Ulnar Nerve Somatosensory Evoked Potentials Research Articles

F111. Isolating C8 nerve root technique with focal digital stimulation

The American Clinical Neurophysiology Society guidelines recommend eliciting upper extremity somatosensory evoked potentials (SSEP) via median or ulnar nerve stimulation at the wrist. However, when monitoring cervical spine surgeries, it may be more appropriate to use ulnar nerve stimulation as the root contribution to the cortical SSEP originates at levels below C7, while median nerve SSEPs have contributions possibly as high as C5. We postulate that wrist stimulation reflects a combination of activated axons from both the median and ulnar nerves secondary to supramaximal stimulation typically used intraoperatively. It’s therefore difficult to precisely determine the level of SSEP nerve root entry. Isolated ulnar stimulation targeting only C8-T1 roots could remedy this issue. The authors tested the feasibility and reliability of pure ulnar C8-T1 activation through 5th digit stimulation. This prospective study consented and enrolled eight patients without known evidence of peripheral neuropathy or cervical radiculopathy undergoing routine intraoperative neurophysiologic monitoring (IONM) including SSEP. Standard adhesive gel electrodes were used for wrist stimulation over the median and ulnar nerves, as well as the 5th digit. Ulnar nerve stimulating electrodes were placed at the wrist and 5th digit on the contralateral side of arterial line placement. Cortical SSEPs (montage of CPc-Fz and CPc-CPi) were analyzed for amplitude, latency and morphology. The intensity, pulse width, and repetition rate of stimulation to elicit the SSEP were also analyzed. SSEPs were obtained throughout surgery to ensure reproducibility. Stimulation of the median nerve at the wrist provided the highest amplitude cortical SSEP. Signals were consistently present in the ulnar nerve at wrist and 5th digit for each patient. The mean ulnar nerve amplitudes at the wrist and 5th digit were 42% and 78% smaller compared to the median amplitude, respectively. The average percent standard deviation of amplitudes for median nerve SSEP was 13.42% compared to 17.87% and 17.35% from ulnar nerve at the wrist and 5th digit, respectively. These data confirm 5th digit stimulation may be a reproducible method of isolating C8-T1 root activation. Despite higher amplitude responses from median wrist stimulation, 5th digit ulnar stimulation remained consistent during surgical procedures and could be used to detect conduction changes. This technique avoids coactivation of the median nerve SSEP which can occur when stimulating for an ulnar nerve SSEP at the wrist, thereby preventing false negative responses. This was further confirmed during this study by the identification of a case, which was subsequently excluded from analysis, of a patient determined to have a preexisting ulnar neuropathy, diagnosed by outpatient nerve conduction studies. Intraoperative cortical SSEP was correctly absent from 5th digit stimulation and incorrectly present from wrist stimulation of the ulnar nerve in this patient.

Causal factors for position-related SSEP changes in spinal surgery.

Somatosensory evoked potentials (SSEPs) are effective in detecting upper extremity positional injuries; however, causal factors for which patient population is most at risk are not well established. To review causal factors for intraoperative SSEP changes due to patient positioning. A case series with retrospective chart analysis was performed. 398 patient charts and intraoperative neurophysiological monitoring data from patients who underwent thoracolumbar and lumbosacral spine surgery were reviewed in a consecutive sequence from 2012 to 2013. Adverse events (AE) with the upper extremity SSEP recordings were compared to the independent variables, sex, positioning, length of procedure, and body habitus. Thoracolumbar and lumbosacral spine surgeries using contemporaneous ulnar and median nerve SSEPs were reviewed. The one-way analysis of variance (ANOVA) test, Chi-square, and independent samples t test were used to determine statistical significance in having an upper extremity SSEP AE to the aforementioned independent variables. The sample consisted of 209 males (52.5%) and 189 females (47.5%) (n=398). AE to the upper extremity SSEP was seen in 44 patients. Sex was found to be statistically significant for isolated ulnar nerve AE (P≤0.001) with males being most at risk (87.5%). AE for isolated median nerve SSEP was statistically significant for supine and prone positions (P=0.043). Length of procedure was statically significant for isolated ulnar nerve SSEP AE (P=0.039). BMI was statistically significant for generalized upper extremity SSEP AE (P=0.016), as well as isolated ulnar SSEP AE (P=0.006), isolated median SSEP AE (P≤0.001) and contemporaneous median and ulnar SSEP AE of the same limb (P≤0.001). Sex, patient positioning, length of procedure, and BMI are determinants for upper extremity neural compromise during thoracolumbar and lumbosacral spine surgeries.

Detection and Prevention of Impending Brachial Plexus Injury Secondary to Arm Positioning Using Ulnar Nerve Somatosensory Evoked Potentials During Transaxillary Approach for Thyroid Lobectomy

ABSTRACT.Robotic assisted surgery is becoming widely used for procedures such as radical prostatectomy. The use of robotic assisted surgery to perform partial and complete thyroidectomies using a transaxillary approach is being investigated in patient populations who wish to avoid a conspicuous cervical scar. Patient positioning for this approach as well as retraction during exposure has the potential to result in postoperative brachial plexopathy similar to what is seen in other types of surgery. We report the use of ulnar nerve somatosensory evoked potentials (SSEPs) to detect and prevent positional related neuropathy in the first pediatric patient in the world to undergo this procedure. We conclude that upper extremity SSEPs should be routinely performed during robotic assisted thyroidectomy.

Clinical Usefulness of Somatosensory Evoked Potentials for Detection of Brachial Plexopathy Secondary to Malpositioning in Scoliosis Surgery

A retrospective longitudinal study of 434 consecutive pediatric patients who underwent surgical correction of scoliosis, while being monitored for positional brachial plexopathy. To evaluate the effectiveness of intermittent monitoring of ulnar nerve somatosensory evoked potentials (SSEPs) for detecting brachial plexus injury caused by malpositioning during scoliosis surgery. Continuous intraoperative SSEP monitoring for spinal cord function has been well reported, and is widely accepted as the standard for spinal deformity correction surgery to detect and avoid neurologic injury. The use of SSEPs for the monitoring of ulnar nerve function intraoperatively as an indicator of brachial plexus function is becoming more accepted as a valid and useful technique to minimize intraoperative neurologic injuries during deformity corrections. A review was conducted to assess the effect of ulnar nerve SSEP monitoring, as a measure of brachial plexus function, during anterior, posterior, or combined approach surgeries. The type of scoliosis, type of surgery and positioning, and surgical event at noted amplitude decrease were included in an analysis of variance for repeated measures, and a Student t test was performed for significant differences. A total of 27 patients had ulnar nerve amplitude decreases of > or =30%, resulting in a point prevalence of 6.2% for positional brachial plexopathy during positioning for all scoliosis surgeries. A significant difference was noted between the types of positioning, with prone positioning accounting for a higher rate of brachial plexopathy compared with anterior approach positioning (P < 0.01). No statistical difference exists as to the type of scoliosis present and the incidence of brachial plexopathy (P < 0.01). Avoidance of neurologic injury to the brachial plexus during scoliosis surgery is possible by early detection with ulnar nerve SSEP monitoring.

Intraoperative SSEP detection of ulnar nerve compression or ischemia in an obese patient: A unique complication associated with a specialized spinal retraction system

Baumann SB, Welch WC, Bloom MJ. Intraoperative SSEP detection of ulnar nerve compression or ischemia in an obese patient: a unique complication associated with a specialized spinal retraction system. Arch Phys Med Rehabil 2000;81:130-2. Objective: To report a case of peripheral nerve compression caused by a specialized spinal retraction system, the Thompson-Farley retractor system, that most likely would not have been detected without intraoperative monitoring of the ulnar nerve. Design: Bilateral median and peroneal nerve somatosensory evoked potentials (SSEPs) were monitored continuously during a C5 corpectomy, as was core body temperature. Results: Within minutes after cervical soft-tissue retraction, the left ulnar nerve SSEP began to decline in amplitude. Peroneal nerve SSEPs were normal throughout the surgery; core body temperature remained at 36° ± 0.2°C. After much effort to reposition the patient, the SSEPs returned to baseline and the Thompson-Farley system was replaced by a self-retracting system. Conclusions: To our knowledge, this is the first report of peripheral nerve compression caused by the Thompson-Farley retractor system. Even with careful positioning on the operating table, obese patients may be particularly at risk for upper arm compression. Continuous monitoring of SSEPs is suggested to prevent postoperative morbidity. © 2000 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

Central conduction in somatosensory evoked potentials: comparison of ulnar and median data and evaluation of onset versus peak methods.

To compare central conduction in ulnar and median nerve somatosensory evoked potentials (SEPs), we recorded SEPs from the neck and scalp elicited by median and ulnar nerve stimulation in 46 normal young adults. We determined the central conduction time (CCT) in each subject from peak-to-peak and onset-to-onset measurements. The mean value of the onset CCT for the ulnar nerve SEP was 6.2 +/- 0.3 msec, and for the median nerve SEP, 5.9 +/- 0.3 msec. Onset CCT was significantly longer for the ulnar nerve SEP, and there was a significant correlation between onset CCT in both median and ulnar nerve SEPs and subject height. In contrast, the mean value of the "conventional" peak CCT for the ulnar nerve SEP was 5.6 +/- 0.6 msec, and for the median nerve SEP, 5.8 +/- 0.5 msec, with no significant difference between them. In addition, the peak CCT was not correlated with subject height in the ulnar or median nerve SEPs. Our findings suggest that onset CCT measurement is superior to the conventional peak CCT measurement for ulnar as well as median nerve SEPs, and confirm that the central conduction pathway for the ulnar nerve SEP is slightly longer than that for the median nerve SEP.

Somatosensory evoked potentials after multisegmental upper limb stimulation in diagnosis of cervical spondylotic myelopathy.

Radial, median, and ulnar nerve somatosensory evoked potentials (SEPs) were recorded, with non-cephalic reference montage, in 38 patients with clinical signs of cervical myelopathy and MRI evidence of spondylotic compression of the cervical cord. Upper limb SEPs are useful in spondylotic myelopathy because SEPs were abnormal in all patients for at least one of the stimulated nerves and SEP abnormalities were bilateral in all patients but one. Reduction of the amplitude of the N13 potential indicating a segmental dysfunction of the cervical cord was the most frequent abnormality; it occurred in 93.4%, 84.2%, and 64.5% of radial, median, and ulnar nerve SEPs respectively. A second finding was that the P14 far-field potential was more sensitive than the cortical N20 potential to slowing of conduction in the dorsal column fibres. The high percentage of N13 abnormalities in the radial and median rather than in the ulnar nerve SEPs correlated well with the radiological compression level, mainly involving the C5-C6 vertebral segments. Therefore the recording of the N13 response is a reliable diagnostic tool in patients with cervical spondylotic myelopathy and P14 abnormalities, though less frequent, can be useful in assessing subclinical dorsal column dysfunction.

The value of ulnar somatosensory evoked potentials (SEPs) in cervical myelopathy

In 57 patients with clinical signs and surgical documentation of compressive myelopathy, ulnar nerve somatosensory evoked potentials (SEPs) were more sensitive (with 74% abnormal) than either median or tibial nerve SEPs. The most frequent abnormalities were reduced or absent neck evoked responses and prolonged central conduction time. All subjects who had an SEP abnormality were identified by combined tibial and ulnar SEPs. Median nerve SEP added no additional information. Normal ulnar and tibial nerve SEPs were also able to exclude major cord damage in patients with cervical radiculopathy but little evidence of myelopathy.

Somatosensory Evoked Potential Quantification of Ulnar Nerve Blockade

We investigated the appropriateness of the somatosensory evoked potentials (SSEPs) as an objective monitor of sensory blockade of the ulnar nerve by comparing the effects of saline and lidocaine infiltration of the ulnar nerve at the elbow, with and without epinephrine. The four treatments were administered in random order to each of eight volunteers who were asked to assess the intensity of sensory blockade on a 10-cm visual analogue scale (SVAS). Concomitantly, ulnar nerve SSEPs were recorded at the Erb's point, over the posterior spine at or rostral to C-7, and over the sensory cortex. All 16 of the saline and saline with epinephrine infiltrations resulted in SVAS scores of 1 or less. In contrast, all the lidocaine and lidocaine with epinephrine injections were associated with SVAS scores in excess of 8. With lidocaine this lasted 83 +/- 22 min, and the duration of lidocaine with epinephrine was 248 +/- 82 min. SSEP responses were considered abnormal when there were 15 min or more of a decrease in amplitude by 50% or more and an increase in latency by 15% or more. After plain lidocaine injection, all the amplitude measurements showed depression over 50%, with the exception of cortical measurements in one subject. Similarly, there were increased latencies at Erb's point and C-7 with lidocaine blocks. In contrast, change in cortical latency was seldom seen. Variable selection by multiple linear regression analysis of the six SSEP measurements gave a statistical model that sensory block duration (SVAS greater than or equal to 8) could be specified by the duration of the reduced C-7 amplitude (r = 0.96).(ABSTRACT TRUNCATED AT 250 WORDS)