The study, “Cognitive decline in Parkinson’s disease: the impact of the motor phenotype on cognition,” was published in the Journal of Neurology, Neurosurgery, and Psychiatry.
Parkinson’s disease is a multisystem neurodegenerative disorder with motor and non-motor features. Rest tremor, slowness of movement (bradykinesia), rigidity, and impairment of posture, balance, and gait are among the motor symptoms and signs, while cognitive decline is a recognized non-motor complication with significant clinical impact.
“There is evidence that approximately 30%–40% of patients with PD [Parkinson’s disease] develop cognitive deficits and may get dementia, starting with mild cognitive impairment and evolving to a more generalized dysfunction of cognition,” the researchers wrote.
Parkinson’s disease has three subtypes of motor symptoms:
- Tremor-dominant, meaning patients experience more tremor than any other motor feature.
- Akinetic-rigid, where patients exhibit slowness of movement accompanied by muscle stiffness and postural instability plus gait difficulty.
- Not-determined, in which patients show mixed symptoms.
“Akinetic-rigid patients have previously been associated with faster cognitive decline, greater risk of dementia and higher sensitivity for depression,” the researchers noted. Scientists believe this is dependent on the affected brain circuit.
Evidence suggests a significant link between compromised working memory and akinetic-rigid patients. Working memory is the type of memory that holds temporary information needed only to accomplish an immediate task. For example, when putting a new contact into a phone, a person temporarily memorizes the digits in the phone number until he or she is done with that task.
Despite the need for a detailed understanding of Parkinson’s motor subtypes, available studies demonstrate inconsistent results about the relation of cognitive function and implemented motor subclassifications.
To address these discrepancies, researchers from RWTH Aachen University in Germany set out to identify distinct cognitive profiles among Parkinson’s patients according to motor subtype. They also examined mental performance within each cognitive diagnosis group, i.e., patients with a normal cognitive profile, mild cognitive impairment, and dementia.
They analyzed data from the DEMPARK/LANDSCAPE study, a multicenter, longitudinal, observational, German cohort study of 538 Parkinson’s patients (ages 50-80 years old), and organized it by motor subtype and cognitive pattern.
In the DEMPARK/LANDSCAPE study, patients were assessed by a series of motor, cognitive, depression, and health status-related questionnaires/scales.
For further cognitive assessment, researchers tested verbal memory, non-verbal memory, attention, executive function (meaning “behavior control,” which includes inhibition, memory, attention, flexibility, planning, and problem-solving), visuospatial function (ability to process and interpret visual information about an object’s position in space), and language skills.
Data analysis showed that compared with tremor-dominant patients, akinetic-rigid patients performed significantly worse in executive functions, including working memory, card sorting, visuospatial skills, and the ability to mentally select and pronounce words meeting certain test-related constraints.
Neuropsychological test scores revealed that executive function and attention were negatively correlated with non-tremor motor scores, which is consistent with previous studies finding that motor symptoms influence cognition.
Parkinson’s patients with the not-determined subtype were three times more likely to develop mild cognitive impairment than tremor-dominant subjects, and twice as likely as akinetic-rigid patients.
In addition, akinetic-rigid patients were eight times more likely to develop dementia than tremor-dominant participants.
“The present results show that akinetic-rigid patients undergo greater cognitive decline in several cognitive domains compared with [tremor-dominant PD] patients, calling for early diagnosis in particular in the higher risk group and individualized therapy interventions at the earliest point,” the researchers concluded.