Investigators from the International Consortium for Freezing of Gait proposed updated clinical and technical definitions of freezing of gait in Parkinson's disease to standardize assessment in clinical practice and research, according to a recent statement.

Freezing of gait (FOG) occurs when patients experience a sudden inability to take an effective step during gait initiation, walking, or turning. Investigators noted that the episodic and heterogeneous nature of the symptom has complicated measurement and contributed to variability across studies.

The consortium introduced a clinical definition describing FOG as “paroxysmal episodes wherein there is an inability to step effectively, despite attempting to do so.” The wording replaced earlier descriptions relying on inferred intent, highlighted observable attempts to step, and specified that no minimum or maximum episode duration exists.

Freezing episodes may occur during gait initiation, turning, or stepping in any direction. Ineffective steps are defined relative to a patient’s typical step pattern under comparable conditions.

The investigators also proposed a technical definition designed for standardized annotation of freezing episodes in video recordings, currently considered the gold standard method for evaluating the presence and severity of FOG in research. Video-based analysis allows measurement of episode frequency, duration, and the percentage of time spent freezing during walking tasks.

Under the updated framework, the onset of a freezing episode occurs when the foot lifts as part of the first ineffective step or when a visible attempt to step occurs without foot movement. Episodes end when two consecutive steps resemble the patient’s typical stepping pattern or when attempts to step cease.

The framework also defines a core phase of freezing, representing periods when foot progression stops completely. A single freezing episode may contain zero, one, or multiple cores.

To improve consistency in video scoring, the investigators simplified the classification of freezing manifestations into three categories:

• Akinetic freezing, with no observable lower limb movement

• Kinetic-trembling freezing, characterized by fast oscillatory leg movements

• Kinetic–no-trembling freezing, including ineffective movements such as paroxysmal shuffling or festinating-freezing.

Continuous gait abnormalities typical of Parkinson's disease such as persistent shuffling or reduced step length aren't classified as freezing because they lack episodic onset.

The consortium also proposed a classification framework for determining whether patients with Parkinson's disease have FOG. Because self-reported symptoms can be affected by recall bias or limited disease insight, classification based only on patient-reported symptoms represents the lowest certainty level. Observation of a freezing episode during examination is sufficient to classify a patient as having definite FOG.

Medication state can influence freezing occurrence. As a result, the investigators recommended evaluating patients both following dopaminergic medication intake and after medication withdrawal to increase classification certainty.

The definitions were developed through a consensus process that included surveys, workshops, and repeated scoring of video-recorded gait tasks demonstrating diverse freezing presentations.

The investigators wrote that standardizing definitions could improve the reliability of FOG measurements and support the validation of emerging objective detection methods.

“[B]y reducing uncertainties when scoring FOG from video, this standardization will result in faster scoring times and improved inter-rater reliability to a degree that a second human rater might no longer be required,” wrote lead statement author Moran Gilat, of the Research Group for Neurorehabilitation  in the Department of Rehabilitation Sciences at KU Leuven in Belgium, and colleagues.

Full disclosures can be found in the published study. 

Source: Nature Reviews Neurology