We found that individuals with PD who experienced nontremor symptoms such as rigidity and gait and balance impairments at diagnosis reported a significantly worse quality of life than individuals who experienced tremor as their initial symptom. This finding was particularly significant in the domain of issues relating to mobility. Although performance on the mobility subscale alone was significantly different in NTD and TD, NTD patients endorsed a worse quality of life than TD patients on all domains except in the domain of social support, and their overall PDQ score indicated significantly worse self-perceived quality of life.
These results are in line with those from Hariz and Forsgren, who found significant differences in PDQ-39 scores in individuals with the TD and NTD subtypes of PD [10
]. Specifically, in their study the subgroup with NTD had significantly worse scores than the TD subgroup for the mobility, ADL, Communication, and Bodily Discomfort subscales as well as for total scores [10
]. We found significant differences between NTD and TD for the total PDQ-39 score as well as for the mobility subscale score. Hariz and Forsgren reported significant differences between the NTD and TD subgroups for total UPDRS scores and Hoehn and Yahr staging, with the NTD group exhibiting more severe deficits than the TD subgroup [10
]. The present study extended these findings by showing that group differences in PDQ-39 scores were independent of motor symptom severity or stage of disease, as no significant differences were found between NTD and TD individuals for scores on the UPDRS examination or Hoehn and Yahr staging in our sample.
Our results also demonstrated that subscales derived from a commonly used quality of life measure, the PDQ-39, correlated significantly with subscales of the UPDRS. In the entire PD group, scores on the Rigidity subscale of the UPDRS were significantly correlated with those on both the ADL and Communication subscales of the PDQ-39, whereas scores on the Tremor subscale did not correlate significantly with those on any PDQ-39 subscale. A separate analysis for TD and NTD patients revealed that the correlations between performance on the PDQ-39 and UPDRS were primarily driven by nontremor symptoms even in TD patients. This finding indicates that nontremor symptoms in general more negatively impact quality of life, regardless of initial motor symptom. The subscales of the UPDRS that pointed to significant relations with the quality of life subscales of the PDQ-39 for TD individuals were for axial symptoms and symptoms of rigidity. That is, for both TD and NTD individuals, nontremor type symptoms appear to negatively impact multiple domains of their quality of life. These results provide evidence that tremor as a singular symptom may not significantly compromise quality of life, whereas symptoms of rigidity and bradykinesia may act as more substantial contributors to deficits in self-reported quality of life.
Our findings provide insight into how the common motor symptoms of PD negatively affect specific aspects of quality of life. For example, performance on the Communication subscale of the PDQ-39 was significantly related to performance on the Facial Expression subscale of the UPDRS, suggesting that difficulties with displaying facial expressions contribute to problems with communication for patients with PD. A recent study examining the effectiveness of a randomized controlled rehabilitation trial for PD used the PDQ-39 to evaluate improvements in quality of life [19
]. Patients demonstrated the greatest improvements in the communication domain of the PDQ-39 [19
]. This finding demonstrated that communication may be significantly related to quality of life and can be particularly receptive to rehabilitation techniques. Accordingly, health care providers should focus on efforts to improve speech and communication skills in individuals with PD. Understanding the relation between specific symptoms of PD and quality of life may inform intervention strategies in order to most effectively improve life quality for individuals with PD.
Additionally, for the entire PD group, scores on the ADL subscale of the PDQ-39 also correlated significantly with Dopamine- (DA-) dependent subscales of the UPDRS, which includes the tremor, rigidity, bradykinesia, and facial expression subscale scores. The non-DA-dependent subscale includes the speech and axial symptom subscale scores of the UPDRS. This finding provides evidence that the physical symptoms of PD may have a larger effect on self-reported quality of life than axial symptoms. This correlation was driven by the NTD patients, indicating that quality of life is more affected by symptoms in NTD individuals rather than individuals with a tremor-dominant symptoms at diagnosis.
Poorer scores on the PDQ-39 may reflect that individuals with the NTD symptom profile (i.e., more rigidity, gait, and balance problems) experience more extensive disease pathology than TD individuals [1
]. While the presence of a tremor may indicate the dopamine depletion in the substantia nigra that negatively affects cortico-striato-thalamocortical circuits, the presence of nontremor symptoms may indicate pathology in additional cortical and subcortical structures of the brain, such as frontal regions and the transentorhinal cortex [1
]. Individuals with NTD may have a wider distribution and greater density of Lewy bodies in the brain than TD individuals, which may contribute to more frequent or more severe nontremor symptoms that ultimately have a negative effect on quality of life. These symptoms may not respond as well to the dopaminergic treatments that have been shown to significantly improve the quality of life in individuals with PD [1
In summary, the present study provides evidence that individuals with nontremor initial symptoms of PD endorse a more negative quality of life than individuals who experience tremor as the initial symptom, despite lack of differences in disease duration or severity. A novel finding of this study is the demonstration of significant correlations between performance on specific subscales of quality of life and specific motor symptom domains. These correlations were driven by nontremor symptoms, even in individuals with tremor as the initial symptom. Determining reliable subtypes and specific motor symptom profiles of PD may aid in prognosis and individualized treatment plans, as well as assist researchers in advancing the understanding of the etiology of PD.