As we rely more and more on technological devices in our daily lives, we started implementing a variety of new developments in clinical and research settings as well. To identify the current technological options for tremor assessment, and the possibility of applying them to the clinic for diagnostic and tracking purposes, we asked three experts actively working on application of objective measures for tremor to share their views.
What are the current limitations for diagnosis and tracking tremor? Are subjective measures used in a clinical setting reliable, and is there a need for objective measures?
Elble: Tremor diagnosis is still based largely on the history and neurological exam. Motion transducers are useful in the diagnosis of functional tremor and primary orthostatic tremor and for distinguishing physiologic and enhanced physiologic tremors from central neurogenic tremors such as Parkinson tremor and essential tremor (ET). Otherwise, motion transducers have limited utility in clinical diagnosis. For the assessment of tremor severity, motion transducers are very sensitive and precise, and they provide linear measures of tremor amplitude and frequency. All rating scales, by contrast, provide crude estimates of tremor amplitude that depend on human perception. However, transducers must be mounted securely to a body part to minimize mechanical motion artifact, and they must be mounted in a way that does not impede body motion. Inertial measurement units (IMUs) record all motion, not just tremor, and accelerometer signals contain gravitational artifact that cannot be removed simply by DC filtering. Multiple IMUs are needed to measure rotation (tremor) at individual joints. Comprehensive assessment of tremor in the head, face, upper limbs, lower limbs and torso is theoretically possible but impractical.
Haubenberger: Use of devices for differential diagnosis is limited to specialized clinics and referral centers, where this technology is established. As for tremor tracking, the main limitation is the lack of available and validated systems that are able to continuously monitor tremor severity in patients’ daily lives. While some platforms to track resting tremor and other movement disorders in Parkinson Disease are used (e.g., Parkinson Kinetograph), no system has yet been shown to reliably and sensitively track action tremor in ET or other action tremor syndromes. The impact of a given tremor amplitude of a body part onto daily life tasks such as writing, drinking from a glass, speaking, etc. is often dependent on the patients subjective rating. While sensors are technically capable to quantify the tremor itself, they often cannot tell us what impact this tremor on a given task actually had.
Nahab: With transducers becoming ubiquitous in our lives, efforts are underway to collect reliable data and use it to drive meaningful change for patients. Unfortunately, how such information should be used clinically has not been established and physicians often have no idea what to do with clinical data patients may share with them. Despite these near-term challenges, I am convinced that tremor assessments will expand beyond the infrequent clinical visits and empower patients to better understand their tremor symptoms, response to treatments, and then better communicate these findings with their physician.
What are the recent technological advances for tremor assessment?
Elble: The expense and size of transducers have dropped precipitously in the last decade. Transducers capable of measuring and recording tremor are contained in most smartphones, smartwatches and activity monitors, and tremor can be recorded continuously for a day or more. Writing and drawing tremor can be measured with commercially-available graphics tables (digitizing tablets) that are used routinely in computer graphics. Commercial software and freeware for data analysis are available.
Nahab: Most objective tremor measures have historically relied on the patient holding or interacting with the sensor(s), though newer sensors (e.g. high frame-rate cameras, proximity sensors, or radar) found in many smartphones now allow the collection of similar information without the need to have the sensor on the body.
Haubenberger: The most exciting development for me is the application of this technology in large-scale, prospective studies in patient populations to validate this technology further as well as gain more insights about the nature of patients’ movement disorders during regular activities of daily living.
Are these objective measures clinically applicable? What are the current challenges?
Haubenberger: In the clinic-setting of an electrophysiological laboratory with the capability to objectively quantify movement, these objective measures are clinically relevant and applicable. The current challenge still lies in the translation of objective motion quantification outside the clinic.
Nahab: If I were to collect a week or year worth of tremor data on one person, I will likely find that the tremor severity that has been measured varies extensively by the second, minute, hour, day, week, month, etc. This biological variability can be explained by many things such as level of excitement, medication use, fatigue or some potentially unknown variability. Getting to the point of understanding the drivers of the variability will lead us to the development of novel therapies, while also allowing us to more accurately detect the effect of a new medication if we’re able to remove the impacts of other variables. For example, tremor severity in many individuals varies based on the time of day. If we don’t control for this, we may either over- or under-estimate the effect of a new medication.
Elble: It is easy to attach an IMU on most locations of the body, but the recorded motion may not accurately reflect the tremor in that body part. For example, an IMU (e.g., activity monitor) on the wrist cannot measure the complex pill-rolling tremor of the hand but will be sensitive enough to detect when tremor is present. ET and other forms of action tremor are best measured by recording tremor during specific tasks in a standardized fashion. If this is done without clinical supervision (e.g., at home), it may be impossible to ensure that the transducer was properly mounted and the tasks properly performed.
Can they ever replace subjective measures in the clinic? Would monitoring tremor in home setting complement the assessment in the clinic?
Nahab: The answer for me is not “if” but “when” this will happen. The field of healthcare is slowly moving toward enhancing patient autonomy and empowerment. This shift may scare some physicians into believing they will be replaced by the “machines”. I can see nothing further from the truth than such a scenario. As objective measures become integrated into the management of health, it will simply mean that patients and physicians spend less time trying to understand and communicate ‘how things are going’ and more time developing management plans and reliably evaluating whether these plans are effective and benefit the patient.
Haubenberger: Some measures will be able to replace subjective measures, some won’t. While sensors may be more sensitive to detect and quantify tremor and its changes over time, the downside would that that sensors may be “too” sensitive, as small changes are clinically often not relevant or an inherent element of a naturally variable symptom as tremor. It has been shown that sensors are equally capable of capturing clinically meaningful change, compared to visual ratings, the advantage of the sensors to quantify tremor is the ability to perform this measurement without a need for a visual rater to be present, as well as the capability to record and store data over long periods (24 hours or more). This opens opportunities for telemedicine application of remote symptom monitoring. Subjective measures will remain relevant in the context of patient reported outcomes on impact of movement onto their daily life tasks, which will ultimately drive clinical decision making.
Elble: I doubt that so-called objective measures (i.e., transducers) will ever replace subjective measures (i.e., rating scales) in the clinic, but it is clear that transducers and scales can be used to corroborate each other, if corroboration is needed (e.g., in a clinical trial). The use of motion transducers is time-consuming, and their validity is limited by the complexities of limb motion. Motion transducers must be used in a standardized fashion to avoid increasing test-retest variability. Data recording and analysis are not completely free of subjectivity. Digital video cameras can be viewed as a form of motion transducer. Digitized images can be analyzed with the same mathematical methods of signal analysis used with data from accelerometers, gyroscopes, digitizing tablets, goniometers, etc. It is conceivable that the eye of the neurologist will someday be replaced by a high-speed computer and complex imaging analysis algorithms.
Based on the views of our experts, technology has potential to aide our tremor assessment inside and outside of the clinic in an objective manner with greater sensitivity and precision than current clinical observation. There are specific diagnostic issues that can be enhanced by having such measures. However, given the natural variability and specific contexts that produce tremor, these devices have so far played a limited role as a diagnostic tool. In addition, the impact of tremor on quality of life depends on many factors such as the severity of the motion, particular body parts involved, and the context of the task being performed. Motion transducers can provide complementary data in the clinic assessment, but the translation to the home setting poses many more challenges. Still, the prospect of home monitoring is very exciting and has significant implications for telemedicine. Technological advances with low cost and wide availability of motion sensor including consumer devices such as watches and phones are likely to impact our diagnosis and treatment of tremor, as well as the quality of life in people with tremor in the near future.
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