A thyroidectomy, the surgical removal of all or part of the thyroid gland, is a common procedure. During this operation, surgeons work in a delicate area of the neck near nerves that control the voice box. To aid in protecting these structures, surgical teams often use intraoperative neural monitoring (IONM), such as the Nerve Integrity Monitoring System (NIMS). Its purpose is to provide real-time feedback to the surgeon, helping to identify and assess nerve function throughout the operation to preserve vocal function.
The Nerves at Risk During Thyroid Surgery
Two specific nerves are situated near the thyroid gland and are at risk during its removal. The first is the recurrent laryngeal nerve (RLN), the primary nerve for voice production and breathing. It controls the movement of the vocal cord muscles, allowing them to open for air and close to produce sound. An injury to the RLN on one side can lead to a weak, breathy, or hoarse voice. If both are damaged, it can cause significant breathing difficulties, a rare but serious complication.
The other nerve is the external branch of the superior laryngeal nerve (EBSLN). This nerve controls the cricothyroid muscle, which adjusts vocal cord tension and length. This function relates to controlling vocal pitch and projecting the voice. Damage to the EBSLN can result in a voice that fatigues easily, has a limited pitch range, or lacks its normal projection.
The anatomical paths of these nerves can vary, and their proximity to the thyroid gland’s blood supply makes them susceptible to injury. Conditions like large goiters or thyroid cancer can further distort the anatomy, increasing the risk. Careful identification and preservation of both nerves are goals of thyroid surgery.
How NIMS Technology Functions
The NIMS system operates by translating nerve activity into signals that the surgical team can monitor. A specialized endotracheal (ET) tube, placed in the patient’s airway for general anesthesia, has small electrodes on its surface positioned to make contact with the vocal cords. These electrodes are designed to detect electromyographic (EMG) activity, the electrical signal produced by muscle contractions.
During the surgery, the surgeon uses a handheld stimulating probe that delivers a low, safe electrical current to the tissues being dissected. If the probe touches a motor nerve, like the RLN or EBSLN, the electrical impulse travels down the nerve and causes the corresponding vocal cord muscle to contract. This muscle contraction generates an EMG signal that is picked up by the electrodes on the ET tube.
The signal is then sent to a monitor in the operating room, which translates this electrical information into both an audible sound and a visual waveform. A distinct “beep” and a corresponding graph on the screen confirm to the surgeon that the stimulated tissue is a nerve and that it is functioning. This feedback allows the surgeon to map the nerve’s location and confirm its integrity without relying solely on visual identification.
Surgical Application and Interpretation
The information from the NIMS system is used in several ways throughout a thyroidectomy. Its first application is for identification. Early in the procedure, the surgeon can use the stimulating probe to help locate the RLN and EBSLN, even before they are visible. This is useful in cases where scarring or the size of the gland obscures anatomical landmarks.
Once identified, the system serves as a dissection aid. The surgeon can periodically stimulate the nerve’s path while separating the thyroid gland from the surrounding structures. This mapping helps the surgeon maintain a safe distance and avoid accidental traction, clamping, or thermal injury, confirming that the nerve remains functional as the surgery progresses.
Another application is predicting postoperative function. After the thyroid lobe has been removed, the surgeon tests the nerve one final time. A strong, stable signal at the end of the procedure is a reliable predictor of normal vocal cord function. Conversely, a “loss of signal” (LOS) alerts the surgeon to potential nerve injury.
In the event of an LOS on the first side of a planned total thyroidectomy, the surgeon must make a decision. An LOS suggests the nerve has been compromised, even if it appears visually intact. Proceeding to remove the second thyroid lobe would put the other nerve at risk, so the surgeon may choose to end the operation and stage the second part for a later date. This strategy uses NIMS to influence the operative plan and enhance patient safety.