Histological Evidence of Intrinsic Muscle Degeneration in Compression Ulnar Neuropathy.

PURPOSE: We investigated the histological characteristics of the tissues in the intrinsic hand muscles of patients with compressive ulnar neuropathy (CUN) to determine how the compromised nerve influences the target muscle.

METHODS

In total, 83 patients underwent surgery for CUN in our institution between March 2015 and August 2017. Of these, 45 patients who met our inclusion/exclusion criteria were initially included in this study. Basic demographic data including age, sex, hand dominance, duration of symptoms, bone mineral density, and clinical stage were evaluated. During the ulnar nerve decompression surgery, a biopsy of the ipsilateral adductor pollicis was performed. Using the biopsy samples, we analyzed the tissue composition and degree of degeneration, and investigated the association with demographic factors and clinical status.

RESULTS

The ratio of muscle/connective tissue/fat was 62.62 ± 8.27%/23.44 ± 4.10%/14.16 ± 6.68% in the affected muscle. The ratio was different than that of samples taken from control groups. In samples containing muscle fibers, although the total fat content remained low, fat was consistently concentrated at the fascicular borders, between fascicles (perifascicular fat, 62.3 ± 18.2% of fascicles), and within fascicles (intrafascicular fat, 35.6 ± 11.8% of fascicles). The proportion of centrally nucleated muscle fibers was also significantly elevated (5.58 ± 4.25%; P < 0.001) compared with that of both controls (1.09 ± 1.26%) and the clinical threshold for abnormal central nuclei (3%). Preoperative scores assessed using Gabel and Amadio criteria were positively correlated with the muscle composition (r = 0.89, P < 0.001). In addition, the clinical status was negatively correlated with the degree of fat accumulation and the proportion of centrally nucleated muscle fibers (r = -0.94, P < 0.001, r = -0.82, P < 0.001).

CONCLUSIONS

We demonstrated that target muscle in CUN underwent degeneration, which was potentially exacerbated by inflammation, and that the degree of degeneration was correlated with the patient's clinical status. Histologically, reversible recovery of the hand muscles may be possible if decompression of the ulnar nerve is performed at earlier stages.