Impaired cognitive performance in Parkinson's disease is related to caudate dopaminergic hypofunction and hippocampal atrophy

Abstract

Frontal lobe dysfunction and other cognitive deficits have been described in Parkinson's disease (PD), which may lead to dementia. Both striatal dopaminergic deficiency and regional or global brain volume loss have been suggested to contribute to cognitive decline in PD. We therefore performed a neuropsychological evaluation, structural brain MRI and Fdopa PET in patients with PD and healthy elderly volunteers. PD patients had impaired cognitive performance in many neuropsychological tests compared to controls, not limited just to frontal lobe function tests. Caudate Fdopa correlated positively with performance in verbal (immediate and delayed) and visual memory. Patients with PD showed atrophy in the hippocampus and the prefrontal cortex and hippocampal atrophy was related to impaired memory. Our findings suggest that striatal dopaminergic depletion and global brain volume loss contribute to cognitive impairment in non-demented PD patients, but dysfunction of extra-striatal dopaminergic or non-dopaminergic systems probably plays a role especially in more generalized cognitive impairment.

Introduction

It has been observed that patients in the early stages of PD already show impaired performance in tests measuring frontal lobe functions such as attention, planning and working memory [1]. In particular, set-shifting, suppressing attention and other attentional deficits are among the first cognitive domains to show deterioration in PD [2], [3], [4]. As the disease progresses there is an apparent increase in the severity and a broadening of cognitive impairments affecting other domains [1].

The cortico-striato-thalamo-cortical loops connect the basal ganglia with the cerebral cortex. The putamen is closely connected with the supplementary motor cortex and is thought to be involved mainly in motor functions. The caudate nucleus, in turn, is connected with the dorsolateral prefrontal cortex and the lateral orbitofrontal cortex, and it has been suggested that the dysfunction in this system contribute to the cognitive impairment in PD. Positron emission tomographic (PET) studies showed that the reduced 6-[¹⁸F]fluoro-l-dopa (Fdopa) uptake [5], [6], especially in the caudate nucleus [7], is associated with the impaired cognitive performance in patients with PD. This suggests the role of the caudate nucleus via connections to frontal areas in the cognitive impairment in PD.

The frontal assessment battery (FAB) was developed to evaluate frontal lobe functions at the bedside [8]. The six FAB subtests explore both cognitive and behavioural domains under the control of the frontal lobes, each of them having been shown to be significantly correlated with frontal lobe metabolic activity measured by 18-fluorodeoxyglucose using PET scan [9]. Performance on the six subtests of the FAB can give a composite global score, which evaluates the severity of the dysexecutive syndrome and may suggest a descriptive pattern of executive function in a given patient. FAB was reported to be able to distinguish patients with frontal lobe dysfunction from control patients with 90% sensitivity [8]. In PD the FAB score was reported to decrease because of frontal lobe dysfunction [10].

The underlying brain pathology of the cognitive deficits in PD is not yet fully understood. A magnetic imaging (MRI) study found that non-demented patients with PD had a significant rate of median/global brain volume loss with no significant loss seen in age-matched controls, and these changes correlated with global measures of cognitive decline [11]. MRI studies reporting hippocampal volumes have shown that PD is associated with hippocampal atrophy [12], and related to impaired memory even in non-demented patients [13], [14]. The patients in these studies were on antiparkinsonian medication and had had PD for at least 5 years.

To explore these issues we performed a neuropsychological evaluation, structural brain MRI and Fdopa PET in patients with PD and healthy elderly volunteers.

We hypothesized that in PD dopaminergic function in the striatum is related to cognitive performance, particularly to frontal lobe functioning. Furthermore, because PD is associated with brain atrophy and cognitive decline, we hypothesized that prefrontal atrophy is associated with frontal lobe functioning and hippocampal atrophy with memory.