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Updated: Sep 24, 2025
Corticospinal Excitability Modulation During Action Observation
Published on: December 31, 2013
Analysis of the compound muscle action potential scan: Step index (STEPIX) and amplitude index (AMPIX)
Sanjeev D Nandedkar1, Paul E Barkhaus2, Erik V Stålberg3
1Natus Medical Inc, Hopewell Junction, New York, USA; Department of Neurology, Medical College of Wisconsin, Milwaukee, Wisconsin, USA.
Objective:
The compound muscle action potential (CMAP) scan is useful to study motor unit (MU) loss. It is of interest to develop simple measurements of the scan.
Methods:
CMAP scan recordings were performed in the abductor pollicis brevis muscle of 20 control subjects and 26 patients with amyotrophic lateral sclerosis (ALS). They were analyzed using two new measurements called Step index (STEPIX) reflecting the number of steps, and Amplitude index (AMPIX) for amplitude of these steps.
Results:
In control subjects, STEPIX ranged from 71 to 172 while AMPIX was 78-158 µV. In ALS patients STEPIX was reduced and AMPIX was increased. The degree of change in STEPIX and AMPIX varied among patients reflecting the success or failure of reinnervation. Follow up studies in 9 muscles demonstrated reduced STEPIX and increased AMPIX despite minimal change in the CMAP.
Conclusions:
STEPIX and AMPIX are deterministic measurements of the CMAP scan made using a spreadsheet program. STEPIX and AMPIX can be inferred as indices for the number of motor units and their size, and demonstrate the expected pattern in ALS patients.
Significance:
The new algorithm for CMAP scan analysis may be useful to study disease progression in patients with ALS.
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