Understanding and treating freezing of gait in parkinsonism, proposed working definition, and setting the stage

Although the term “freezing of gait” (FOG) was not used by early authors and notably not by Parkinson himself,1 the typical propulsive high frequency stepping associated with this gait disturbance was described by him as a feature of Parkinson’s disease (PD). Martin2 also reported examples of an inability to initiate locomotion accompanied by disturbed stepping patterns in postencephalitic parkinsonism with a dramatic response to visual cues. It was not until the early 1970s that FOG started to get increased attention, based on the realization that its response to levodopa was more complex than that of bradykinesia and rigidity. During those early days, it was first suggested that levodopa can sometimes induce or even worsen FOG.3 Based on those reports and the occurrence of FOG in atypical parkinsonism, it has long been considered a levodopa-resistant symptom. It took 30 years of experience with levodopa treatment to understand that this is a misconception. Schaafsma et al. demonstrated that “off”-related FOG episodes were significantly shorter in duration and markedly fewer in frequency when turning from “off” to “on.”4 However, the concept of a complex relationship with levodopa treatment still holds, as is evidenced by the continued manifestation of FOG in the “on” period, and its relationship with other levodopa-resistant symptoms such as postural instability.5,6 Despite its fascinating and unique nature, its common appearance among people with advanced PD, and its significant contribution to the development of major disability and frequent falls,7 research about the pathophysiology and treatment of FOG moved slowly forward. One possible reason for that delay is the unpredictable and episodic nature of freezing, which makes it very difficult to capture true spontaneous episodes. In addition, FOG appears most frequently at home during unobserved behavior and in response to specific environmental triggers8 and rarely in the gait lab.9 Another difficulty that might have slowed down FOG research is its lack of definition. This is of special importance, considering the fact that FOG is very heterogeneous in nature and can frequently be confused with bradykinesia or akinesia. Taking all those difficulties together, we thought it is time to join forces and move research about FOG forward to a better understanding of its mechanisms and hopefully with time leading to more effective treatment. This supplement is the result of a satellite symposium which was held just prior to the Kyoto International Movement Disorders Congress in late October 2006. In this meeting, a number of state-of-the-art presentations were put forward, summarizing the most recent clinical and research findings. All speakers and two additional leading figures in the field of FOG research were subsequently invited to contribute to this first ever supplement devoted to FOG in parkinsonism. As part of the introduction to this supplement, we propose a working definition of FOG. We are aware of the difficulties inherent to this task but believe that this first step has to be taken to improve communication and upgrade the quality of scientific terminology among researchers. The most common feature associated with FOG is the unique subjective feeling of patients describing that “their feet get glued to the ground.” As suggested in one of the supplement papers on the clinimetrics of FOG,10 this characteristic feeling may aid in accurate history *Correspondence to: Alice Nieuwboer, Departement Revalidatiewetenschappen, Faculteit Bewegingsen Revalidatiewetenschappen, Katholieke Universiteit Leuven, Tervuursevest 101, 3001-B Leuven (Heverlee), Belgium. E-mail: alice.nieuwboer@faber.kuleuven.be Received 26 November 2007; Accepted 29 November 2007 Published online 25 July 2008 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/mds.21927 Movement Disorders Vol. 23, Suppl. 2, 2008, pp. S423–S425 © 2008 Movement Disorder Society