Lawrence Yao MD, Presenter: Stockholder, Amgen Inc

Neville Gai PhD, Abstract Co-Author: Nothing to Disclose

Ultrasound studies suggest that carpal tunnel syndrome (CTS) is associated with enlargement of the median nerve, either within or just proximal to the carpal tunnel. Preliminary experience with diffusion tensor MRI (DTI) suggests that the fractional anistropy (FA) of the median nerve may have diagnostic value in CTS, and that FA of the median nerve proximal to the tunnel may differ from that within the carpal tunnel in normal subjects. This study seeks to establish normative values for median nerve caliber and FA proximal to and within the carpal tunnel.

20 wrists in 17 healthy volunteers underwent MRI of the wrist at 3 Tesla, using a multi-channel wrist coil. 3D isotropic turbo spin echo imaging (0.3 mm3 voxel size) was performed on all wrists. In 15 wrists, DTI was performed, using a multi-shot, spin-echo prepared echo planar sequence at a B value of 600 mm2/sec. DTI was acquired with sensitivity encoding (SENSE) at a voxel size of 1 x 0.7 x 3 mm. The median nerve was analyzed at standardized anatomic levels proximal to and within the carpal tunnel.

Mean (stdev) median nerve caliber proximal to and within the carpal tunnel was 8.8 (2.6) and 10.2 (3.3) mm2, respectively. The mean (stdev) FA of the median nerve was 0.64 (0.14) and 0.64 (0.15) proximal to and within the carpal tunnel respectively. The mean (stdev) ADC of the median nerve was 1.18 (0.4) and 1.25 (0.5) x 10-3 mm2/sec proximal to and within the carpal tunnel respectively. Only the caliber (and not ADC or FA) of the median nerve was significantly (p<.05) different at these two anatomic levels.

Our results suggest 90% (single tail) upper limits for normal median nerve caliber at and proximal to the carpal tunnel of 12. 1 and 14.5 mm2, respectively. These values are higher than corresponding normal ranges reported by ultrasound studies. We did not observe a difference in FA in the median nerve between anatomic levels proximal to and within the carpal tunnel, as suggested by a recent published study. Further research will determine whether changes in FA or diffusivity as measured by DTI are predictive of compressive median neuropathy at the carpal tunnel.

Caliber but not fractional anisotropy of the median nerve varied between anatomic levels proximal to and within the carpal tunnel, in asymptomatic subjects.

Yao, L, Gai, N, Median Nerve Caliber and MRI Diffusion Characteristics: Normative Values at the Carpal Tunnel.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6010627.html