Have You Tried the APS Electrode for Neuromonitoring in Thyroid Surgery?
I have! While the design could use some tweaking, I think the electrode can be a valuable asset to monitoring the RLN during thyroid surgery.
The Medtronic APS electrode is a circumferential clip that is placed around the nerve of interest. Electrical stimulation is delivered through a small, round contact that is flush with the inner surface of the circumferential clip, and compound muscle action potentials (CMAPs) are recorded from the muscle(s) innervated by the nerve.
The APS electrode has been released by Medtronic for trial on a limited basis, and it comes in 2 sizes: The 2.0 mm model is designed for nerves ranging from 2-3 mm in diameter, and the 3.0 mm model is designed for nerves ranging from 3-4 mm in diameter.
APS is an acronym for Automated Periodic Stimulation. Medtronic states that the APS Electrode is for exclusive use with their NIM Monitors, but we all know that’s hollow business propaganda. You can use any standard IONM system to deliver the current and record the CMAPs.
Here’s how it works in practice, using recurrent laryngeal nerve (RLN) monitoring during thyroid surgery as an example.
In thyroid surgery, the recurrent laryngeal nerve (RLN) is at risk for injury, which can result in post-op dysphonia. Typically, we map the course of the nerve with a hand-held probe, recording CMAPs from the vocalis muscles, but this technique has its limitations. One such limitation is the inability to monitor RLN function between stimulation/mapping. The APS electrode is useful to overcome this limitation. It can be placed on the main trunk of the vagus nerve in the carotid sheath. Continuous stimulation necessarily activates the ipsilateral RLN, allowing for nearly constant monitoring as you record CMAPs from the vocalis muscle. This video is a pretty good demonstration.
For EMG recordings, have anesthesia place an appropriately-sized NIM endotrachial tube, verifying that the blue wire electrodes are in contact with the left vocalis muscle, and the red wire electrodes are in contact with the right vocalis muscle. As a control, you should place twisted pair needle electrodes in the upper trapezius muscles, bilaterally. Be sure to do a “tap test” to make sure you can record the artifact from the electrodes. You will also need to place a single needle electrode in one of the deltoid muscles as a stimulus return. Obviously, you’ll need a ground. Make sure you run a train-of-four (from the hand is fine) to prove normal conduction across the neuromuscular junction. You will give the surgeon the APS electrode and a hand-held monopolar probe.
Note: the use of automated or semi-automated EMG monitoring devices is dangerous and not recommended. Electrical stimulation of nervous system tissue and recording of EMG activity should be performed by a professional neurophysiologist.
The surgeon will open the carotid sheath on one side and clip the APS electrode onto the main trunk of the vagus nerve. I would recommend delivering a 50-100 µsec constant current pulse at 0.5 Hz. While supramaximal stimulation is often recommended for nerve monitoring, this may or may not be possible with the main trunk of the vagus nerve in the carotid sheath due to possible cardiac side-effects. In practice, you can try using 0.2 mA, which is low but seems to work. I would recommend at least finding an intensity that produces a maximal response amplitude. NOTE: Be very careful here. You must be ready to discontinue stimulation immediately if any problems arise.
You should see a CMAP occur approximately 8-10 msec after stimulus onset. This response should come exclusively from the ipsilateral vocalis muscle. The amplitude of the response is somewhat variable, maybe 10-30%, but the variability may be due to the (required) low stimulation intensity if you CMAP amplitude isn’t maxed out. The latency is quite stable within subjects. Because you are stimulating every 2 seconds, you can evaluate the RLN constantly as the surgeon dissects (as long as electrocautery is not in use). I don’t know what Medtronic recommends for stimulation frequency, but several surgeons have asked me to stimulate every 6 seconds. I wouldn’t recommend this. First, the frequency with which you monitor nerve function is based on the stimulus frequency. Second, due to amplitude variability a couple of trials may be required for one to accurately interpret the data. If you stimulate at 0.5 Hz, you can give the surgeon feedback in 4-6 seconds. If you stimulate at 0.17 Hz, the surgeon may have to wait 12-18 seconds. I know very few surgeons that will wait that long. So, stimulate frequently.
As the procedure continues, the surgeon may need to map the course of the RLN, or differentiate neural vs non-neural tissue. You should stop stimulating with the APS and switch to the monopolar probe. You can use your standard method for this. I find threshold to CMAP and maximum amplitude with 50 µsec constant current pulse at 2.1 Hz. Because you are now stimulating much closer to the vocalis muscle, your latency should be shorter, maybe 3-4 msec. Encourage the surgeon to stimulate often. I recommend the methods employed by Chiang et al (2010).
The most obvious benefit to using the APS electrode is being able to evaluate the RLN constantly as the surgeon dissects and removes tumor. Free-running EMG tells you almost nothing about nerve function when EMG is quiet. Indeed, you may see no EMG activity with complete transection of a nerve (Kartush, 1989; Nelson and Vasquez, 1995). Also, infrequent or periodic nerve stimulation may miss nerve injury. So, frequent, real-time nerve monitoring has an obvious benefit. In theory, no change in the amplitude of the response from pre- to post-resection should be indicative of preserved RLN function post-op, but I am unaware of any data relating APS electrode use to post-operative outcomes. Nevertheless, in terms of using stimulus-triggered EMG to predict post-op vocal cord function, I would point you toward Taha (1995), Mandpe (1998) and Genther (2014).
The P in APS stands for “periodic”. While I used to the term “continuous” above, our ability to monitor the RLN is limited by the frequency of stimulation/recording. This is another reason why I would not recommend stimulating every 6 seconds. Just be aware, though, that you are not monitoring continuously.
My biggest problem with the APS electrode is the size. It is a challenge for surgeons to estimate the diameter of the vagus nerve in situ. As a consequence, I have found that we waste 1 electrode about 25% of the time after finding it doesn’t fit around the nerve. When the clip is too small, the problem is obvious because it won’t fit over the nerve. Far more problematic is when the clip is too big. This will not be obvious, and the consequence is that the small electrode on the inside of the clip will make variable contact with the nerve, making your CMAP variably present, and complicating IONM.
While my experience with the APS electrode has been largely successful, I have had problems with low intraoperative specificity. In other words, manipulation of the nerve can cause the CMAP to disappear. This doesn’t mean that the patient will wake with dysphonia, but it does mean that every once-in-a-while (and sometimes frequently) you will have to tell the surgeon to chill out for a minute and let the CMAP return to baseline.
The final limitation, which is more of a methodological challenge, is finding the appropriate stimulation intensity for the most reliable CMAP.
Pearls of Wisdom:
Communication is critical. The surgeon should always tell you what he/she is stimulating before you deliver current. Some surgeons want to play games and test you. This is dangerous and, in my experience, dramatically reduces the probability of positive post-op outcome. Likewise, you need to keep the surgeon informed of the status of the nerve.
Stimulate often. Some surgeons think that a single, post-resection stimulation of the RLN is sufficient to establish function. That’s just crazy. I recommend taking at look at Chiang et al (2010) and finding a way to incorporate their methods into your protocol.
The Medtronic APS electrode is a valuable asset to thyroid surgery when the RLN is at risk for injury. The electrode can be a bit finicky. One must take time to establish safe stimulation parameters appropriate to the patient. Prognostication will require analysis of data on a large scale. Utility of the APS electrode is not necessarily limited to thyroid surgery.
- Chiang FY, Lee KW, Chen HC, Chen HY, Lu IC, Kuo WR, Hsieh MC, Wu CW. Standardization of intraoperative neuromonitoring of recurrent laryngeal nerve in thyroid operation. World J Surg. 2010 Feb;34(2):223-9.
- Dionigi G, Donatini G, Boni L, Rausei S, Rovera F, Tanda ML, Kim HY, Chiang FY, Wu CW, Mangano A, Rulli F, Alesina PF, Dionigi R. Continuous monitoring of the recurrent laryngeal nerve in thyroid surgery: a critical appraisal. Int J Surg. 2013;11 Suppl 1:S44-6.
- Genther DJ, Kandil EH, Noureldine SI, Tufano RP. Correlation of final evoked potential amplitudes on intraoperative electromyography of the recurrent laryngeal nerve with immediate postoperative vocal fold function after thyroid and parathyroid surgery. JAMA Otolaryngol Head Neck Surg. 2014 Feb;140(2):124-8.
- Jonas J, Boskovic A. Intraoperative Neuromonitoring (IONM) for Recurrent Laryngeal Nerve Protection: Comparison of Intermittent and Continuous Nerve Stimulation. Surg Technol Int. 2014 Mar;24:133-8.
- Kartush JM. Electroneurography and intraoperative facial monitoring in contemporary neurotology. Otolaryngol Head Neck Surg. 1989 Oct;101(4):496-503.
- Mandpe AH, Mikulec A, Jackler RK, Pitts LH, Yingling CD. Comparison of response amplitude versus stimulation threshold in predicting early postoperative facial nerve function after acoustic neuroma resection. Am J Otol. 1998 Jan;19(1):112-7.
- Nelson KR, Vasconez HC. Nerve transection without neurotonic discharges during intraoperative electromyographic monitoring. Muscle Nerve. 1995 Feb;18(2):236-8.
- Taha JM, Tew JM Jr, Keith RW. Proximal-to-distal facial amplitude ratios as predictors of facial nerve function after acoustic neuroma excision. J Neurosurg. 1995 Dec;83(6):994-8.