Freezing of gait in Parkinson's disease.

Freezing of gait (FOG) is a highly debilitating, poorly understood manifestation of Parkinson’s disease (PD) that severely limits patients’ mobility. When coupled with postural instability in PD patients, FOG is a common cause of falls, complications of which can occasionally lead to death. Freezing of gait was originally defined as an inability to move from a spot ‘‘as if nailed to the floor’’ [1]. The definition was later broadened to include sudden, short-lasting inhibition in executing a movement or in switching from one movement pattern to another [2]. Freezing is typically induced by visual or proprioceptive stimuli [1], is not associated with plasma dopa levels, and can occur when patients are in an ‘‘on’’ or ‘‘off’’ state [1]. The symptom may also be referred to as akinesia paradoxica [1], hypotonic freezing [1], motor blocks [2], trepidant abasia, and lower-body parkinsonism [3,4]. ‘‘On’’ FOG does not respond to treatment with levodopa (L-dopa) or any other currently available PD therapies. FOG may occur as an independent entity called primary-progressive freezing of gait (PPFG), in which FOG is the initial manifestation of disease and remains the predominant symptom throughout its course. In the majority of PPFG patients, the tendency to fall begins within 3 years of disease onset, retropulsion occurs within 4 years, and confinement to a wheelchair occurs within 5 years [5]. In PPFG patients, when FOG occurs without other disabling symptoms, the potential impact of freezing episodes on the lives of affected individuals can be seen. This article reviews our current understanding of the prevalence, risk factors, pathophysiology, and treatment of FOG and, by highlighting gaps in that understanding, attempts to point toward appropriate avenues for research.