Early-Stage Diabetic Neuropathy Reduces Foot Strength and Intrinsic but Not Extrinsic Foot Muscle Size

Background. Tracking progression of diabetic peripheral polyneuropathy (DPN) is usually focused on sensory nerves and subjective testing methods. Recent studies have suggested that distal muscle atrophy may precede sensation loss. Methods to objectively measure distal muscle size and strength are needed to help understand how neuropathy affects muscle function. Purpose. To evaluate individual intrinsic and extrinsic foot muscle sizes and functional foot strength in participants with DPN. Methods. Thirty individuals participated in this cross-sectional study (15 DPN and 15 matched controls). Sizes of 10 separate muscles of the lower leg and foot were measured using ultrasound imaging. Functional foot strength was also quantified using custom great toe and lateral toe flexion tests along with a doming test. Muscle size and strength metrics were compared between groups using ANOVAs and paired t-tests (alpha=0.05). Correlations between strength and relevant muscle sizes were also evaluated. Results. The sizes of all four intrinsic foot muscles were smaller in individuals with DPN (p <= 0.03), while only one (toe extensor) of the six extrinsic muscles was smaller (p<0.01). Great toe flexion (p=0.03) and lateral toe flexion (p<0.01) strengths were decreased between groups and showed moderate to strong correlations (0.43 <= r <= 0.80) with several corresponding intrinsic muscle sizes. The doming strength test did not show any difference between groups and was moderately correlated with one muscle size (r=0.59). Conclusion. Diabetic peripheral polyneuropathy affects intrinsic muscles before extrinsics. Ultrasound imaging of individual muscles and functional toe flexion tests can be used clinically to monitor DPN progression and foot function. Participants need to be trained in the doming test before a relationship can be established between this test and DPN foot function. Future studies should include muscle quality measurements to better understand characteristics of affected muscles.