Muscle assessment in brachial plexus injury: A reliability study

Motor outcome following a brachial plexus injury is frequently measured to evaluate the success of surgical interventions. We aimed to identify whether the manual muscle testing using the Medical Research Council (MRC) method in adults with C5/6/7 motor weakness was reliable and whether its results correlated with functional recovery.

Two experienced clinicians examined 30 adults with C5/6/7 weakness following proximal nerve injury. The examination included using the modified MRC to assess motor outcome in the upper limb. Kappa statistics were calculated to evaluate inter-tester reliability. Correlation coefficients was calculated to explore the correlation between the MRC and the Disabilities of the Arm Shoulder and Hand (DASH) score and each EQ5D domain.

We found that grades 3-5 of the modified and unmodified MRC motor rating scales have poor inter-rater reliability when assessing C5/6/7 innervated muscles in adults with a proximal nerve injury. The Deltoid Posterior and the Extensor Carpi Radialis Longus were the only muscles (using the modified MRC) to achieve a Kappa over 0.6 indicating substantial reliability. Higher combined MRC scores correlated significantly with a lower DASH and vice versa. Similarly, higher combined scores of MRC correlated significantly with a higher rating of overall health on the EQ5D VAS.

This study demonstrates that the MRC motor rating scale has poor inter-rater reliability when assessing C5/C6/C7 innervated muscles in adults following proximal nerve injury. Other methods of assessing motor outcome following proximal nerve injury need to be considered.