Michael Hausman, MD
Robert K. Lippmann Professor of Orthopedic Surgery; Vice-Chairman of the Department of Orthopedics at Mount Sinai; Chief of Hand and Upper Extremity Surgery for the Mount Sinai Health System
The treatment of peripheral nerve injuries caused by stretching, crushing or non-transecting penetrating trauma remains a major challenge. The decision whether to perform a neurolysis or proceed to nerve resection and grafting or nerve transfer is challenging, especially in the case of a neuroma-in-continuity with a complete palsy by physical exam.
In such cases, imaging studies, such as MR or ultrasound cannot provide clarity on the best approach. Electrophysiological testing would, hypothetically, be helpful, but more often than not, the conclusion is that there is a severe nerve injury with fasciculation, indicating axonal injury, and no voluntary muscle response. In other words, it confirms that the nerve isn’t working but does little to clarify the decision of neurolysis versus reconstruction with graft or transfer.
Intraoperative assessment with a reliable nerve stimulator, providing a variable, reproducible stimulus, can be of help in assessing the function of a motor nerve. Direct, intraoperative stimulation and assessment by the surgeon can add important and useful information, particularly because the surgeon is looking directly at and has the ability to touch the nerve, thus eliminating thick layers of intervening tissue, inference and the inherent problem of a poor signal-to-noise ratio that compromises conventional electrophysiological information.
For instance, we know from experience that direct surface stimulation of a peripheral nerve with a physiologic, biphasic stimulus of 0.5 mA and a pulse width of 25-50 microseconds produces a vigorous and unambiguous muscle contraction. The lack of response to stimulus in this range indicates some degree of nerve dysfunction, which may be subtle. An example is axillary nerve dysfunction after trauma or a failed previous surgery. If the surgeon plans a revision procedure to a reverse total shoulder prosthesis, which is dependent upon a healthy, strong deltoid, confirming axillary nerve normal function would be important.
It is our practice to routinely use the Checkpoint stimulator to identify and check the axillary nerve. If 0.5 mA does not produce a strong contraction, we perform an external neurolysis and test again. Consistently, in the absence of a more serious injury to the nerve, incising or stripping the thickened epineurium improves the function and a strong contraction in the 0.5 mA range is achieved.
Similarly, it is important to prevent overstretching the nerve, especially as rTSA is dependent upon some degree of stretch for stability. (Fig. 1) To assess this, we establish the “threshold stimulus”, which is the lowest stimulus intensity required to produce any deltoid response. This is determined by placing the stimulator in the 0.5 mA range and gradually increasing the pulse width with the slide switch until a muscle twitch is observed. This is marked, on the stimulator, as the “threshold stimulus”. The test is repeated after inserting the trial components. If the threshold is increased (meaning a stronger stimulus is needed to produce a muscle response), then the nerve is being stretched and the components should be downsized and stability and nerve function reassessed.
Intraoperative nerve testing with the Checkpoint stimulator is also helpful with more serious nerve injuries. For example, this 22 year old underwent a Latarjet procedure for recurrent shoulder instability and awoke from anesthesia with inability to flex his elbow and a complete, isolated musculocutaneous nerve injury. (Fig. 2) Electrodiagnostic studies showed no distal conduction in the distribution of the musculocutaneous nerve and fibrillations in the brachialis and biceps muscles.
During surgical exploration of the brachial plexus, a neuroma-in-continuity was found in the musculocutaneous nerve,(Fig 3) with a discrete area of thickening and scarring. Testing proximal to the lesion produced no response, even in the stronger, 2.0 mA range. Accordingly, the operating microscope was used to perform an internal neurolysis. Damage to multiple fascicular groups was seen, but some fascicles could be traced through the scarred area. After dissection, the nerve was stimulated again proximal to the lesion and a response could be observed in the elbow flexors in the 2.0 mA range. From this finding, we can infer that the patient will recover at least antigravity elbow flexion and that neurolysis, rather than grafting or Oberlin transfer, is indicated. After further dissection, the nerve is tested again and a very weak, but unambiguous twitch is seen in the brachialis in the 0.5 mA range. The presence of any response in the 0.5 mA range implies that the patient will have a complete recovery, which, in this case, was complete by 6 weeks after surgery
Thus, intraoperative nerve testing can help with the nerve reconstruction decision tree by applying the “Rule of 2”. While not validated with a rigorously performed study, there is an evolving consensus that with lesions-in-continuity, a distal response in the 2.0 mA range indicates that with neurolysis, recovery will occur at, at least, motor grade III level and, therefore, neurolysis is warranted. A response in the 0.5 mA range implies that complete recovery is likely, while failure to observe any response in the 2.0 mA and maximum pulse width range implies that the nerve is sufficiently damaged that no recovery will occur and that grafting or nerve transfer should be performed.
The Checkpoint Stimulator is a single-use, sterile device intended to provide electrical stimulation of exposed motor nerves or muscle tissue to locate and identify nerves and to test nerve and muscle excitability. Do not use the Checkpoint Stimulator when paralyzing anesthetic agents are in effect, as an absent or inconsistent response to stimulation may result in an inaccurate assessment of nerve and muscle function.
Please note: Case reports and white papers are company funded and not peer reviewed.