Reduced StartReact effect and freezing of gait in Parkinson's disease: two of a kind?

Freezing of gait (FOG) is a disabling feature of Parkinson's disease. Emerging evidence suggests that dysfunction of the pedunculopontine nucleus (PPN) and pontomedullary reticular formation (pmRF) plays a role in the causation of FOG. These brainstem structures can be examined by the StartReact paradigm, which utilizes a startling stimulus to accelerate reaction times (StartReact). Here, we examined gait initiation in PD patients with and without FOG using this paradigm. Twenty-six patients with Parkinson's disease (12 freezers and 14 non-freezers) and 15 controls performed two tasks: rapid gait initiation in response to an imperative 'go' signal; and a control condition, involving a simple reaction-time task involving ankle dorsiflexion. During both tasks, a startling acoustic stimulus was combined with the imperative signal in 25 % of trials. In controls, the startle accelerated gait initiation and shortened the onset latency of tibialis anterior responses during ankle dorsiflexion. This acceleration was intact in non-freezers, but was significantly attenuated in the freezers. Independent of the occurrence of a startle, freezers showed a reduced length of the first step compared to non-freezers and controls. The diminished StartReact effect in freezers probably reflects deficient representation or release of motor programs at the brainstem reticular level due to dysfunction of the PPN, the pmRF, or both. These brainstem structures are presumably involved in integrating anticipatory postural adjustments with subsequent stepping movements. We suggest that with time-varying demands, these structures may no longer be able to coordinate the integration of anticipatory postural adjustments with steps, leading to FOG episodes.