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.
Surgery, to include partial or complete removal of the thyroid gland, is the treatment of choice for patients with differentiated thyroid cancer. In certain circumstances, patients will also receive radioiodine therapy after thyroidectomy. During surgery, key structures must be identified and preserved in order to avoid serious complications, to include voice issues, breathing difficulty, aspiration, trouble swallowing, and low calcium.
It is also important to be certain that all thyroid tissue has been adequately removed. For papillary thyroid cancer, certain enzymes such as thyroglobulin will be followed after surgery to ensure both complete removal of the gland, and to monitor for recurrence/ growth of cancer.
Papillary Thyroid Cancer
Papillary thyroid cancer affects roughly 5 to 15 per 100,000 individuals. In the mid 1900s, the use of radiation treatment for various childhood head and neck conditions lead to a spike in thyroid cancer during the late 1900s. Nuclear power plant accidents such as in Chernobyl, as well as radiation exposure in Japan after the bombing of Hiroshima/Nagasaki, have both led to spikes in cases of thyroid cancer in the following decades.
This disease affects females more than males, with a 2.5 to 1 ratio, and is most common in ages 40-60. In addition, patients with genetic disorders such as multiple endocrine Neoplasia Type 2, Werner syndrome, and Cowden syndrome have a higher likelihood of developing thyroid cancer.
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.