Extensor digitorum brevis bulk and associations with fibular motor nerve conduction amplitude
Josh Levin1,2, Anita Lowe1, Lyndly Tamura1
1Department of Orthopaedic Surgery, Stanford University, Stanford, California, USA.
PM & R : the Journal of Injury, Function, and Rehabilitation
|April 20, 2021
Summary
Assessing extensor digitorum brevis (EDB) muscle bulk is a reliable indicator of fibular compound motor action potential (CMAP) amplitude, helping to identify neuromuscular impairment and fall risk.
Area of Science:
- Neurology
- Physical Medicine & Rehabilitation
- Electromyography
Background:
- Fibular compound motor action potential (CMAP) amplitude below 4.0 mV is linked to impaired ankle proprioception and increased fall risk.
- The extensor digitorum brevis (EDB) muscle may serve as a clinical surrogate for CMAP amplitude.
Purpose of the Study:
- To assess the inter-rater reliability of EDB muscle bulk evaluation.
- To determine the association between EDB muscle bulk inspection and fibular CMAP amplitude.
Main Methods:
- Prospective inter-rater reliability study conducted in outpatient electromyography clinics.
- Evaluated 52 adult participants (102 feet) using EDB bulk grading and fibular CMAP amplitude measurements.
- Analyzed the association between EDB bulk, distal symmetric polyneuropathy, lumbar radiculopathy, and CMAP amplitude.
Main Results:
- Moderate inter-rater reliability was found for EDB bulk grading (kappa: 0.65).
- Mean fibular CMAP amplitude varied significantly with EDB bulk: 5.9 mV (normal), 3.4 mV (diminished), and 0.6 mV (atrophied).
- EDB bulk, distal symmetric polyneuropathy, and lumbar radiculopathy were all associated with CMAP amplitude. EDB atrophy specifically indicated CMAP amplitude < 4.0 mV in 100% of cases.
Conclusions:
- EDB muscle bulk assessment is moderately reliable and associated with fibular CMAP amplitude.
- EDB atrophy is a highly specific indicator of reduced fibular CMAP amplitude (< 4.0 mV).
- Evaluating EDB bulk offers a simple clinical surrogate for assessing CMAP amplitude and distal neuromuscular function.
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