T2-Signal of Ulnar Nerve Branches at the Wrist in Guyon's Canal Syndrome

Objective: To evaluate T2-signal of high-resolution MRI in distal ulnar nerve branches at the wrist as diagnostic sign of guyon's-canal-syndrome (GCS). Materials and Methods: 11 GCS patients confirmed by clinical/electrophysiological findings, and 20 wrists from 11 asymptomatic volunteers were prospectively included to undergo the following protocol: axial T2-weighted-fat-suppressed and T1-weighted-turbo-spin-echo-sequences (3T-MR-scanner, Magnetom/Verio/Siemens). Patients were examined in prone position with the arm extended and wrist placed in an 8-channel surface-array-coil. Nerve T2-signal was evaluated as contrast-to-noise-ratios (CNR) from proximal-to-distal in ulnar nerve trunk, its superficial/sensory and deep/motor branch. Distal motor-nerve-conduction (distal-motor-latency (dml)) to first dorsal-interosseus (IOD I) and abductor digiti minimi muscles was correlated with T2-signal. Approval by the institutional review-board and written informed consent was given by all participants. Results: In GCS, mean nerve T2-signal was strongly increased within the deep/motor branch (11.7 +/- 4.8 vs. controls: -5.3 +/- 2.4; p = 0.001) but clearly less and not significantly increased in ulnar nerve trunk (6.8 +/- 6.4vs. -7.4 +/- 2.5; p = 0.07) and superficial/sensory branch (-2.1 +/- 4.9vs. -9.7 +/- 2.9; p = 0.08). Median nerve T2-signal did not differ between patients and controls (-9.8 +/- 2.5vs. -6.7 +/- 4.2; p = 0.45). T2-signal of deep/motor branch correlated strongly with motor-conduction-velocity to IOD I in non-linear fashion (R-2 = -0.8; p < 0.001). ROC-analysis revealed increased nerve T2-signal of the deep/motor branch to be a sign of excellent diagnostic performance (area-under-the-curve 0.94, 95% CI: 0.85-1.00; specificity 90%, sensitivity 89.5%). Conclusions: Nerve T2-signal increase of distal ulnar nerve branches and in particular of the deep/motor branch is highly accurate for the diagnostic determination of GCS. Furthermore, for the first time it was found in nerve entrapment injury that T2-signal strongly correlates with electrical-conduction-velocity.