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Quantitative methods of assessing disability in patients with movement disorders provide measures that can be clinically relevant, reproducible, and allow for rigorous mathematical and statistical scrutiny. These techniques also can assist in diagnosis through precise determination of the timing and organization of movements,1 and help in the understanding of underlying brain pathophysiology.2 Methods for assessment have increased in scope and breadth over the last decade—with almost twice as many publications relating to clinical motor quantification methods and motion analysis in peer-reviewed journals since 2000 compared with the prior decade. Advances in the sensitivity and precision of sensors, improvements in processor speeds, computer graphics, and the development of neural interface systems3 have all contributed to faster, more accurate, more meaningful methods of assessing motor disability, as well as uncovering the relationships between brain activity and motor behavior.
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Importantly, quantitative techniques often contribute to improvement in quality of life or medical care,4–6 not just assign numerical values to motor findings. For example, with the analysis of early or mild conditions such as Parkinson disease (PD),7 methods have been developed with guidelines for their predictive utility8 as well as general clinical use. Movement disorder laboratories now assess motor disabilities with proprietary hardware and software, and evaluate a multitude of questions in motor control for clinical investigations and routine clinical service. New devices are also available commercially, many of which are designed for ambulatory and home use9–13, with the caveat that they should be monitored by health care professionals for accuracy and interpretation. As movement disorders may have overlapping or subtle clinical features, discerning the presence of and differentiating conditions, for example PD from essential tremor (ET) or dystonia (DYT), with more certainty and objectivity is important for therapeutic and prognostic reasons.
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Descriptive comments or ratings with four-point (0–3) or five-point (0–4) scales are typically used clinically to characterize disabilities, such as tremors, and other motor signs and symptoms of movement disorders such as PD, e.g. 1+ (for mild) to 4+ (for severe). Multiple scores are rated in the various scales. While scales differ in length, with the shorter scales focused on decreasing subject burden and assessment time, many also include subscales of varying structures. Though validated and used by clinicians, rating scales are subjective, and are variable from examiner to examiner. Not all scales are easily reproducible and clinical scales may be insensitive to small changes—particularly in the “close to normal” range. Furthermore, while many scales are ordinal, they are not necessarily linear. A logarithmic relationship has been found between clinical rating scales of tremor and tremor amplitudes, suggesting that a one-point change in clinical scores represents a substantial change in tremor amplitude.14
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This chapter first provides a brief overview of clinical rating scales and then describes some of the currently available techniques for quantitative and diagnostic measurement of movement disorders. These include uses of reaction and movement ...