The tibialis anterior response revisited.

The idiomuscular response to direct percussion is rarely tested nowadays because of its uncertain mechanism and significance. While performing neurological examination, we observed a brisk ankle dorsiflexion response on direct muscle percussion of m. tibialis anterior in patients with acute inflammatory demyelinating polyradiculoneuropathy (AIDP). In contrast, in patients with upper motor neuron lesions, an ankle inversion response was seen. In this article we describe our findings in patients with bilateral lower limb weakness. We assessed 73 consecutive patients with bilateral lower limb weakness. A strong dorsiflexion response to percussion of m. tibialis anterior was seen in 11 out of 14 patients with AIDP (sensitivity 78.6%). None of the other patients showed a strong dorsiflexion response (specificity 100%). An inversion response was seen in 11 out of 13 patients with UMN involvement (sensitivity 92.3%). It was also noted in two of 46 patients without proven UMN involvement (specificity 96.7%). The idiomuscular response to percussion of m. tibialis anterior can be useful in the assessment of patients with lower limb weakness of unclear cause.