Neuropsychiatric symptoms (NPS) are common and range from mild disturbances to severe impairment, comprising abnormalities in cognition, mood, behavior, or thought which can interfere with daily activities, reduce quality of life, and increase the risk for admission to a nursing home. Some of them, such as depression and anxiety, are known to precede characteristic motor signs by up to several years and may herald the development of PD, while most of them worsen as the disease progresses.[37] Research indicates, that patients with more severe motor symptoms are at higher risk for any NPS. Conversely, NPS can worsen PD.[38][39]
Depression is the most common NPS and occurs in nearly half of all patients. It features low mood and lack of pleasure and is more prevalent in females. The diagnosis can be challenging, since some symptoms of depression, such as psychomotor retardation, memory problems, or altered appetite, share similarities with psychiatric signs caused by PD.[38] It may result in suicidal ideation which is more prevalent in PD. Nonetheless, suicidal attempts themselves are lower than in general population.[40]
Apathy is characterized by emotional indifference and arises in about 46 percent of cases. Diagnosis is difficult, as it may become indistinct from symptoms of depression.[38]
Anxiety disorders (AD) develop in around 43 percent of cases.[38] The most common are panic disorder, generalized anxiety disorder, and social anxiety disorder.[37] Anxiety is known to cause deterioration in the symptoms of PD.[39]
Parkinson's disease psychosis (PDP) is present in around 20 percent of cases[41] and comprises hallucinations, illusions and delusions. It is associated with dopaminergic drugs used to treat the motor symptoms, higher morbidity, mortality, a decrease in health-promoting behaviors, and longer nursing home stays. Additionally, it correlates with depression and may herald onset of dementia in advanced stages. Unlike other psychotic forms, PDP typically presents with a clear sensorium.[42] It might overlap with other psychiatric symptoms, making the diagnosis challenging.[43]
Impulse-control disorders (ICD) can be seen in approximately 19 percent of all patients[38] and, in the context of PD, are grouped along with compulsive behavior and dopamine dysregulation syndrome (DDS) within the broader spectrum of impulsive and compulsive behaviors (ICB). They are characterized by impulsivity and difficulty to control impulsive urges and are positively correlated with the use of dopamine agonists.[44]
Cognitive
Cognitive disturbances can occur in early stages or before diagnosis, and increase in prevalence and severity with duration of the disease. Ranging from mild cognitive impairment to severe Parkinson's disease dementia, they feature executive dysfunction, slowed cognitive processing speed, and disrupted perception and estimation of time.[45]
Sleep
Sleep disorders are common in PD and affect about two thirds of all patients.[46] They comprise insomnia, excessive daytime sleepiness (EDS), restless legs syndrome (RLS), REM sleep behavior disorder (RBD), and sleep-disordered breathing (SDB) and can be worsened by medication. RBD may begin years prior to the initial motor symptoms. Individual presentation of symptoms vary, although most of people affected by PD show an altered circadian rhythm at some point of disease progression.[47][48]