Nancy A. Melville

May 13, 2016

CHICAGO — The incidence of nerve injury resulting from positioning in spine surgery is relatively low, but intraoperative monitoring has poor sensitivity in predicting the position-related nerve deficit, according to a large database study.

"This is the largest data set [of position-related neurapraxia] studied to date, with 5000 patients," first author Gurpreet Surinder Gandhoke, MD, from the University of Pittsburgh in Pennsylvania, told Medscape Medical News.

"We found the incidence of position-related neurapraxia is lower than what has been quoted in the literature."

"[While] intraoperative restive neurophysiological monitoring has the potential to detect and potentially mitigate a position-related nerve injury, monitoring has a high specificity but low sensitivity in detecting position-related nerve injuries."

Dr Gandhoke presented the study findings here at the American Association of Neurological Surgeons (AANS) 84th Annual Meeting.

Peripheral nerve injuries make up approximately a third of all anesthesia-associated medicolegal claims in the United States, with studies showing rates of spine surgery position-related neurapraxia to range from 1.8% to as high as 15%, Dr Gandhoke explained.

No previous large database studies have looked at the incidence of the condition, however; most studies ranged from only about 20 patients to 1000.

For the new study, Dr Gandhoke and colleagues reviewed data from spine surgeries performed on all adult patients at the University of Pittsburgh center between 2010 and 2012. Patients had an average age of 56 years, with about 50% under age 40 and 25% over age 65. Approximately 70% of patients had a body mass index above 26.

Intraoperative monitoring was optimized according to the level of spinal surgery, such as from bilateral median nerve stimulation for cervical procedures or bilateral posterior tibial nerve for select lumbar surgeries, Dr Gandhoke said.

The study showed that among 4489 patients, most operating positions were arms abducted and flexed at the elbow (n = 2904 [64.7%]), followed by arms tucked at the side (n = 1570 [35%]) and the lateral position (n = 15 [0.3%]).

The incidence of new position-related complications was low: Only 13 of the 4489 patients (0.29%; 95% confidence interval [CI], 0.15% - 0.49%) developed a new positioning-related peripheral nerve deficit, with 7 (54%) developing meralgia paresthetica and 6 (46%) developing ulnar neuropathy.

Seventy-two (1.6%) of the patients had intraoperative somatosensory evoked potential (SSEP) changes relating to positions, and all subsequently underwent a repositioning maneuver. Only 1 of those patients (1.3%) went on to develop a new position-related nerve deficit.

Among the remaining 4417 patients (98.4%) who did not develop position-related intraoperative monitoring changes, 12 (0.3%) did develop a new position-related nerve deficit.

Overall, the results showed intraoperative monitoring had a sensitivity of 7.69% in detecting a new position-related nerve deficit and a specificity of 98.41%.

Whereas the positive predictive value for intraoperative monitoring was 1.39%, the negative predictive value was 99.73% (95% CI, 99.53% - 99.86%).

A multivariate analysis showed no association between length of surgery and the development of new neurapraxia. Although 49% of patients had hypertension, 18% had diabetes mellitus, and 11% had coronary artery disease, patient-related variables also were not associated with the development of a new neurapraxia.

"Our large population series has shown position-related neurapraxia to be significantly lower (0.3%) than the reported rates of 1.8% to 15% in smaller series," Dr Gandhoke said.

"In addition to that, the modification of the positioning is a mitigating factor that averted postoperative permanent neurological deficits in (71/72 [98.6%]) of our patients."

Limitations include the inability of knowing, among the patients with intraoperative SSEP changes who underwent repositioning, whether the measure did indeed prevent a deficit or whether the nerve was really under pressure or stretch.

"The results should be interpreted in light of the fact that intraoperative changes were made to patient positioning based on intraoperative monitoring findings," Dr Gandhoke said.

Previous studies have in fact shown that in addition to patient comorbidities, factors previously linked to position-related neurapraxia include type of anesthesia and surgery.

One recent study showed that an intraoperative mean arterial pressure of less than 55 mmHg for a total duration of at least 5 minutes while the patient was under general anesthesia was also associated with position-related neurapraxia.

Dr Gandhoke said further studies should take a closer look at those and various other factors.

"We want to look prospectively at the incidence of position-related neurapraxia after spine surgery and the role of SSEP monitoring in predicting the development of such a deficit," he said.

"Also, we should look at whether this is cost-effective — we should carry out an analysis to calculate the cost per quality-adjusted life-year of using intraoperative SSEP monitoring to prevent a position-related peripheral nerve injury."

In commenting on the study, Frederick G. Barber II, MD, a neurosurgeon with Massachusetts General Hospital in Boston, agreed that an important limitation is the change in positioning related to the intraoperative monitoring readings.

"We can speculate that the repositioning that always followed a monitoring change was an effective salvage therapy in preventing many nerve palsies, but we can't prove it," Dr Barber said.

"Because there are no internal controls treated without using monitoring or repositioning, it's not a strong design to prove efficacy or effectiveness. This shows the difficulty of studying an intervention after it has already become the standard of care," he said.

Discussant Allan D. Levi, MD, PhD, chief of neurosurgery at the University of Miami Hospital, Florida, added that the study importantly addresses the questions regarding position-related neurapraxia.

"On the positive side, there are a tremendous number of patients, but it is a retrospective chart review in which position-related neurapraxia was not an a priori primary consideration," he said.

He also underscored the importance of considering which nerves are being monitored or stimulated during surgery. "[In the study], it was the median nerve for the cervical spine and tibial nerve for lumbar surgery, and that will affect the ability to detect position-related neurapraxia."

Dr Levi added that the study’s findings on risk factors were not consistent with previous studies.

"It was surprising that neither the length of the surgery nor things like obesity, which have been shown in the past to be important factors, were shown here to be important — that's distinct from the prior literature," Dr Levi said.

"And the much lower incidence of position-related neurapraxia than has been reported may be, in part, related to the methods of diagnosis."

The authors have disclosed no relevant financial relationships. Dr Levi has received an honorarium from Medtronic.

American Association of Neurological Surgeons (AANS) 84th Annual Meeting. Abstract 702. Presented May 3, 2016.

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