Abstract

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: Independently and prior to nerve conduction studies (NCS), two investigators graded EDB bulk. Next, fibular NCS were completed.SETTING: An academic center's outpatient Physical Medicine & Rehabilitation EMG clinics.PARTICIPANTS: 52 adult participants (102 feet) PREDICTOR: EDB bulk graded as 1)normal, 2)diminished, or 3)atrophied 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 above or below a 4.0 mV threshold.RESULTS: Inter-rater reliability of EDB bulk grading was moderate (kappa: 0.65 [95% C.I. 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 vs abnormal in detecting CMAP amplitude above or below 4.0mV were 0.86 [95% C.I. 0.78 - 0.94] and 0.71 [95% C.I. 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.

View details for DOI 10.1002/pmrj.12608

View details for PubMedID 33876583