MRI can evaluate morphology of the ulnar nerve in patients with CuTS while EDX evaluates functional aspects of the nerve. This study demonstrated that fair or poor outcomes after surgery was strongly associated with increased ulnar nerve CSA in MRI (OR = 11.15 at 1 cm distal from the ME and OR = 16.01 at 1 cm proximal from the ME), while the association was minimal with preoperative EDX (OR = 0.92) and no association was seen with preoperative symptom severity as McGowan grades. This result suggests that morphologic changes of the ulnar nerve can better predict delayed nerve recovery after surgery for CuTS than preoperative symptom severity or EDX examination.
In our study, ulnar nerve CSA was increased 1 cm proximal to the ME, at the level of the ME, and at 1 cm distal to the ME, which is consistent with previous studies of MR neurography or ultrasonography.[3, 15–17] An MRI study by Bäumer et al reported that the mean (± SD) ulnar nerve CSA across the cubital tunnel was 15.4 ± 0.9 mm2, whereas the mean (± SD) of healthy control subjects was 11.0 ± 0.7 mm2. They also suggested that nerve caliber enlargement discriminated severe from mild disease preoperatively and that the mean (± SD) ulnar nerve CSA was significantly higher in patients with severe disease (19.4 ± 2.5 mm2) than the mean (± SD) ulnar nerve CSA in patients with mild disease (12.7 ± 1.2 mm2).[15]
Previous animal model studies have suggested that in compressive neuropathy, peripheral nerves compressed at a local level interfere with both anterograde and retrograde axonal transport and undergo perineurial thickening, fibrosis, and swelling proximal and distal to the compressed segment.[18, 19] Studies have also postulated that increases in nerve caliber are associated with extrinsic injury in which peripheral nerves undergo degenerative demyelination and compensatory remyelination, increasing myelin sheath thickness.[20, 21] In CuTS, these pathological alterations occur along the ulnar nerve proximal and distal to the cubital tunnel, and thus increase the ulnar nerve CSA.[17] In this study, fair or poor outcomes were associated with increased ulnar nerve CSA, which reflected morphological changes in the nerve and the severity of the disease. This association has been reported in previous studies using ultrasound.[22, 23] Beekman et al.[22] showed that significant nerve enlargement found during sonography at the time of diagnosis was associated with a poor outcome with an odds ratio of 2.9. A similar study by Domenico et al.[23] also showed that increased ulnar nerve CSAs in preoperative ultrasounds were associated with poor outcomes.
We found that decreased mNCV correlated with fair or poor surgical outcomes in univariate analysis. However, this correlation disappeared in the multivariate model 1 and the OR was only 0.92 in the multivariate model 2. Previous studies have reported on the relationship between preoperative EDX and CSA or between EDX and grip/pinch strength.[16, 19] The relationship between preoperative EDX and postoperative outcomes has been contradictory. A systematic review of the predictors of surgical outcomes after anterior transposition of the ulnar nerve for CuTS found that out of nine articles available, four case series and one cohort study reported no association between preoperative EDX and outcomes, whereas one randomized controlled trial, two cases series, and one cohort study reported a significant association.[24]
There were several limitations to this study. First, this study involved patients who underwent surgery, and thus the MRI findings may not apply to the general population of CuTS patients, including those with early symptoms. In addition, many patients had to be excluded due to the exclusion criteria, which may limit the applicability of the study findings. Second, the 6-month follow up period is short to evaluate the final surgical outcomes. Hironori et al.[25] reported that symptoms had improved in 14 out of 15 patients within 4.5 years and nerve conduction velocity had recovered two years after surgery. However, the 6-month outcome can be helpful for patient consultation regarding their expectations about recovery speed.