Why Parkinson's Disease patients have Neuropathic Pain?
Parkinson's disease (PD) is a neurodegenerative disorder, associated with the degeneration of dopaminergic neurons in the substantia nigra pars compacta. It affects approximately 1% of the population over 65 years of age, occurs more frequently in men than women, and represents an important and consistently increasing economic burden for families and countries with increasingly aging populations. Although the diagnosis of PD is based on motor disorders, recently, it has become evident that in this condition a wide range of non-motor symptoms are also present. These include cognitive disorders, sleep disorders, autonomic, neuropsychiatric disorders, and sensory manifestations such as internal tremor, restless legs syndrome, numbness, paraesthesia, visual disturbances, and pain.
Pain is a well-recognized and important non-motor manifestation in Parkinson disease (PD). Painful or unpleasant sensations in PD can be classified as musculoskeletal, dystonia, akathisia, radicular, and central or primary pain; the last two are associated with neuropathic pain. Particularly, neuropathic pain in PD has not been fully clarified; therefore, it goes somewhat unnoticed, and the affected patients do not receive adequate pain treatment.
Although there has been considerable advance in the understanding of pain in PD, the mechanisms underlying neuropathic pain from PD remain unclear. Jose Luis Cortes-Altamirano et al did a review to identify the incidence of neuropathic pain in PD and the participation of dopamine in this type of pain through the integration of information from various lines of research.
They reviewed relevant literature in relation to PD and neuropathic pain in the last 30 years through a systematic search of original research articles, reviews, systematic reviews, and meta-analysis published between January 1990 and August 2020.
The studies showed that central neuropathic pain is present in 4% to 11% of PD patients, and the pain phenotype could be intermittent or persistent and is characterized by being diffuse, with cramps and numbness and not well defined to a part of the body, is generally lateralized to the side of the body most affected by the disease. It has been suggested that central neuropathic pain is caused by a stimulus to the thalamus–cortical circuitry and sensory basal ganglia or diencephalon-spinal.
Neuropathic pain, however, could not be classified as a prominent motor symptom, and may not even be a link between pain and motor symptoms.
Their research showed that in the early stages, pain can be the result of shoulder stiffness, or dystonia. In advanced stages, it can be caused by motor fluctuations (dyskinesia or dystonia out of period), and it can be radicular or due to musculoskeletal problems. In some patients, pain cannot be attributable to an apparent cause although it is proposed to be related to central neuropathic pain.
Although the pathophysiology of central neuropathic pain in patients with PD is not defined, the evidence of the involvement of basal ganglia suggests that alterations at this level could also lead to alterations in the processing of nociceptive inputs.
Nociceptive signals reach the basal ganglia through direct or indirect inputs from the sensorimotor cortex, the medial and posterior thalamus, the parabrachial area, the raphe nuclei, the superior colliculus, the hypothalamus, and the amygdala. Individual neurons in the substantia nigra, neostriatum, and globus pallidus can selectively respond to nociceptive stimuli, many of which encode the intensity of the painful stimulus, nociceptive stimuli increase the activity of mesocortical and mesolimbic neurons, and the local release of dopamine.
Review of studies showed that Dopamine is a key neurotransmitter in the modulation of pain perception. The degradation of dopamine producing cells in the substantia nigra can affect analgesia by interrupting the descending and ascending pathways of nociceptive signals from the spinal cord. However, experiments with rodents showed that dopamine is involved in descending antinociception mechanism mediated in the rostral agranular insular cortex. Deregulation of dopamine in subcortical areas could be the basis for clinical characteristics of central neuropathic pain during early-stage PD. Dopamine could cause hypersensitivity to pain, either indirectly through modulatory effects on affective pain processing and/or directly by affecting the neural activity in the key areas of the brain that modulate pain, such as the thalamus, basal ganglia, insula, anterior cingulate cortex, and periaqueductal gray matter.
Studies indicate that central neuropathic pain in PD is exceedingly difficult to diagnose due to its etiology; however, it is proposed that it may be mediated in part by the spinoreticulothalamic pathway, derived from neurophysiological changes in the basal ganglia due to degradation of dopaminergic neurons in the substantia nigra pars compacta, which in turn directly affects the cognitive–emotional function of patients with PD. Early neuropathology of PD has been reported to begin in key brain structures in the limbic circuitry, including the amygdala and the intralaminar nuclei of the thalamus. This could result in a generalized hypersensitivity to noxious stimulus, which can influence the onset and/or exacerbate painful symptoms in PD.
However, many of these mechanisms do not depend on the cause of the disease, the same mechanism can be found in different diseases. In an individual patient, different mechanisms may be involved and different mechanisms may lead to the same symptom.
The authors concluded that more preclinical and clinical research is needed to evaluate the structures and more neurotransmitters that have been proposed in this review, which could allow for better diagnosis and treatment for neuropathic pain in patients with PD, which could ultimately lead to an improvement in the QoL of patients.
Source - Cortes-Altamirano et al, Neuropathic Pain in Parkinson's Disease, Neurology India, September-October 2022, DOI: 10.4103/0028-3886.359257
