% of tumors are benign, and 25% are malignant, roughly. Pleomorphic adenoma, although slow growing and benign, has a high propensity for local recurrence if the entirety of the tumor is not removed. The facial nerve is an anatomic landmark running between the superficial and deep lobes of the parotid gland. It is vital to preserve the function and integrity of the facial nerve during surgery, to prevent a permanent facial weakness on one side.
The Medtronic NIM monitor is an example of a continuous intraoperative nerve monitor, with applications in a number of Head and Neck Cancer surgeries. For thyroidectomy, the NIM EMG endotracheal tube provides both an airway for intraoperative ventilation, as well as nerve monitoring for the vagus or recurrent laryngeal nerves. It does so by detecting movement of the muscles of the larynx.
Cross bands on the tube guide its placement during intubation, and should be placed at the level of the vocal cords. After identification of the recurrent laryngeal nerve, confirmation can be achieved via stimulation with the NIM probe, and evaluation of vocalis muscle activity response on the monitor.
The continuous monitoring system will also notify the surgeon of a change in nerve status in the event of an accidental transection, at which time re-anastomosis can be attempted. Immediate identification of nerve injury can potentially save the patient from another dedicated surgery for nerve repair.