T2-signal of ulnar nerve branches at the wrist in guyon's canal syndrome

Abstract

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.

Figure 1. ROC plots of sensitivity versus specificity for the discrimination between GCS and healthy status.

Nerve T2-signal was evaluated for its performance of diagnostic classification, empirical and fitted values are plotted. High diagnostic performance (AUC = 0.94) for nerve T2-signal of the deep motor branch was observed, while diagnostic of the ulnar nerve trunk (AUC = 0.70) and the superficial branch were significantly inferior (AUC 0.69).

Figure 2. Proximal-to-distal mapping of nerve T2-signal.

Mean T2-weighted CNR values and corresponding SEM (error bars) for the ulnar nerve trunk (left), the deep motor (middle) and the superficial sensory (right) branches. Graphs are plotted separately for GCS patients (red lines and dots) and asymptomatic controls (black lines and dots). Note the strong, statistically significant difference (p = 0.001) of increased T2-signal within the deep motor branch just distal to the osseus center of the pisiform bone for GCS patients as compared to healthy controls. T2-signal of the ulnar trunk more than the sensory branch was increased likewise compared to controls but without statistical significance. Median nerve T2-signal within each subject was not any different from ulnar nerve T2-signal (given in text of results section, not shown here). On the lower half of this graph array, an anatomical schematic drawing (according to Shea and McClain, 1969, with permission from The Journal of Bone and Joint Surgery, Volume 51, pages 1095–1103) is supposed to indicate the anatomical position of each slice with reference to the osseous center of the pisiform bone (slice position  = 0) as anatomical landmark. Slices proximal to center were assigned negative numbers indicating the distance (in mm) to center, slices distal to center were assigned positive numbers.

Figure 3. Representative findings of nerve T2-signal increase on single subject level.

one GCS patient with isolated motor symptoms (left column), another with combined motor and sensory symptoms (middle column), both compared to one asymptomatic control (right column). Three image sections from distal (A) to proximal (C) are given and image positions indicated on the anatomical schematic drawing on the left (with permission from The Journal of Bone and Joint Surgery, Volume 51, pages 1095–1103). The deep motor branch at the level of the hook of the hamate is encircled in red and found in the upper image row (A = slice position 10). Just distal to the bifurcation of the ulnar nerve motor and sensory branches are encircled in red and green, respectively (middle image row, B, slice position +3). Just proximal to the bifurction the ulnar nerve trunk is encircled in yellow (lower image row, B = slice position -3). In GCS with isolated motor symptoms (left column, GCS motor only) increased nerve T2-signal was most noticeable within the deep motor branch (red arrow) and extended over a short distance proximally into the ulnar nerve trunk (yellow circle in C), while the T2-signal was normal within the superficial sensory branch (green arrow). In case of combined motor and sensory symptoms both distal ulnar nerve branches clearly show an incresed nerve branch T2-signal (red and green circle without arrows in level B, middle column, GCS motor+sensory).

Figure 4. Correlation between deep motor branch T2-signal (y-axis) and its electrical conduction time in ms (x-axis).

Electrical conduction time through the deep motor branch was measured as distal motor latency to its most distal target muscle which is the first dorsal interosseus muscle (IOD I). Note how the empirical data (black dots) strongly correlate (R² = −0.8; p<0.001) in a non-linear fashion (non-linear fit of asymptotic growth, red dashed line). This finding indicates that nerve T2-signal and electrical nerve conduction show a distinctive behavior under physiological and pathological condition, in that the former exhibits its largest dynamic range near the normative reference value of dml IOD I (which is given in the literature at 3–5 ms), and that electrical conduction exhibits its largest dynamic range in severe pathological states (>5 ms). This may point toward a strong diagnostic value of nerve T2-signal as a novel marker for the early detection of nerve injury.