Cognitive impairment, dementia and behavioral disorders can also be symptoms of Parkinson’s disease.
Historically, Parkinson’s disease has been eminently associated with movement problems in older people. But we know that the reality is quite different and that beyond tremor, rigidity or slowness, Parkinson’s also associates a significant risk of developing cognitive impairment and dementia, as well as other non-motor cognitive and behavioral symptoms.
These symptoms include difficulties in the ability to maintain attention, to plan tasks or remember events, memory problems, word finding difficulties, depression, anxiety, loss of motivation, development of visual hallucinations, obsessive behaviors and addictive behaviors. No less important is the fact that many of these symptoms often go unnoticed for years, causing havoc and incomprehension in all those who experience them closely.
What causes cognitive impairment and behavioral changes in Parkinson’s disease?
Parkinson’s disease is caused by the loss of a type of neuron responsible for the production of a neurotransmitter, dopamine. We do not know why these neurons die, but we know that certain environmental and genetic factors and age play a role in this process.
At the same time, in the brains of patients affected by this disease, aggregates of an “abnormal” protein called alpha-synuclein are observed, which forms foreign bodies known as Lewy bodies and which play a role in neurodegeneration and, therefore, in the clinical manifestations of the disease.
Dopamine plays a fundamental role in the optimization of movement production, since it modulates the functioning of a complex brain network where certain regions of the cerebral cortex called “motor” interact with a set of structures called basal ganglia. Therefore, the loss of dopamine leads to the dysfunction of this circuit and explains the development of the motor symptoms of Parkinson’s disease: resting tremor, slowing, rigidity and postural instability.
But dopamine also modulates the functioning of other circuits that allow us to plan, maintain attention, generate abstract reasoning or access information in our memory store, as well as others related to the ability to control our impulses, motivation, emotions or decision-making processes. Therefore, it is to be expected that, in a disease characterized by a massive loss of dopamine, we will find not only symptoms derived from the involvement of the motor circuit, but also cognitive or behavioral symptoms derived from the involvement of these other circuits.
Already at the time of diagnosis, all patients with Parkinson’s disease present some type of cognitive difficulty, especially in processes related to attentional capacity, planning and memory. In fact, 30% of newly diagnosed patients meet diagnostic criteria for what we call “mild cognitive impairment (MCI) associated with Parkinson’s disease”.
The concept of MCI was born in the field of Alzheimer’s disease to define the transitional state between cognitive normality and dementia that all patients affected by this disease go through. But in Parkinson’s disease, MCI does not always anticipate dementia, and, unlike what we see in Alzheimer’s disease, the characteristics and the way in which cognitive impairment progresses in Parkinson’s disease are highly variable among patients.
Even so, after three years of disease progression, MCI already affects 57% of patients with Parkinson’s disease and after four years, we find that up to 36% of cases have developed dementia. This means that having Parkinson’s disease poses a six times higher risk of developing dementia than that found in the general population.
In this sense, it is extremely important to be able to identify cognitive functioning profiles that allow us to anticipate which patients are at greater or lesser risk of developing a more aggressive pattern of cognitive impairment or dementia in the future. We know that the initial cognitive alterations that we find in most Parkinson’s patients as a consequence of dopamine loss are not good indicators of possible progression to dementia.
Beyond cognitive problems, although closely related to them, in Parkinson’s we find a wide range of neuropsychiatric symptoms that can also, in some cases, be predictors of a higher risk of developing dementia and therefore, detecting and treating them is crucial. Among these, motivation disorders or apathy and the development of visual hallucinations stand out.
Loss of motivation, initiative and emotional flattening define apathy, an entity distinct from depression, which is found in up to 70% of patients with Parkinson’s disease. In some cases this apathy is a consequence of the patient requiring more dopaminergic medication, but in other cases, the apathy persists and worsens despite efforts to treat it. In these cases, we know that apathy seems to be a consequence of further neuronal damage. Thus, detecting it may, even when cognitive performance is still eminently normal, be an early indicator of the existence of a more aggressive neurodegenerative pattern.
The development of hallucinations in Parkinson’s disease is also a frequent phenomenon, puzzling for many patients and also tends to progress. It is common for up to 42% of newly diagnosed patients to have “minor” hallucinations or visual illusions in the form of sensations of nearby presences, unclear visions of moving shadows and other “simple” hallucinatory phenomena. We also know that up to 33% of patients already had these experiences between 7 months and 8 years before the diagnosis of the disease.
Over time, it is common for these hallucinations to progress in frequency and complexity, acquiring very realistic and extravagant forms such as perceptions of animals, people or fantastic beings. In this sense, we know that there is a parallelism between the progression in frequency and complexity of hallucinations, the aggressiveness of neurodegeneration and the risk of developing dementia. Therefore, detecting the first manifestations of visual hallucinations and their progression is again indispensable in the prognosis of possible cognitive changes.
Is dementia then an inevitable outcome in patients with Parkinson’s disease?
Parkinson’s is not just a tremor disease, nor does it necessarily lead to dementia. Not all patients will have these problems and even many of those who do have some of these symptoms will never develop dementia. But detecting these symptoms early allows us to anticipate the future and to continue working on the development of treatments, such as those we have today to address many of the non-motor symptoms of the disease.