Ultrasonographic evaluation of ulnar innervated muscles in ulnar neuropathy at the elbow

Background Quantitative muscle ultrasound (qMUS) provides morphological measurements that can support clinical observation of muscle atrophy and weakness. As the use of qMUS continues to expand, and its potential for broader implementation in neuromuscular evaluation grows, there is a clear need for further validation, particularly in pathologies frequently encountered in electrodiagnostic laboratories, such as cubital tunnel syndrome caused by ulnar neuropathy at the elbow (UNE). Objective To evaluate the validity of qMUS evaluation of ulnar‐innervated intrinsic hand muscles. Design Cross‐sectional study. Participants Individuals 18 years or older with chronic UNE and healthy controls (HC). Main Outcome Measurement qMUS parameters (muscle thickness [MT], cross‐sectional area [CSA], echogenicity) were measured in the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) and were correlated to corresponding negative peak amplitude of the compound muscle action potential measured (CMAP) and key pinch grip strength. Results Sixteen adults (8 HC, 8 UNE) participated in the study. Key pinch strength was significantly correlated with FDI MT ( r = 0.45, p = .03 [95% CI, 0.05–0.73]) and echogenicity (rho = −0.60, p = .002 [95% CI, −0.82 to −0.24]). FDI MT and echogenicity explained 20% and 36% of pinch strength variance, respectively. Echogenicity was significantly correlated with corresponding CMAP amplitudes (FDI: rho = −0.59, p = .004 [95% CI, −0.81 to −0.21]; ADM: rho = −0.62, p = .002 [95% CI, −0.83 to −0.26]). No statistically significant relationship was found between key pinch strength and FDI CSA ( r = 0.35 p = .104 [95% CI, −0.08–0.67]). Furthermore, FDI CMAP was not significantly associated with FDI MT (rho = 0.14, p = .55 [95% CI, −0.32–0.54]) or FDI CSA (rho = 0.041, p = .86 [95% CI, 0.04–0.47]). Conclusion qMUS measures of echogenicity and MT (indicators of muscle fibrosis and size, respectively), correlated with key pinch strength and CMAP amplitudes in the FDI and ADM, in a heterogenous group including UNE and HC limbs. qMUS parameters may provide useful complementary markers and deeper insight into neuromuscular health in entrapment neuropathy.