Science News: Extensor Digitorum Brevis Bulk and Associations With Fibular Motor Nerve Conduction Amplitude

Submitted by: Rebecca O’Bryan, MD
Edited by: Nakul Katyal, MD

Levin J, Lowe A, Tamura L, et al. Extensor digitorum brevis bulk and associations with fibular motor nerve conduction amplitude. PM R. 2022;14(4):428-433. doi:10.1002/pmrj.12608

Summary: Prior studies demonstrate fibular compound motor action potential (CMAP) < 4.0 mV predicts impairment of ankle proprioceptive precision and increased fall risk. The authors posit that evaluation of extensor digitorum brevis (EDB) may present a simple surrogate for CMAP amplitude. This study aims to correlate physician evaluation of EDB muscle bulk with CMAP amplitude on nerve conduction study (NCS). This was a prospective trial of 102 feet from 52 patients (30 male, 22 female) ages 21-82 (mean age 57.8). All patients ≥18 years of age, capable of providing consent, referred for an electrodiagnostic study were included. Exclusion criteria included body mass index (BMI) >40, pitting pedal edema, and inability to tolerate NCS. Lead investigator and a trainee independently assessed and graded muscle bulk using a Likert scale (Grade 1: Normal: easily visualized and or palpable muscle mass, Grade 2: Diminished: muscle bulk present but difficult to palpate, Grade 3: Severe atrophy: no visual or palpable muscle mass). The trainee could be either a resident or a fellow. AANEM guidelines were used to perform fibular motor NCS.
 
72 feet (71%) were graded 1, 22 feet (22%) were graded 2, and 8 (8%) were graded 3. Mean CMAP was about 6 when muscle bulk was normal, about 3.5 when diminished, and 0.6 when severe atrophy was present. Adjustment for distal symmetric polyneuropathy, lumbar radiculopathy, age, sex, and BMI still yielded significant association between CMAP amplitude and muscle bulk. EDB grade 3 was a highly specific indicator that CMAP amplitude would be abnormal. All patient with grade 3 EDB had abnormal CMAP. 88% of those graded normal had a normal CMAP.

Comments: While this study has limitations in terms of design and execution, this is a very interesting finding that is not surprising, but may not be a physical finding clinicians are specifically aiming to measure. Intrinsic foot musculature is often commented in general, but the specificity of this finding with an objective measure associated with functional outcome is very interesting.