An unusual cause of foot clonus: spasticity of fibularis longus muscle.

The functional consequences of spasticity can be corrected by local, pharmacological or surgical treatments once the spastic muscle has been identified. However, this diagnosis can be tricky when the muscle in question is rarely involved in spasticity or when its mechanical action is unusual or poorly characterized. Here, we present the case of a man presenting with left hemiplegia after an ischaemic stroke. His gait was perturbed by foot clonus in the sagittal plan, which persisted after selective neurotomy of the gastrocnemius and soleus but disappeared after neurotomy of the peroneus longus. Clonus triggered by pushing up under the whole of the forefoot in the direction of dorsiflexion may not be related to spasticity of the triceps surae. We recommend screening for foot clonus by first pushing up on the sole of the foot under all five metatarsals. In a second step, selectively pushing up under the first metatarsal joint enables the physician to evidence spasticity of the peroneus longus.