Side of onset of motor symptoms and cognitive functioning in Parkinson's disease

Objective: While lateralisation of cognitive functions at a cortical level is well described, less is known about sub-cortical lateralisation. Parkinson’s disease (PD) is a degenerative, sub-cortical neurological disorder characterised by a large number of motor and non-motor symptoms. The non-motor symptoms range from mild cognitive impairment to PD dementia, while the cardinal motor symptoms include muscle rigidity, tremor, bradykinesia, loss of postural reflexes and akinesia. These motor symptoms initially present unilaterally or bilaterally. The aim of the current study was to investigate sub-cortical lateralisation of cognitive functioning by examining possible links between the lateralisation of onset of motor symptoms and subsequent cognitive decline.

Participants and Methods: 50 participants with idiopathic PD were recruited from the Royal Hobart Hospital outpatient clinic and assessed using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS), the Trail Making Test, and the National Adult Reading Test (NART). Side of motor symptom onset information was also recorded.

Results: Participants were split into three groups; right, left, and bilateral motor symptom onset and initial analyses indicated no significant age or pre-morbid IQ (NART) differences. One-way ANOVAs with Tukey’s post-hoc measures were used to detect significant differences between the three groups. Significantly lower scores were found for the right-side motor onset group on the digit span task (p = .013), and for the left-side motor onset group on the figure copy task (p = .008) and for the overall RBANS Visuospatial/Constructional index score (p = .004). In addition to this, the results, while not significant, trend towards suggesting increased impairment effects across most tasks for patients with bilateral symptom onset.

Conclusion: PD patients with right side motor symptom onset are likely to experience greater difficulties with left hemisphere specific tasks, while patients with left side motor symptom onset are likely to experience greater difficulties with right hemisphere specific tasks. These results indicate that there is some sub-cortical lateralisation of cognitive functions to mirror that which has already been demonstrated at the cortical level. Further research associated with this study will examine specific measures of motor functioning using a finger tapper and hand dynamometer. Additional research should investigate a wider range of cognitive functions.