Extensor digitorum brevis bulk and associations with fibular motor nerve conduction amplitude

Introduction Prior work demonstrates that fibular compound motor action potential (CMAP) amplitude <4.0 mV predicts impairment of ankle proprioceptive precision and increased fall risk. Extensor digitorum brevis (EDB) inspection may present a simple clinical surrogate for CMAP amplitude. Objective (1) To estimate the inter-rater reliability of assessment of EDB bulk. (2) To determine whether inspection of EDB bulk is associated with fibular CMAP amplitude. Design Prospective inter-raterreliability study. Setting Academic center outpatient Physical Medicine & Rehabilitation electromyography (EMG) clinics. Participants Fifty-two adult participants (102 feet). Main Outcome Measures (1) Inter-rater reliability of assessment of EDB bulk. (2) Mean fibular CMAP amplitude. (3) A binary measure of fibular CMAP amplitude at/above or below a 4.0 mV threshold. Results Inter-rater reliability of EDB bulk grading was moderate (kappa: 0.65 [95% confidence interval (CI) 0.48-0.82]). The mean CMAP value was 5.9 +/- 2.2 mV when bulk was normal, 3.4 +/- 2.1 mV when diminished, and 0.6 +/- 0.9 mV when atrophied. A multivariable analysis demonstrated that EDB bulk, distal symmetric polyneuropathy (DSP), and lumbar radiculopathy were all associated with CMAP amplitude. The sensitivity and specificity of grading muscle bulk as normal versus abnormal in detecting CMAP amplitude above or below 4.0 mV were 0.86 (95% CI 0.78-0.94) and 0.71 (95% CI 0.54-0.88), respectively. An atrophied EDB was a highly specific indicator that CMAP amplitude was abnormal (<4.0 mV) in 100% of cases (8/8). Conclusions EDB bulk was associated with fibular CMAP amplitude. Atrophy was a highly specific indicator for CMAP amplitude below 4.0 mV. Evaluation of EDB bulk may represent a quick and easy clinical surrogate marker for CMAP amplitude and distal neuromuscular impairment.