The dimensionality of fatigue in Parkinson’s disease

Between 30 and 70% of individuals with Parkinson’s disease (PD) experience undue fatige everyday [1‐4]. It is a symptom which may appear several years before overt symptoms prompt the disease diagnosis. Although it significantly impacts quality of life, understanding its origin or mechanisms has remained elusive. Early evidence also suggests that fatigue in PD is not due to dopamine deficiency. Instead, the serotonin system and possibly autonomic disorder may play a role. Pharmaceutical interventions such as Rasagiline [5, 6], doxepin and modafanil [7] provide some relief but do not prevent the symptom from progressing.

No evidence-based guidelines are available to treat fatigue in PD. So little is known about the symptom that it is not even clear how to classify it, whether it should be a central, physical, cognitive, peripheral or mixed/complex phenomenon. Researchers and clinicians have attempted to document the perception of fatigue among PD patients through surveys. Many scales have been used, of which nine of the most commonly ones were reviewed by the Movement Disorders Society (MDS) task force [3]. Since not all the scales were developed for PD, recommendations were based on available reports to estimate their validity for use in this population. The scales were judged on their ease of use, namely the number of items and time needed to administer the questions. All the scales were rated as Listed (i.e., acceptable), Suggested or Recommended. None received a Not-Recommended rating. Neither was any scale given a Highly-Recommended ranking. Thus, despite best attempts to come up with a method of quantifying the perception of fatigue in PD, it remains unclear what the best way is to implement it.

Here in this pilot study, we sought to examine the dimensionalities of the fatigue scales reviewed by the MDS task force. We wanted to determine whether consolidation of these commonly used scales may be achieved, or abandon them and instead develop an entirely new scale. Importantly, we wanted to also determine how the assessment of fatigue relates to willingness to engage in activities of daily living. It is intuitive to reason that higher levels of fatigue perception negatively impacts quality of life. We looked at three scales rated as Recommended for screening fatigue in PD by the task force: the Fatigue Severity Scale (FSS), the Functional Assessment of Chronic Illness Therapy-Fatigue Scale (FACIT-F) and the Parkinson Fatigue Scale (PFS). In addition, we looked at the Fatigue Visual Analog Scale (VAFS). This one-item interval scale was rated as acceptable. It did not receive a higher rating due to lack of research regarding its sensitivity to change. Nevertheless, we included it because it is the simplest crude test for assessing fatigue perception. It has been used to validate a number of other PD fatigue scales. Our hope is to enable better documentation of fatigue in PD through the use of as few items as possible that are highly correlated with quality of life measures. Our goal is also consistent with the quest to develop measurements that are easily accessible, easy to use and universally interpretable across the health science disciplines [8].

The new finding in this study relates to the identification of the VAFS as a potential reliable estimate of assessing the overall sensation of excessive fatigue experienced by individuals with PD. Our finding of the VAFS being a highly consistent and reliable measure of fatigue in PD is consistent with other studies which used a similar visual analog scale to evaluate other conditions of fatigue [10] and subjective report such as pain [11]. It is intuitive to realize that when a visual analog scale is used to document severity or a change in condition, the individuals being assessed are often basing their response on how they feel overall. Thus, although lengthy questionnaires that are specific and detailed may serve their purposes, it appears that a simple question asking subjects how they feel overall (such as fatigue or pain) succinctly captures the status of their well-being.

Our finding of high internal consistency among the 16 items from the Parkinson Fatigue Scale (PFS), Fatigue Severity Scale (FSS) and Visual Analogue Fatigue Scale (VAFS) is similar to a previous report on the Modified Fatigue Impact Scale (MFIS) [12]. The MFIS is a newer Parkinson fatigue scale and was therefore not included in the MDS task force’s reviews. The MFIS has 21 items which asks about cognitive, physical and non-physical components. Their analyses showed a two-factor structure, similar to the current analyses on the PFS, FSS and VAFS fatigue scales. Unfortunately, their Eigen values were not reported. Nevertheless, similar to the current study, the authors also found high internal consistency in all their 21 items of at least 0.95. Thus, despite the lack of consensus about the definition of fatigue in PD and the root cause of it, available evidence suggests that the constructs of fatigue in PD may be appreciated as unidimensional, at least in terms of how it impacts quality of life.

Despite our smaller sample, the results of the study appear to provide useful pilot data, out of which further understanding of fatigue in Parkinson’s disease (PD) may be advanced. The fatigue scales which we used in the study and recommended by the MDS task force were associated with many aspects of quality of life. Indeed, the PDQ-39 summary index was found to show the highest correlation with the fatigue scales, while possible confounders of fatigue such as daytime sleepiness and depression had lower correlations [13, 14]. Our findings are also consistent with published data on the prevalence of fatigue as well as the ranking of fatigue as the top three most bothersome PD symptoms.

Another potential limitation in the study is the use of caregivers as control subjects. Certainly, caregiver strain may produce undue fatigue and confound the study. In addition to the convenience in recruitment, there are several advantages of using caregivers as control subjects: (1) Examination of the data showed that only 3 subjects rated their fatigue at < 5 on the VAFS. The rest of the control subjects did not exceed the threshold for significant caregiver stress. We did not find it surprising that the control subjects reported low-level caregiver strain as our PD subjects were mostly in stage 2 of the H&Y and were therefore still ambulatory and living independently. Perhaps the use of the term caregiver may be a bit overstated in this case. (2) The second advantage was that by asking the spouses to estimate the severity of fatigue experienced by their PD spouse, we were able to determine that the spouses could accurately detect and perceive the severity of their PD spouse’ fatigue. (3) The third advantage of using caregivers as control subjects was that we were able to estimate how much more fatigue the PD subjects experienced compared to a family member who lived under similar housing/neighborhood conditions and lifestyles. Overall, the results showed that PD subjects experienced a lot more fatigue than their caregivers. The fatigue experienced by many of the PD subjects was excessive and unusual.

The potential use of a single measure such as the VAFS in PD that is reliably correlated with quality of life measures is consistent with the pursuit to develop clinical tests and measurements that are accessible, easy to use and universally interpretable across the health science disciplines [8]. Although it may appear that the VAFS does little in terms of helping us understand the severity of fatigue in PD, we hope that the results from this pilot study will stimulate further investigations that lead to a better understanding of the mystifying sensation of fatigue in Parkinson’s disease and its impact on quality of life.