Determinants of the Maximal Nerve Cross-Sectional Area and Its Role in the Separation of Entrapment and Compression Ulnar Neuropathies at the Elbow.

INTRODUCTION/AIMS: Nerve cross-sectional area (CSA) is the most robust and sensitive ultrasonographic indicator of nerve pathology. The study aimed to identify predictors of maximal CSA (CSAmax) in ulnar neuropathy at the elbow (UNE) and to assess its utility in elucidating the mechanism of UNE, which is potentially important for patient referral to a surgeon.

METHODS

In a group of prospectively recruited UNE patients, 11 demographic, clinical, and electrodiagnostic features were recorded. CSAmax in the elbow segment was measured ultrasonographically. Explanatory variables for CSAmax were determined using a backward stepwise method of multiple linear regression analysis. To determine the CSAmax threshold with the best differentiation of retrocondylar compression and cubital tunnel entrapment (i.e., the UNE mechanism), a ROC curve was constructed.

RESULTS

We studied 130 patients (94 men), aged 27-88 years (median 62 years). The only significant predictor of CSAmax was the mechanism of UNE (CSAmax = 7.49 + 6.89 UNE mechanism, SE 1.04, (adjusted) R 2 = 0.25, p < 0.0001). The best differentiation between UNE mechanisms was obtained at the CSAmax of 16 mm2 (sensitivity 0.80, positive predictive value 0.71).

DISCUSSION: In UNE, the main determinant of CSAmax was the mechanism of neuropathy. In the UNE of unclear mechanism, CSAmax larger than 16 mm2 is more in favor of a cubital tunnel entrapment. These findings suggest that US is more sensitive in the diagnosis of entrapment compared with compression neuropathies.