Parkinson’s disease: the flip side of the coin

The human brain might be perceived as an organ with two main strategic tasks: goal-directed motor behavior, and mental functioning in order to work out that goal. These two main functions have two prototypical diseases: Alzheimer disease, in case of mental function, and Parkinson’s disease, with motor function.

Following its inception as an entity, Parkinson’s disease (PD) was long perceived to be a purely motor disorder with unimpaired mental functions. This was partly influenced by its initial description by James Parkinson, who wrote that the intellect and senses remain intact. Although some of his contemporaries challenged this view and suggested that mental functions do get impaired in patients with advanced PD, this observation did not receive much attention for many decades. It’s ironic that modern medical treatment, which has dramatically changed the lives of PD patients, came as a mixed blessing; trading effective management of motor symptoms and longer patient survival with mental dysfunction (and some other non-motor features of the disease) becoming more apparent.

Research in the last few decades has established that cognitive impairment and dementia are an integral part of the disease process. Several prospective as well as cross-sectional studies revealed that milder forms of cognitive impairment can even be detected at the early stages of the disease, if appropriately looked for. In fact, it has become apparent that some non-motor symptoms may precede classical motor symptoms by many years, such as anosmia, depression, constipation and dream-enacting behavior. When properly assessed, a variety of non-motor symptoms can be identified throughout the disease. Dementia develops particularly in patients with advanced age and severe disease; twenty years after the diagnosis, mental dysfunction of varying degrees is present in practically all patients.

In earlier years the assumption was that dementia in PD may simply represent co-incident Alzheimer disease (AD). Research on the profile of dementia associated with PD (PD-D), however, demonstrated that, in a typical patient, its features are different than that of AD, and it constitutes a dementia syndrome of its own. Biochemical deficits and pathological features associated with PD-D were also worked out, which lead to the first rationally designed treatment trials. These studies bore fruit and the first specific drug for PD-D became available. In parallel, clinical diagnostic criteria were described for PD-D as well as criteria for mild cognitive impairment associated with PD (PD-MCI). Efforts are now ongoing to better understand the neurobiological basis of cognitive impairment and to find biomarkers which can identify patients at risk in earlier stages. Once the chrono-biology of mechanisms are better understood, scientists may be able to design treatment interventions to halt the progression of mental dysfunction, as well as the progression of the disease itself.

We have entered a new phase in Parkinson’s disease research: its molecular pathology is being disentangled; the first vaccination trial is on the way along with efforts to in vivo image the accumulation of alpha synuclein molecules, the hallmark pathological feature of PD. We are looking forward to exciting times, which we hope to come sooner rather than later.

Featured image: PET image by Jens Maus. Public domain via Wikimedia Commons.