Cognitive characteristics in Chinese non-demented PD patients based on gender difference

Parkinson’s disease (PD) has been considered a debilitating motor disorder, and the non-motor symptoms are gaining more and more attention. Cognitive impairment is a major non-motor symptoms, which greatly influence the quality of life [1]. It is estimated that 25% untreated ‘de novo’ patients have cognitive impairment of varying degrees. Some changes in cognition are subtle thus inconspicuous to the patients and their caregivers [2, 3]. Various studies have been conducted to measure specific cognitive functions in PD patients, such as executive abilities, working memory, visuospatial processing, language and attentional processes [4‐6]. However, no agreement has been reached as to a definite neuropsychological profile of non-demented PD patients. Besides, it is reported that more men than women are diagnosed with PD, suggesting a gender difference in PD [7, 8]. Although a few studies addressed the gender differences in PD as well as the influence of estrogen on dopaminergic neurons and related pathways in the brain, most of them adopted general cognitive screening tools such as Mini-mental State Examination (MMSE) or Montreal Cognitive Assessment (MocA), little is known as to the specific cognitive domains influenced by gender [9‐12]. Therefore, knowledge about differences in cognition between men and women with PD and about the pathophysiology underlying those differences may enhance the accuracy and effectiveness of clinical assessment and treatment of the disease.

The current study examined the five domains of cognitive function in non-demented PD patients who were not treated with anticholinergic medications and normal controls, with special emphasis on the comparison between male and female patients. Meanwhile, we performed sub-group analysis regarding medical treatment of PD patients, aiming to eliminate the possible confounding effects of medication and making the groups more comparable. As the effect of anti-parkinsonism medication on cognitive function was complicated and controversial [13‐15].

PD patients frequently encounter neuropsychological problems. The present study has confirmed the previously reported cognitive impairment in cognitive domains including attention/executive function, visuospatial function, verbal memory and language. These aspects of cognition were all affected by the disease to varying degrees.

Dysexecutive syndrome is the most prominent prototype of early cognitive impairment in PD [22]. Deficits in this domain could be sensitively detected by measures of SDMT, which showed abnormality in this study. Our result was also in accordance with previous studies documenting visuospatial impairments by evidence of poor performance of PD patients in CDT [23]. Poor performance on free recall tasks but near normal performance on recognition and cued recall tasks in our study concur with the hypothesis that verbal memory impairment in PD has been manifested as retrieval difficulty more than encoding problems [22]. Although the majority of studies showed that language remain relatively intact in PD, we found it was impaired compared to the control group in the verbal fluency test. Verbal fluency combines the ability to retrieve the correct information and suppress the incorrect response. According to O’Brien’s report, dysfunction of various domains does not occur in isolation, but presents in association with each other [24]. In fact, though impairment in substantia nigra is most pronounced in PD, areas affected by the disease are widespread, including ventral tegmental area, dorsal raphe nucleus, hypothalamus, thalamus, hippocampus, cerebral cortex, the temporal, frontal, anterior cingulate and insular cortices [25]. Thus, it is not surprising that the cognitive deficits are due to cortical pathology and subcortical circuitry dysfunction as a whole and deficits in language may be the result of deterioration of the other cognitive functions as a whole. In addition to that, PD patients with anti-parkinsonism medication had extra deficits in Rey-copy score and stroop3 test score, which may due to the longer disease duration (p = 0.000) or medical effects. These results displayed a possible vulnerability of PD patients to the effects of disease duration and medication on Rey-copy score and stroop3 test.

In regard to gender differences in PD, epidemiological survey showed that the ratio of men and women who had the disease is approximately 2:1, suggesting a biological diversity [18]. However, not many studies considered gender when examining cognition in PD patients. There were some studies investigated the gender differences with a remarkable number of participants, but the lack of control groups did not permit to determine if these differences could be specific to PD patients [26, 27]. The present study highlights the role of gender differences associated with cognitive functions. The conclusion was strengthened by the study design of age and education matched control groups. Our study showed a disparity between male and female patients in two domains of cognition. Male patients surpassed female patients on BNT, a measure less commonly used to assess frontal lobe dysfunction [28], while female patients were superior on verbal retrieval test, reflecting the impairment of hippocampus [29]. Since no significant differences were observed in these two measures between male and female controls, it is reasonable to infer that gender-based differences existed in PD patients. In a cross-sectional study of the effect of gender on BNT which recruited 1111 healthy elderly subjects, there was also a tendentiously while non-significantly higher score of males (p = 0.08) [30]. Meanwhile, it reveals that age and educational level had more powerful effect on BNT. On the other hand, other studies found no effect of gender on the BNT. Therefore, there was possibility that the gender difference in BNT was due to the natural difference between men and women, which was unrelated to PD.

One consideration centers on our results is the role estrogen plays in the pathogenesis of PD. Even though the menopausal condition at experiment might be heterogeneous of the female PD patients in our cohort, the lifetime cumulative level of estrogen also played an important part in the pathogenesis of Parkinson’s disease [31, 32]. According to other’s reports, it is still a mystery as to the mechanism of estrogen acting on the dopaminergic system [33, 34]. In addition, the changes in other neurotransmitter systems, such as cholinergic, noradrenergic, serotonergic, also contribute to the multiple neuropsychological impairments. The interactions of these neurotransmitter systems make the role of estrogen even more complex. Another frequently-used theory to explain the gender differences is “cognitive reserve”, which posits that premorbid condition may generate distinctions in clinical presentation [35‐37]. Although the subjects in our study have been adjusted for education, it may not be sufficient to rule out the impact of other socioeconomic factors such as occupation, income and social status. Thus better performance in verbal memory by female patients may indicate a larger cognitive reserve in this aspect. Likewise, advantages in naming by male patients may suggest a later onset of impairment and greater reserve of prefrontal function than female counterpart. The gender differences might also be associated with neural organization [38]. Some studies shown that greater bihemispheric representation were more prominent in women taking verbal memory task and men taking visuospatial task [39, 40]. Others found right hemispheric lateralization for males and bilateralization for females [41, 42]. Though no consensus has been reached, differences in neural lateralization render certain aspects of cognition more sensitive to neuropathological changes in a gender-specific manner, which lead to dissimilar manifestation in male and female patients.

When interpreting these data, several limitations should be acknowledged. First of all, the relatively small sample size may limit the generalization of these data. As several studies pointed out, normal elderly women performed better in tests involving verbal components [37, 43].Thus, whether female PD patients’ better performance in RVLT-long delayed cued recall was due to the influence of the disease needs further study with larger sample size. Secondly, as Cronin-Golomb described, side of disease onset may also influence the cognition of the patients [44]. Should this be the case, more detailed division of participants by both gender and side of disease onset would provide stronger evidence. Finally, another limitation is the influence of medication. Though anticholinergic medications were ruled out and levodopa equivalent dose were well matched between groups, the underlying effect of dopaminergic medication may still change the natural pathological development of neurodegeneration.

In conclusion, our study indicates that cognitive impairment was common in PD patients even in the absence of dementia. PD patients who underwent anti-parkinsonian treatment had worse cognitive impairment than untreated ones. In light of the above mentioned observations, we hypothesized that genders may have a different presentation of cognitive impairment in PD patients. Sex influences on brain anatomy, chemistry and functions are poorly understood. Increased knowledge on possible gender effects in PD would provide an enhanced insight in underlying pathological mechanisms, and has potential implications for the diagnosis and treatment of PD.