AANEM News Express

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Science News: A New Tool for Assessment of Motor Axon Excitability. A Multicenter Prospective Study

Submitted by: Eman Tawfik, MD
Edited by: Marcus Pai, MD, PhD

Tyberghein M, Grapperon AM, Bouquiaux O, Puma A, Attarian S, Wang FC. iMAX: A new tool for assessment of motor axon excitability. A multicenter prospective study. Clin Neurophysiol. 2022;133:20–28.

Summary: This study is a multicenter study that assessed a novel electrodiagnostic test, ‘iMAX’, to test the motor axon excitability. The study was performed in four centers in Belgium and France. The authors aimed to evaluate the test reliability and consistency among the four centers, provide normative values, and determine its ability to detect impaired motor axon excitability in patients with peripheral neuropathies. The centers recruited 28 healthy volunteers and 32 patients with Charcot-Marie-Tooth (CMT), Guillain-Barr√© syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP), and axonal polyneuropathies (PN). The new test was performed using the usual EMG machine and routine motor nerve conduction technique stimulating the median nerve (with recording from the abductor pollicis brevis), ulnar nerve (with recording from the adductor digiti minimi), and fibular nerve (with recording from the tibialis anterior), using a stimulus duration of 1 ms. The test steps included:
1. Measuring the ‘minimal motor threshold’, which is the minimum stimulus intensity eliciting a reproducible motor response.
2. Then the intensity was increased gradually by 1 mA increments to determine the lowest stimulus intensity that evoked a maximal response. This intensity is called ‘iUP'.
3. Then the intensity was progressively lowered with a decrement of 0.1 mA until compound muscle action potential amplitude decreased followed by increasing the intensity again with increments of 0.1 mA until reaching the maximum response again. This stimulus intensity is called ‘iMAX’. 
The results showed no significant difference between the four centers for the measured parameters, which enabled the authors to pool the results and provide the normative values of the test.
Comparing healthy controls and patients, the iMAX test parameters were significantly higher in the CMT and CIDP groups compared to the healthy volunteers denoting decreased nerve excitability in these disorders. On the other, there was no significant difference in the iMAX parameters between patients with axonal PN and healthy volunteers.

Comments:  The new test is worth consideration given its simplicity and ease of use. The provided normative values can be helpful given the fact that they are derived from four centers. The ability of the test to detect impairment in nerve excitability in demyelinating disorders like CIDP and CMT may make the test of value in daily clinical practice.  

Article of similar interest:
Letter to the Editor: iMAX: A new tool to assess peripheral motor axonal hypoexcitability. Clin Neurophysiol. 2017;128:2382-83.

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