Perspectives for the association between olfactory disturbances and depression in Parkinson’s disease

Non-motor disturbances in Parkinson’s disease: Globally, population aged 60 or over is growing and considering the World Bank predictions for the next 20 years it is expected that the number of Parkinson’s disease (PD) cases will double at the end of this period thus, reaching an impressive 13–39 million patients worldwide. This scenario is potentially associated with a significant global negative impact on public health systems particularly in countries with increased ageing populations such as the European countries, Asia and Americas. The pathophysiology of PD involves the progressive degeneration of dopaminergic neurons of the substantia nigra pars compacta (SNpc) that triggers denervation of the nigrostriatal pathway and consequent significant reduction of dopamine in the dorsal striatum. Such process leads to a critical motor impairment scenario characterized by bradykinesia, rigidity, resting tremor, and postural instability (Emamzadeh and Surguchov, 2018). However, several other non-motor disturbances develop earlier, thus, being considered as prodromal signs of the neurodegeneration. In this sense, the literature is increasingly emphasizing the importance of investigating mood and olfactory disruptions as high sensitive benchmarks of the early-phase disease. In fact, the need for innovative early-phase diagnostic tools, as well as the elucidation of the pathophysiological mechanisms of such disturbances, is essential priority in the field of investigation of PD. Remarkably, in the year of 2015, a woman was known for an intriguing ability of “smell PD”. Her husband had lived with the disease for twenty years and during this process she noticed that his odor slowly changed to a musky smell. Interestingly, this woman could associate this particular smell with PD after meeting different people with this distinct odor in a charity for PD patients. In fact, this clever observation is aligned with studies showing that hyposmia is found decades before the motor onset. Another type of anecdotal story, frequently reported by the patients, is the often cases of burnt food during cooking. They just fail to sense the burnt smell generating a very frustrating feeling of incapacity