Characterizing freezing of gait in Parkinson's disease: models of an episodic phenomenon.

Freezing of gait (FOG) is a very disabling and common gait disorder in Parkinson's disease (PD). The first aim of this article was to provide a methodological and critical review of the most common research approach to understand FOG, ie, comparing the behavior of freezers with that of non-freezers. The review demonstrates that studies often fall short in clearly defining the freezer\non-freezer groups and in controlling for disease severity and other confounders. These problems complicate data interpretation on FOG. The second aim of this article was to summarize the literature on the potential mechanisms behind the episodic nature of FOG in the following four models: (1) The threshold model assumes that FOG occurs when the accumulation of various motor deficits reinforce each other to a point of motor breakdown; (2) the interference model proposes that FOG represents an inability to deal with concurrent cognitive, limbic, and motor input, causing an interruption of locomotion; (3) the cognitive model views FOG as induced by a failure to process response conflict, leading to behavioral indecision; and (4) the decoupling model sees FOG as a disconnection between preparatory programming and the intended motor response as a result of which automatic movement generation gets stuck. These four theoretical premises are still incomplete and do not fully explain FOG. The depletion of motor and cognitive reserves and an increasingly complex response to levodopa with disease progression will also have an impact on the emergence of FOG episodes.