Foot small muscle atrophy is present before the detection of clinical neuropathy

OBJECTIVE - To characterize structural changes and the metabolic profile of foot muscles and correlate them with diabetic neuropathy measurements using phosphorus-31 (P-31) rapid acquisition with relaxation enhancement (RARE) magnetic resonance imaging (MRI). RESEARCH DESIGN AND METHODS - We studied 12 control subjects, 9 nonneu-ropathic diabetic patients, and 12 neuropathic diabetic patients using P-31 RARE and proton (H-1) MR1 at 3 Tesla. The ratio of the total cross-sectional area of the foot to that of the muscle tissue was calculated from transaxial H-1 and P-31 images. The average P-31 concentration across the metatarsal head region was measured from the P-31 images. RESULTS - The muscle area-to-total area ratio differed among all three groups (means +/- SD): 0.55 +/- 0.04 vs. 0.44 +/- 0.05 vs. 0.06 +/- 0.06 for control, nonneuropathic, and neuropathic subjects, respectively (P < 0.0001). The average P-31 concentration also differed among all groups: 27.7 +/- 3.8 vs. 21.7 +/- 4.8 vs. 7.9 +/- 8.8 mmol/l for control, nonneuropathic, and neuropathic subjects (P < 0.0001). The muscle area-to-total area ratio strongly correlated with clinical measurements: Neuropathy Disability Score, r = -0.83, P < 0.0001; vibration perception threshold, r = -0.79, P < 0.0001; and Semmes-Weinstein monofilaments, r -0.87, P < 0.0001. CONCLUSIONS - Small muscle atrophy is present in diabetes before clinical peripheral neuropathy can be detected using standard clinical techniques. The P-31 RARE MRI method evaluates the severity of muscle atrophy, even in the early stages when neuropathy is absent. This technique may prove to be a useful diagnostic tool in identifying early-stage diabetic foot problems.