Nerve Excitability Assessment in Chemotherapy-induced Neurotoxicity

The overall goal of the following experiment is to identify abnormalities in nerve function in patients undergoing chemotherapy treatment using axonal excitability studies. This is achieved by stimulating the median nerve at the wrist and recording the compound sensory action potentials from the second digit or compound motor action potentials from the abductor lysis brevis muscle. Next, a stimulus response curve is produced to determine the changes in threshold current required to achieve a target amplitude, which is tracked online using threshold tracking techniques.

Multiple nerve excitability parameters are recorded in response to different stimulus patterns, which assess markers of peripheral ion channel function and membrane potential in order to evaluate nerve function results are obtained that show the longitudinal changes in sensory axonal excitability with chemotherapy treatment and provide a method to detect these changes earlier than conventional techniques. The main advantage of this technique over conventional nerve conduction studies is that it provides information about the intrinsic function of the axonal membrane, including constituent conductances rather than simply amplitude and conduction velocity. Demonstrating this technique will be Dr.Cindy Lin, a senior lecturer in neurosciences, and Dr.Susanna Park, a postdoc from my laboratory For this study.

Protocol patients are typically referred by an oncologist for baseline nerve excitability testing prior to commencement on chemotherapy. All study procedures should be approved by a hospital or institution, internal review board screen patients to assure they have no history nor baseline neurophysiological evidence of peripheral neuropathy that they have not received prior neurotoxic chemotherapy treatment and have no contraindications for excitability testing. Undertake sensory and motor excitability protocols on the median nerve using the semi-automate computerized system Q track S, and an isolated linear bipolar constant current stimulator and amplifier.

Begin preparation by using an abrasive gel or pad to prepare the skin surface at the wrist and forearm to reduce skin resistance, and then apply an alcohol wipe. Next, prepare the recording site for motor recordings by placing non-polar electrodes on the muscle belly of the abductor lysis Brevis. Also place a reference electrode four centimeters distal to this location to record compound motor action potentials.

Now prepare the recording site for sensory recordings using ring electrodes placed at the proximal and distal interphalangeal joints as active and reference electrodes respectively to record compound sensory action potentials. Also place an electrosurgical neutral earth plate in the palm with conductive gel and tape the fingers to stabilize them. At this point, be sure to remove as much electrical noise in the recording setup as possible.

Using a humbug 50 hertz to 60 hertz noise eliminator, then place a repositionable stimulating electrode at the median nerve at the wrist. Place the anode electrode 10 centimeters proximal from the stimulating electrode over bone. Now stimulate the median nerve.

Use the repositionable bipolar electrode to locate the site of lowest threshold, which will be used as the stimulation site for subsequent steps in this protocol. Once the site is located, place a non-polar electrode. To begin the excitability protocol record a stimulus response curve by incrementally increasing the stimulus until the response is maximal and does not augment when stimulus intensity is further increased.

The target amplitude for threshold tracking should set automatically to 40%of maximal amplitude corresponding to the area of steepest slope on the stimulus response curve. Changes in threshold current required to achieve the target amplitude should be tracked online. Record multiple excitability parameters, including threshold, electro tous recovery cycle and current threshold relationship Assess threshold electro tous using 100 millisecond sub-threshold.

Polarizing currents Record the change in threshold current required to maintain target response amplitude. Following both D and hyperpolarization threshold, electro tous provides an assessment of intra nodal conductances and membrane potential with responses in the hyperpolarizing direction at the end of the polarizing pulse strongly associated with membrane potential. Next, assess the recovery cycle using a paired pulse paradigm with an initial supra maximal conditioning stimulus followed at different intervals via a test stimulus ranging from 2.5 milliseconds to 200 milliseconds.

Following the supra maximal stimulus, it is more difficult to generate a subsequent response termed raciness reflecting the inactivation of vaulted gated sodium channels. Then following the refractory period, a period of facilitation known as super excitability occurs. Finally, assess the current threshold relationship using polarizing currents of 200 milliseconds, varying in strength from plus 50%to minus 100%of threshold.

Using these techniques, patients can be assessed both acutely and longitudinally across chemotherapy treatment to assess acute neurotoxicity, have patients return for post chemotherapy assessment within 48 hours of treatment and compare to pre-treatment results. To examine chronic neurotoxicity assessments taken prior to chemotherapy infusion can be compared longitudinally across treatment cycles. In addition to axonal excitability testing, conventional clinical grading scales should be used to assess chemotherapy induced neurotoxicity, including the National Cancer Institute, common criteria for adverse events, neuropathy, sensory subscale, total neuropathy score, and patient reported outcome assessment.

Key parameters for assessment include raciness super excitability, extent of threshold change in threshold electro tous in addition to conventional parameters such as peak amplitude and latency. To assess overall change in excitability parameters across treatment, calculate a composite excitability score. The change in these three parameters should be summed from initial to final treatments to give an overall marker of change.

Here we see examples of excitability changes in sensory axons in oxaliplatin treated patients with baseline recordings shown in black and post-treatment recordings shown in white following four to six months of oxaliplatin treatment. These changes are thought to reflect widespread axonal damage and membrane potential change. Once master, this technique takes less than 10 minutes if performed properly, after watching this video, you should have a good understanding of how to perform external excitability techniques.