Shoulder Rotation Test: A New Test for Discriminating Between Functional and Structural Weakness

While many positive signs have been reported for diagnosing functional weakness (FW), additional signs are needed to enhance diagnostic accuracy. This study aims to describe and validate a new test, the "shoulder rotation test", to discriminate between FW and structural weakness (SW).

We retrospectively analyzed patients with FW or SW who presented with upper limb weakness. Inclusion criteria were that MRC scores of the shoulder internal rotation (IR) and external rotation (ER) were recorded, and that either or both were scored 4 or lower. The combination of the MRC scoring of IR and ER was named the "shoulder rotation test" and the results were classified into "weak IR with normal (MRC score of 5) ER", "weak IR and ER", and "normal IR with weak ER". The former two were combined into "weak IR".

In total, 29 patients with FW and 44 with SW were identified. Weak IR showed 100% sensitivity and 86% specificity for FW. Conversely, normal IR with weak ER, the complementary sign of weak IR, showed 86% sensitivity and 100% specificity for SW. Weak IR with normal ER showed 66% sensitivity and 100% specificity for FW.

IR was preferentially weakened in FW, which is thought to be due to the preferential impairment of an "active" movement. In contrast, IR was not easily weakened in SW. The shoulder rotation test is a promising new test for discriminating between FW and SW of the upper limbs.