Right Recurrent Laryngeal Nerve Shows Greater Stimulation in Spine Surgery; May Indicate Greater Risk

PHOENIX-The right recurrent laryngeal nerve showed greater stimulation than the left during anterior cervical spine surgery on 40 patients, researchers have shown in a study intended to promote greater understanding and prevention of damage to the recurrent nerve during the procedure.

Researchers at Albany Medical Center in New York said the findings indicate that the right recurrent laryngeal nerve might be at a higher risk of injury during the surgery.

They found one case of vocal fold paralysis among the 40 patients and no correlation between the electrical signaling and vocal fold injury, leaving the controversy over which side is best for the operation continuing to smolder.

The right recurrent laryngeal nerve does show greater stimulation during the surgery, said Karen Bellapianta, MD, who presented the findings at the 130th Annual Meeting of the American Laryngological Association, conducted as part of the Combined Otolaryngology Spring Meeting. It may be at increased risk, but it will take future study in order to prove this.

Cervical spine surgery is important to laryngologists because of the risk of injury. Monitoring electrical signals might give an early indication of a problem during the surgery, possibly helping to prevent injuries.

We, as otolaryngologists, care about anterior cervical spine surgery because the most common complication is temporary or permanent recurrent largyngeal nerve paralysis, or paresis, recorded in the literature, anywhere from 1 to 24 percent, Dr. Bellapianta said.

Which Side Is Safer? Controversy Continues

In Surgical Exposures in Orthopedics: The Anatomic Approach, Hoppenfeld and de Boer wrote that the recurrent laryngeal nerve is the most important structure at risk during the [anterior cervical] approach.

But which side is the best for performing the surgery is a question that has not been settled. There’s a lot of controversy in the literature about which side of the neck is safer to operate on, Dr. Bellapianta said.

The left recurrent laryngeal nerve is more consistent in its position. It loops around the aorta and is found in the tracheal esophageal groove, ascending along a direct vertical trajectory. The right recurrent laryngeal nerve is inconsistent in its position, entering from a more lateral position after looping around the subclavian artery.

Nerve Stimulation Studied

In the study, a nerve integrity monitor endotracheal silicone tube was used, with four stainless steel wires exposed about 30 mm above the cuff. They gave off a signal when the recurrent laryngeal nerve was stimulated.

In anterior cervical spine surgery, a horizontal incision is made in the neck, the retractors are inserted, deflecting the trachea in an anterior direction. Then an X-ray is taken to confirm the position of the spine, which is then plated.

The study involved 40 patients who had the operation performed, 25 of them from the left side and 15 of them from the right side. One surgeon did the right-sided procedures, and another did the left. The intraoperative nerve monitoring involved continuous free-run electromyography recorded from the recurrent laryngeal nerve. A technologist monitored visual and auditory information in the operating room.

A board-certified clinical neurophysiologist then analyzed the data independently, noting the number of signals lasting less than 10 seconds, those lasting 10 to 30 seconds, and those lasting more than 30 seconds.

The one case of paralysis that was found was from a right-sided surgery, but that was not statistically significant.

More interesting, researchers said, was the greater amount of signaling of the right true vocal fold with right-sided surgery than with the left.

Of the nine nerve integrity monitoring signals during the right-sided surgeries, four of them (44.4%) lasted longer than 30 seconds. That compared to one out of 24 signals (4.17%) during the left-sided surgeries that lasted longer than 30 seconds and two (8.33%) that lasted between 10 and 30 seconds.

The greater activity on the right side dovetailed with some prior research that found more susceptibility for injury on the right side.

In 1997 (Weisberg et al. Otolaryngol Head Neck Surg 1997;116:317-26), researchers who examined 10 cadavers found that because the left recurrent laryngeal nerve is redundant along its course, it was impossible to place any linear tension on it. But on the right nerve, which has minimal redundancy, researchers were to achieve a C4 stretch in three of the 10 and a C7 stretch in all 10 cadavers.

There is clear evidence that the right side is at greater risk of being stretched and, in our study, stimulated, Dr. Bellapianta said. However, this does not translate into a higher risk of being injured and that has yet to be proven.

Is Nerve Monitoring Necessary?

There is even lingering question over whether nerve monitoring during the surgery is even necessary. Dr. Bellapianta said that some research has indicated that it might be useful.

A 2007 study (Shindo et al. Arch Otolaryngol Head Neck Surg 2007;133:481-5), involving 684 patients undergoing a thyroidectomy with and without nerve monitoring, found a 5.8% injury rate in the monitored group and a 6.6% injury rate in the unmonitored group.

Similarly, in 2007 (Terris et al. Arch Otolaryngol Head Neck Surg 2007;133:1254-7), researchers found that, in 137 patients undergoing minimally invasive thyroid surgery, there was a 4.3% rate of temporary paresis in the group that was monitored and a 6.0% rate in those who were not monitored.

While neither of these groups showed a statistically significant difference between monitored and unmonitored groups, both authors still support the use of it, giving them additional monitoring during their surgeries, Dr. Bellapianta said. Recurrent laryngeal nerve injuries can cause significant voice dysfunction and morbidity if they occur during the anterior cervical spine surgery. And thus I feel it does warrant figuring out ways to make this a safer surgery.

After the presentation, an audience member said that another study found that the rate of injury is more related to the endotracheal tube’s cuff pressure than stretching, the main problem being the pinching of the soft tissue where the nerve lies between the cuff and the retractor blade. He said that the researchers have been reducing the pressures to the minimum needed, and their rate of paralysis has virtually disappeared.

Dr. Bellapianta agreed that there is still no clear answer. A follow-up study, she said, found that patients for whom cuff pressure was reduced fared no better than those for whom the pressure was not reduced (Audu et al. Anesthesiology 2006;105:898- 901). They concluded that perhaps cuff pressure, deflating it or regulating it, doesn’t make a difference, she said. So it still is controversial as to what exactly is causing it.

Marvin Fried, Chair of the Department of Otorhinolaryngology at Montefiore Medical Center and the current President of American Laryngological Association, said he was struck mainly by the small number of patients in the Albany study.

I believe it is too early to make conclusions based on this study, Dr. Fried said. As with many similar endeavors, more patients are needed.

Dr. Bellapianta acknowledged that was a limitation of the study. She also said that a deficiency in this area of research is that most studies have been retrospective. No study, she said, has actually looked specifically at left versus right in a prospective manner.

©2009 The Triological Society