Time to Move Upstream? A Framework for Expanding the Focus of Freezing of Gait to Prodromal, Subclinical, and Pre‐FOG Stages

ABSTRACT

Freezing of gait (FOG) is one of the most disabling motor symptoms of Parkinson's disease, yet a fundamental question remains unresolved: why does FOG occur at any given moment? Most research has focused on studying FOG once it is already manifest. The present work aims to synthesize recent observations and propose a conceptual framework in which FOG reflects a dynamic process unfolding across multiple stages and time scales. Subtle changes in gait dynamics and physiology can precede episodes by several seconds (Stage −1), and experimental paradigms show that individuals without clinically recognized freezing may exhibit brief episodes under sufficiently sensitive conditions (Stage −2). At longer time scales, prospective studies indicate that vulnerability to freezing may be detectable years before onset (Stage −3). This framework reflects the idea that FOG emerges from progressive instability across interacting motor, cognitive, autonomic, and environmental systems, rather than as an isolated event. The proposed framework includes a prodromal stage, a subclinical stage, an immediate prefreezing state, and the freezing episode itself (Stage 0). This framework integrates findings from wearable sensing, physiology, and clinical studies and emphasizes that FOG remains latent in Stage −3, may be elicited only under specific conditions in Stage −2, and becomes manifest as instability evolves toward imminent and overt freezing (Stages −1 and 0, respectively). Reframing FOG as a multistage process that includes an upstream phase raises new questions regarding mechanisms, classification, and intervention, and may, ultimately, inform strategies aimed at predicting and preventing freezing rather than simply reacting to it.