Pains in Parkinson disease—many syndromes under one umbrella

The gut-brain connection in the pathogenicity of Parkinson disease: Putative role of autophagy

Parkinson disease (PD) is a progressive movement functionality disorder resulting in tremor and inability to execute voluntary functions combined with the preponderant non-motor disturbances encompassing constipation and gastrointestinal irritation. Despite continued research, the pathogenesis of PD is not yet clear. The available class of drugs for effective symptomatic management of PD includes a combination of levodopa and carbidopa. In recent past, the link between gut with PD has been explored. According to recent preclinical evidence, pathogens such as virus or bacterium may initiate entry into the gut via the nasal cavity that may aggravate lewy pathology in the gut that eventually propagates and progresses towards the brain via the vagus nerve resulting in the prodromal non-motor symptoms. Additionally, experimental evidence also suggests that alpha-synuclein misfolding commences at a very early stage in the gut and is transported via the vagus nerve prior to seeding PD pathology in the brain. However, this progression and resultant deterioration of the neurones can effectively be altered by an autophagy inducer, Trehalose, although the mechanism behind it is still enigmatic. Hence, this review will mainly focus on analysing the basic components of the gut that might be responsible for aggravating lewy pathology, the mediator(s) responsible for transmission of PD pathology from gut to brain and the important role of trehalose in ameliorating gut dysbiosis related PD complications that would eventually pave the way for therapeutic management of PD.

The Parkinson disease pain classification system: results from an international mechanism-based classification approach

ABSTRACT

Pain is a common nonmotor symptom in patients with Parkinson disease (PD) but the correct diagnosis of the respective cause remains difficult because suitable tools are lacking, so far. We developed a framework to differentiate PD- from non-PD-related pain and classify PD-related pain into 3 groups based on validated mechanistic pain descriptors (nociceptive, neuropathic, or nociplastic), which encompass all the previously described PD pain types. Severity of PD-related pain syndromes was scored by ratings of intensity, frequency, and interference with daily living activities. The PD-Pain Classification System (PD-PCS) was compared with classic pain measures (ie, brief pain inventory and McGill pain questionnaire [MPQ], PDQ-8 quality of life score, MDS-UPDRS scores, and nonmotor symptoms). 159 nondemented PD patients (disease duration 10.2 ± 7.6 years) and 37 healthy controls were recruited in 4 centers. PD-related pain was present in 122 patients (77%), with 24 (15%) suffering one or more syndromes at the same time. PD-related nociceptive, neuropathic, or nociplastic pain was diagnosed in 87 (55%), 25 (16%), or 35 (22%), respectively. Pain unrelated to PD was present in 35 (22%) patients. Overall, PD-PCS severity score significantly correlated with pain's Brief Pain Inventory and MPQ ratings, presence of dyskinesia and motor fluctuations, PDQ-8 scores, depression, and anxiety measures. Moderate intrarater and interrater reliability was observed. The PD-PCS is a valid and reliable tool for differentiating PD-related pain from PD-unrelated pain. It detects and scores mechanistic pain subtypes in a pragmatic and treatment-oriented approach, unifying previous classifications of PD-pain.

Cellular and synaptic mechanisms for Parkinson's disease-related chronic pain

Parkinson’s disease is the second most common neurodegenerative disorder after Alzheimer’s disease. Chronic pain is experienced by the vast majority of patients living with Parkinson’s disease. The degeneration of dopaminergic neuron acts as the essential mechanism of Parkinson’s disease in the midbrain dopaminergic pathway. The impairment of dopaminergic neurons leads to dysfunctions of the nociceptive system. Key cortical areas, such as the anterior cingulate cortex (ACC) and insular cortex (IC) that receive the dopaminergic projections are involved in pain transmission. Dopamine changes synaptic transmission via several pathway, for example the D2-adenly cyclase (AC)-cyclic AMP (cAMP)-protein kinase A (PKA) pathway and D1-G protein-coupled receptor kinase 2 (GRK2)-fragile X mental retardation protein (FMRP) pathway. The management of Parkinson’s disease-related pain implicates maintenance of stable level of dopaminergic drugs and analgesics, however a more selective drug targeting at key molecules in Parkinson’s disease-related pain remains to be investigated.

Measurement and Correction of Stooped Posture during Gait Using Wearable Sensors in Patients with Parkinsonism: A Preliminary Study

Stooped posture, which is usually aggravated during walking, is one of the typical postural deformities in patients with parkinsonism. However, the degree of stooped posture is difficult to quantitatively measure during walking. Furthermore, continuous feedback on posture is also difficult to provide. The purpose of this study is to measure the degree of stooped posture during gait and to investigate whether vibration feedback from sensor modules can improve a patient's posture. Parkinsonian patients with stooped posture were recruited for this study. Two wearable sensors with three-axis accelerometers were attached, one at the upper neck and the other just below the C7 spinous process of the patients. After being calibrated in the most upright posture, the sensors continuously recorded the sagittal angles at 20 Hz and averaged the data at every second during a 6 min walk test. In the control session, the patients walked with the sensors as usual. In the vibration session, sensory feedback was provided through vibrations from the neck sensor module when the sagittal angle exceeded a programmable threshold value. Data were collected and analyzed successfully in a total of 10 patients. The neck flexion and back flexion were slightly aggravated during gait, although the average change was <10° in most patients in both measurement sessions. Therefore, it was difficult to evaluate the effect of sensory feedback through vibration. However, some patients showed immediate response to the feedback and corrected their posture during gait. In conclusion, this preliminary study suggests that stooped posture could be quantitatively measured during gait by using wearable sensors in patients with parkinsonism. Sensory feedback through vibration from sensor modules may help in correcting posture during gait in selected patients.

Towards optimising experimental quantification of persistent pain in Parkinson's disease using psychophysical testing

People with Parkinson's disease (PD) may live for multiple decades after diagnosis. Ensuring that effective healthcare provision is received across the range of symptoms experienced is vital to the individual's wellbeing and quality of life. As well as the hallmark motor symptoms, PD patients may also suffer from non-motor symptoms including persistent pain. This type of pain (lasting more than 3 months) is inconsistently described and poorly understood, resulting in limited treatment options. Evidence-based pain remedies are coming to the fore but therapeutic strategies that offer an improved analgesic profile remain an unmet clinical need. Since the ability to establish a link between the neurodegenerative changes that underlie PD and those that underlie maladaptive pain processing leading to persistent pain could illuminate mechanisms or risk factors of disease initiation, progression and maintenance, we evaluated the latest research literature seeking to identify causal factors underlying persistent pain in PD through experimental quantification. The majority of previous studies aimed to identify neurobiological alterations that could provide a biomarker for pain/pain phenotype, in PD cohorts. However heterogeneity of patient cohorts, result outcomes and methodology between human psychophysics studies overwhelmingly leads to inconclusive and equivocal evidence. Here we discuss refinement of pain-PD paradigms in order that future studies may enhance confidence in the validity of observed effect sizes while also aiding comparability through standardisation. Encouragingly, as the field moves towards cross-study comparison of data in order to more reliably reveal mechanisms underlying dysfunctional pain processing, the potential for better-targeted treatment and management is high.

Sympathetic and sensory nerve fiber function in multiple system atrophy and idiopathic Parkinson's disease

OBJECTIVE

To explore small fiber somatosensory and sympathetic function in PD and MSA.

METHODS

We recruited 20 PD patients (7 women, median age 65.5 years; IQR 54.75-70.0), 10 MSA patients (4 women; median age 68 years; IQR 66.25-74.0), and 10 healthy subjects (HC; 4 women, median age 68; IQR 59.0-71.0 years). Autonomic testing included forehead cooling, intradermal microdialysis of norepinephrine (NE; 10^-5; 10^-6; 10^-7; and 10^-8), and orthostatic hypotension (OH); somatosensory testing included quantitative sensory testing (QST) according to the protocol of the German Research Network on Neuropathic Pain (DFNS).

RESULTS

OH occurred more frequently in PD (p = 0.018) and MSA (p = 0.002) compared to HC. Vasoconstriction responses were stronger in PD compared to MSA during forehead cooling (p = 0.044) and microdialysis of physiologically concentrated NE solutions (10^-7; 10-8; p = 0.017). PD and MSA had impaired cold (PD: p < 0.01; MSA: p < 0.05) and warm detection thresholds (PD and MSA, both p < 0.05). The mechanical detection threshold was higher in PD (p < 0.01). Conversely, mechanical pain thresholds were decreased in PD and MSA (both p < 0.001), indicating mechanical hyperalgesia.

CONCLUSION

In contrast to MSA, we found evidence of peripheral adrenoreceptor hypersensitivity in PD, probably caused by peripheral sympathetic denervation. Sensory testing revealed peripheral neuropathy and central pain sensitization in PD and MSA. Jointly, our data demonstrate autonomic and somatosensory dysfunction in PD and MSA.

Trans-synaptic spreading of alpha-synuclein pathology through sensory afferents leads to sensory nerve degeneration and neuropathic pain

Pain is a common non-motor symptom of Parkinson’s disease (PD), with current limited knowledge of its pathophysiology. Here, we show that peripheral inoculation of mouse alpha-synuclein (α-Syn) pre-formed fibrils, in a transgenic mouse model of PD, elicited retrograde trans-synaptic spreading of α-Syn pathology (pSer129) across sensory neurons and dorsal nerve roots, reaching central pain processing regions, including the spinal dorsal horn and the projections of the anterolateral system in the central nervous system (CNS). Pathological peripheral to CNS propagation of α-Syn aggregates along interconnected neuronal populations within sensory afferents, was concomitant with impaired nociceptive response, reflected by mechanical allodynia, reduced nerve conduction velocities (sensory and motor) and degeneration of small- and medium-sized myelinated fibers. Our findings show a link between the transneuronal propagation of α-Syn pathology with sensory neuron dysfunction and neuropathic impairment, suggesting promising avenues of investigation into the mechanisms underlying pain in PD.

D2 receptor activation relieves pain hypersensitivity by inhibiting superficial dorsal horn neurons in parkinsonian mice

Chronic pain is a common and undertreated nonmotor symptom in Parkinson's disease (PD). Although chronic pain is improved by L-dopa in some PD patients, the underlying mechanisms remain unclear. In this study, we established PD mice by unilateral microinjection of 6-OHDA in the medial forebrain bundle to investigate the contribution of spinal cord dopamine receptors to parkinsonian pain hypersensitivity. The von Frey filament tests and thermal pain tests revealed that these PD mice displayed decreased nociceptive thresholds in both hindpaws; intrathecal injection of L-dopa or apomorphine significantly increased the mechanical and thermal nociceptive thresholds, and the analgesic effect was mimicked by ropinirole (a D2 receptor agonist), but not SKF38393 (a D1/D5 receptor agonist), and blocked by sulpiride (a D2 receptor antagonist), but not SKF83566 (a D1/D5 receptor antagonist). Whole-cell recordings in lumber spinal cord slices showed that superficial dorsal horn (SDH) neurons in PD mice exhibited hyperexcitability, including more depolarized resting membrane potentials and more action potentials evoked by depolarizing current steps, which were mitigated by ropinirole. Furthermore, ropinirole inhibited the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in SDH neurons more strongly in PD mice than in control mice. However, sulpiride caused less disinhibition of sEPSCs in PD mice than in control mice. Taken together, our data reveal that pain hypersensitivity in PD mice is associated with hyperexcitability of SDH neurons, and both events are reversed by activation of spinal D2 receptors. Therefore, spinal D2 receptors can be promising therapeutic targets for the treatment of PD pain.

Antinociceptive effects of rotigotine-loaded microspheres and its synergistic interactions with analgesics in inflammatory pain in rats

Rotigotine-loaded microspheres (RoMS) are sustained-release formulations with prolonged anti-Parkinson's effects. Given that pain is a non-motor symptom of Parkinson's disease, this study investigated the antinociceptive effects of RoMS and their synergistic effects with analgesics on inflammatory pain. A model of inflammatory pain was prepared by intraplantarly injecting male Sprague-Dawley rats with carrageenan. The antinociceptive effects of RoMS, acetaminophen, and tramadol, both alone and in combination, were evaluated using the hind paw withdrawal latency in the hot plate test and Randall-Selitto test. The rotigotine concentrations in serum and tissues were assayed using ultra-performance liquid chromatography-tandem mass spectrometry. Isobolographic analysis was performed to evaluate the nature of the interactions of RoMS with acetaminophen or tramadol. The results showed that hind paw withdrawal latency to thermal and mechanical stimuli was significantly increased on day 3 and 7 after administered RoMS. Rotigotine could be detected in serum and tissues 3 and 7 days after an intramuscular injection of RoMS. However, the rotigotine concentration fell the detection limit of the assay on day 14 after administration. RoMS produced synergistic antinociceptive effects in the inflammatory pain model when RoMS is combined with acetaminophen or tramadol. These findings suggest that RoMS can relieve inflammatory pain in rats. Furthermore, the combination of RoMS with acetaminophen or tramadol produces synergistic antinociception, which may be clinically worthy because combination therapies may reduce the drug doses required for antinociception.

Sheth M, Viswanath O, Urits I. Chronic Pain Treatment Strategies in Parkinson's Disease

Neurological disorders, including Parkinson's disease (PD), have increased in prevalence and are expected to further increase in the coming decades. In this regard, PD affects around 3% of the population by age 65 and up to 5% of people over the age of 85. PD is a widely described, physically and mentally disabling neurodegenerative disorder. One symptom often poorly recognized and under-treated by health care providers despite being reported as the most common non-motor symptom is the finding of chronic pain. Compared to the general population of similar age, PD patients suffer from a significantly higher level and prevalence of pain. The most common form of pain reported by Parkinson's patients is of musculoskeletal origin. One of the most used combination drugs for PD is Levodopa-Carbidopa, a dopamine precursor that is converted to dopamine by the action of a naturally occurring enzyme called DOPA decarboxylase. Pramipexole, a D2 dopamine agonist, and apomorphine, a dopamine agonist, and Rotigotine, a dopamine receptor agonist, have showed efficacy on PD-associated pain. Other treatments that have shown efficacy in treating pain of diverse etiologies are acetaminophen, Nonsteroidal anti-inflammatory drugs (NSAIDs), and cyclooxygenase-2 (COX-2) inhibitors. Opioids and opioid-like medications such as oxycodone, morphine, tramadol, and codeine are also commonly employed in treatment of chronic pain in PD. Other opioid related medications such as Tapentadol, a central-acting oral analgesic with combined opioid and noradrenergic properties, and Targinact, a combination of the opioid agonist oxycodone and the opioid antagonist naloxone have shown improvement in pain. Anticonvulsants such as gabapentin, pregabalin, lamotrigine, carbamazepine and tricyclic antidepressants (TCAs) can be trialed when attempting to manage chronic pain in PD. The selective serotonin and noradrenaline reuptake inhibitors (SNRIs) also possess pain relieving and antidepressant properties, but carry less of the risk of anticholinergic side effects seen in TCAs. Deep brain stimulation (DBS) of the subthalamic nucleus (STN) has been shown in multiple studies to be effective against various types of PD associated pain symptoms. Massage therapy (MT) is one of the most common forms of complementary and alternative medicine. Studies have shown that pressure applied during MT may stimulate vagal activity, promoting reduced anxiety and pain, as well as increasing levels of serotonin. In a survey study of PD patients, rehabilitative therapy and physical therapy were rated as the most effective for pain reduction, though with only temporary relief but these studies were uncontrolled. Yoga has been studied for patients with a wide array of neurological disorders. In summary, PD pathology is thought to have a modulating effect on pain sensation, which could amplify pain. This could help explain a portion of the higher incidence of chronic pain felt by PD patients. A treatment plan can be devised that may include dopaminergic agents, acetaminophen, NSAIDs, opioids, antidepressants, physical therapies, DBS and other options discussed in this review. A thorough assessment of patient history and physical examination should be made in patients with PD so chronic pain may be managed effectively.

Chronic pain in Parkinson's disease: Clinical and pathophysiological aspects

Pain is an increasingly recognized non-motor symptom of Parkinson's disease (PD), with significant prevalence and strong impact on quality of life of patients. Moreover, pain can occur with various features in PD and several subtypes may coexist in a same patient, leading to a complex presentation and difficult diagnosis and treatment. In this paper we review the clinical manifestations of painful phenomena in PD, with focus on classifications and algorithms allowing to standardize the diagnosis of pain and PD. We also discuss the pathophysiological mechanisms underlying pain in PD, particularly parkinsonian central pain, in regard to recent clinical, neurophysiological and imaging studies.

Management of Pain in Parkinson's Disease

Pain is a very frequent symptom with influence on the quality of life in Parkinson’s disease (PD), but is still underdiagnosed and commonly treated only unsystematically. Pain etiology and pain character are often complex and multi-causal, and data regarding treatment recommendations are limited. Pain can be primarily related to PD but frequently it is associated with secondary diseases, such as arthrosis of the spine or joints. However, even basically PD-unrelated pain often is amplified by motor- or non-motor PD symptoms, such as akinesia or depression. Beyond an optimization of anti-parkinsonian treatment, additional pain treatment strategies are usually needed to properly address pain in PD. A careful pain history and diagnostic work-up is essential to rate the underlying pain pathophysiology and to develop a targeted therapeutic concept. This review gives an overview on how pain is treated in PD patients and how patients assess the effectiveness of these therapies; here, the manuscript focuses on pathophysiology-driven suggestions for a multimodal pain management in clinical practice.

Inhibition of NADPH oxidase within midbrain periaqueductal gray decreases pain sensitivity in Parkinson's disease via GABAergic signaling pathway

Hypersensitive pain response is observed in patients with Parkinson's disease (PD). However, the signal pathways leading to hyperalgesia still need to be clarified. Chronic oxidative stress is one of the hallmarks of PD pathophysiology. Since the midbrain periaqueductal gray (PAG) is an important component of the descending inhibitory pathway controlling on central pain transmission, we examined the role NADPH oxidase (NOX) of the PAG in regulating exaggerated pain evoked by PD. PD was induced by central microinjection of 6-hydroxydopamine to lesion the left medial forebrain bundle of rats. Then, Western Blot analysis and ELISA were used to determine NOXs and products of oxidative stress (i.e., 8-isoprostaglandin F2alpha and 8-hydroxy-2'-deoxyguanosine). Pain responses to mechanical and thermal stimulation were further examined in control rats and PD rats. In results, among the NOXs, protein expression of NOX4 in the PAG of PD rats was significantly upregulated, thereby the products of oxidative stress were increased. Blocking NOX4 pathway in the PAG attenuated mechanical and thermal pain responses in PD rats and this was accompanied with decreasing production of oxidative stress. In addition, inhibition of NOX4 largely restored the impaired GABA within the PAG. Stimulation of GABA receptors in the PAG of PD rats also blunted pain responses. In conclusions, NOX4 activation of oxidative stress in the PAG of PD rats is likely to impair the descending inhibitory GABAergic pathways in regulating pain transmission and thereby plays a role in the development of pain hypersensitivity in PD. Inhibition of NOX4 has beneficial effects on the exaggerated pain evoked by PD.

Effects of safinamide on pain in Parkinson's disease with motor fluctuations: an exploratory study

Pain is a common and disabling non-motor symptom (NMS) of Parkinson's disease (PD), which occurs through the course of the disease, often unrecognized and undertreated. For this study, we evaluated the efficacy and safety of safinamide to reduce pain in PD patients with motor fluctuations. A total of 13 PD patients with pain receiving safinamide (Xadago®, 100 mg/daily) were prospectively evaluated for 12 weeks. The primary outcome measures were changes in the total score of the King's Pain Scale for Parkinson's Disease (KPPS), Brief Pain Inventory (BPI) Intensity and Interference, and the Numeric Rating Scale (NRS). Secondary outcomes were the proportion of pain responders, changes in the Clinical Global Impression of Change (CGI), the Parkinson's disease Quality of Life 39 (PDQ39), the Unified Parkinson's Disease Rating Scale parts III and IV (UPDRS III and IV), and laser-evoked potentials (LEPs). LEPs were used to assess potential changes in the central processing of nociceptive inputs. The safety profile was evaluated based on the occurrence of treatment-emergent side effects and the dropout rate. After 12 weeks of add-on safinamide therapy, a significant improvement was noted in the primary (KPPS, BPI Intensity and interference, and NRS) and the secondary outcomes (UPDRS III, IV, CGI, and PDQ39). No significant changes in LEP complexes were observed. All patients completed the study and no treatment-emergent side effects were reported. Our preliminary findings suggest that safinamide 100 mg/day may be effective for the management of pain in PD patients with motor fluctuations and is safe. Further randomized controlled trials are needed to confirm its efficacy.

Effects of Heating-needle Stimulation in Restoration of Weakened Descending Inhibition of Nociception in a Rat Model of Parkinson's Disease

Here we investigated variations of endogenous descending modulation of nociception and therapeutic effects of intramuscular (i.m.) heating-needle stimulation in early stage of Parkinson's disease (PD) induced by unilateral microinjection of 3.5 μl of 2.5 μg/μl 6-hydroxydopamine into the rat striatum. Paw withdrawal reflexes to noxious mechanical and heat stimuli in PD rats with and without exposure to i.m. 5.8% saline induced muscle nociception were evaluated. Experimental PD had no influence on mechanical or heat sensitivity in the baseline condition, whereas descending facilitation was stronger and descending inhibition was weaker in PD rats than vehicle-treated or naive rats during muscle nociception (P < 0.05). Striatal administration of 5 μg of dopamine failed to reverse the PD-associated changes in descending facilitation or inhibition, whereas dopamine in the thalamic mediodorsal (MD) nucleus and ventromedial (VM) nucleus significantly decreased the increase in descending facilitation and reversed the attenuation in descending inhibition, respectively (P < 0.05). I.m. 43 °C of heating-needle stimulation had no effects on the enhanced descending facilitation in PD rats, but it markedly increased descending inhibition and reversed the increase in the number of apomorphine-induced body rotations (P < 0.05), which effects were dose-dependently attenuated by raclopride, a dopamine 2 receptor antagonist, in the thalamic VM nucleus (P < 0.05). The results indicate that the early-stage PD is associated with enhanced descending facilitation and weakened descending inhibition. From clinical perspective, 43 °C heat therapeutic regime promises to selectively enhance descending inhibition that is accompanied by improvement of motor dysfunction in PD.

Reversal of hyperactive subthalamic circuits differentially mitigates pain hypersensitivity phenotypes in parkinsonian mice

Treatments for pain symptoms in patients with Parkinson’s disease (PD) show inconsistent efficacy across clinical trials, largely owing to our limited understanding of the mechanisms underlying PD pain. Here, we demonstrate that overactivation of subthalamic nucleus (STN) neurons and their projections is adequate to produce a pain hypersensitivity phenotype, and that such overactivation is essential for the hypersensitivity in pain processing pathways and the maintenance of pain hypersensitivity observed in parkinsonian mice. These results suggest that inhibition of STN neurons may be a potential therapeutic strategy for pain relief in PD. Our finding that individual STN projections differentially regulate mechanical and thermal pain thresholds raises the possibility that individual STN projections may be optimal therapeutic targets for different pain phenotypes.

Although pain is a prevalent nonmotor symptom in Parkinson’s disease (PD), it is undertreated, in part because of our limited understanding of the underlying mechanisms. Considering that the basal ganglia are implicated in pain sensation, and that their synaptic outputs are controlled by the subthalamic nucleus (STN), we hypothesized that the STN might play a critical role in parkinsonian pain hypersensitivity. To test this hypothesis, we established a unilateral parkinsonian mouse model with moderate lesions of dopaminergic neurons in the substantia nigra. The mice displayed pain hypersensitivity and neuronal hyperactivity in the ipsilesional STN and in central pain-processing nuclei. Optogenetic inhibition of STN neurons reversed pain hypersensitivity phenotypes in parkinsonian mice, while hyperactivity in the STN was sufficient to induce pain hypersensitivity in control mice. We further demonstrated that the STN differentially regulates thermal and mechanical pain thresholds through its projections to the substantia nigra pars reticulata (SNr) and the internal segment of the globus pallidus (GPi)/ventral pallidum (VP), respectively. Interestingly, optogenetic inhibition of STN-GPi/STN-VP and STN-SNr projections differentially elevated mechanical and thermal pain thresholds in parkinsonian mice. In summary, our results support the hypothesis that the STN and its divergent projections play critical roles in modulating pain processing under both physiological and parkinsonian conditions, and suggest that inhibition of individual STN projections may be a therapeutic strategy to relieve distinct pain phenotypes in PD.

Widespread Pressure Pain Hyperalgesia Is Not Related to Pain in Patients with Parkinson's Disease

OBJECTIVE

Pain is one of the most frequent nonmotor impairments in Parkinson's disease (PD) and is hypothesized to be associated with altered nociceptive pain processing. Our aims were to investigate differences in widespread pressure pain sensitivity between PD patients with and without pain and healthy controls and to assess the relationship of health-related quality of life and sleep quality with pressure pain sensitivity.

METHODS

Nineteen PD patients with pain (12 men, age = 68 ± 9 years), 19 PD patients without pain (11 men, age = 69 ± 8 years), and 19 matched controls participated. Pressure pain thresholds (PPTs) were assessed bilaterally over the cervical spine, the second metacarpal, and the tibialis anterior by an assessor blinded to the subject's condition. Patients were assessed in a dopamine-medicated (ON) state. Pain intensity (numerical pain rating scale, 0-10), health-related quality of life (39-item Parkinson's Disease Questionnaire), and sleep quality (Pittsburgh Sleep Quality Index) were also assessed.

RESULTS

No significant differences existed between PD patients, with or without pain, and healthy controls on PPTs over the cervical spine, the second metacarpal, or the tibialis anterior muscle (all P > 0.3). PPTs were lower in females than in males in all groups (P < 0.01). In PD patients with pain, worse quality of sleep was associated with higher widespread pressure pain sensitivity (-0.607 < r < -0.535, P < 0.05). No other significant association was observed.

CONCLUSIONS

This study revealed no differences in widespread pressure hyperalgesia between PD patients with or without pain (ON state) and controls. Although dopamine may modulate pain responses, other mechanisms seems to also be implicated in altered nociceptive pain processing in patients with PD.

The phenomenology of pain in Parkinson's disease

Background

Parkinson's disease (PD) is a neurodegenerative disorder that is the second most common disorder after Alzheimer's disease. PD includes both "motor" and "non-motor" symptoms, one of which is pain. The aim of this study was to investigate the clinical characteristics of pain in patients with PD.

Methods

This cross-sectional study included 250 patients diagnosed with PD, 70% of which had mild to moderate PD (stages 2/3 of Hoehn and Yahr scale). The average age was 67.4 years, and the average duration since PD diagnosis was 7.1 years. Relevant data collected from PD patients were obtained from their personal medical history.

Results

The prevalence of pain was found to be high (82%), with most patients (79.2%) relating their pain to PD. Disease duration was correlated with the frequency of intense pain (R: 0.393; P < 0.05). PD pain is most frequently perceived as an electrical current (64%), and two pain varieties were most prevalent (2.60 ± 0.63). Our findings confirm links between pain, its evolution over time, its multi-modal character, the wide variety of symptoms of PD, and the female sex.

Conclusions

Our results demonstrated that the pain felt by PD patients is mainly felt as an electrical current, which contrasts with other studies where the pain is described as burning and itching. Our classification is innovative because it is based on anatomy, whereas those of other authors were based on syndromes.

Cannabidiol increases the nociceptive threshold in a preclinical model of Parkinson's disease

Medications that improve pain threshold can be useful in the pharmacotherapy of Parkinson's disease (PD). Pain is a prevalent PD's non-motor symptom with a higher prevalence of analgesic drugs prescription for patients. However, specific therapy for PD-related pain are not available. Since the endocannabinoid system is expressed extensively in different levels of pain pathway, drugs designed to target this system have promising therapeutic potential in the modulation of pain. Thus, we examined the effects of the 6-hydroxydopamine- induced PD on nociceptive responses of mice and the influence of cannabidiol (CBD) on 6-hydroxydopamine-induced nociception. Further, we investigated the pathway involved in the analgesic effect of the CBD through the co-administration with a fatty acid amide hydrolase (FAAH) inhibitor, increasing the endogenous anandamide levels, and possible targets from anandamide, i.e., the cannabinoid receptors subtype 1 and 2 (CB1 and CB2) and the transient receptor potential vanilloid type 1 (TRPV1). We report that 6-hydroxydopamine- induced parkinsonism decreases the thermal and mechanical nociceptive threshold, whereas CBD (acute and chronic treatment) reduces this hyperalgesia and allodynia evoked by 6-hydroxydopamine. Moreover, ineffective doses of either FAAH inhibitor or TRPV1 receptor antagonist potentialized the CBD-evoked antinociception while an inverse agonist of the CB1 and CB2 receptor prevented the antinociceptive effect of the CBD. Altogether, these results indicate that CBD can be a useful drug to prevent the parkinsonism-induced nociceptive threshold reduction. They also suggest that CB1 and TRPV1 receptors are important for CBD-induced analgesia and that CBD could produce these analgesic effects increasing endogenous anandamide levels.

The challenge of pain in the pharmacological management of Parkinson's disease

Introduction: Pain is a common symptom in Parkinson's disease (PD), impairing quality of life. The clinical appearance and the underlying etiologies are diverse. Different subtypes of pain may occur, with musculoskeletal pain considered to be the most frequent. Often there is also a combination of different causes of pain. There is a lack of controlled studies addressing pain therapy in PD. Areas covered: In this review the authors analyzed the currently available data, taking into account the available publications in the databases, especially PubMed. The authors further provided their expert perspectives on the challenges of treating pain in PD patients. Expert opinion: There is both nociceptive and neuropathic pain and in patients with PD, some PD-related pain and some unrelated. Diagnosis requires a thorough and differentiated history and examination, and targeted diagnostics. Therapeutically, many drugs are used, but the data is unfortunately limited and not specific. Medications used include Parkinson-related, mainly dopaminergic drugs, as well as opioids and non-opioid analgetics, anticonvulsives, antidepressants, and more recently cannabinoids. Currently, therapy is performed nonspecifically, without taking into account the special requirements of PD. Unfortunately, in many cases, pain is resistant to these therapies. In the future, both diagnostic and therapeutic efforts should be made to address this issue.

Sensory neuropathy and nociception in rodent models of Parkinson's disease

Parkinson's disease (PD) often manifests with prodromal pain and sensory losses whose etiologies are not well understood. Multiple genetic and toxicity-based rodent models of PD partly recapitulate the histopathology and motor function deficits. Although far less studied, there is some evidence that rodents, similar to humans, develop sensory manifestations of the disease, which may precede motor disturbances and help to elucidate the underlying mechanisms of PD-associated pain at the molecular and neuron circuit levels. The present Review summarizes nociception and other sensory functions in frequently used rodent PD models within the context of the complex phenotypes. In terms of mechanisms, it appears that the acute loss of dopaminergic neurons in systemic toxicity models (MPTP, rotenone) primarily causes nociceptive hyperexcitability, presumably owing to a loss of inhibitory control, whereas genetic models primarily result in a progressive loss of heat perception, reflecting sensory fiber neuropathies. At the molecular level, neither α-synuclein deposits alone nor failure of mitophagy alone appear to be strong enough to result in axonal or synaptic pathology of nociceptive neurons that manifest at the behavioral level, and peripheral sensory loss may mask central ‘pain’ in behavioral tests. Hence, allostatic combinations or additional challenges and novel behavioral assessments are needed to better evaluate PD-associated sensory neuropathies and pain in rodents.

Summary: Rodent models of Parkinson's disease partially develop prodromal somatosensory and olfactory dysfunctions reminiscent of sensory neuropathies in patients and reveal mechanistic insight, but data are incomplete and fragmented.

Chronic pain and pain processing in Parkinson's disease

Pain is experienced by the vast majority of patients living with Parkinson's disease. It is most often of nociceptive origin, but may also be ascribed to neuropathic (radicular or central) or miscellaneous sources. The recently validated King's Parkinson's Disease Pain Scale is based on 7 domains including musculoskeletal pain, chronic body pain (central or visceral), fluctuation-related pain, nocturnal pain, oro-facial pain, pain with discolouration/oedema/swelling, and radicular pain. The basal ganglia integrate incoming nociceptive information and contribute to coordinated motor responses in pain avoidance and nocifensive behaviors. In Parkinson's disease, nigral and extra-nigral pathology, involving cortical areas, brainstem nuclei, and spinal cord, may contribute to abnormal central nociceptive processing in patients experiencing pain or not. The dopamine deficit lowers multimodal pain thresholds that are amenable to correction following levodopa dosing. Functional brain imaging with positron emission tomography following administration of H₂¹⁵O revealed abnormalities in the sensory discriminative processing of pain (insula/SII), as well as in the affective motivational processing of pain (anterior cingulate cortex, prefrontal cortex). Pain management is dependent on efforts invested in diagnostic accuracy to distinguish nociceptive from neuropathic pain. Treatment requires an integrated approach including strategies to lessen levodopa-related response fluctuations, in addition to other pharmacological and non-pharmacological options such as deep brain stimulation and rehabilitation.

Relationship of Nocturnal Sleep Dysfunction and Pain Subtypes in Parkinson's Disease

Background

Little research has been conducted regarding the relationship between sleep disorders and different pain types in Parkinson's disease (PD).

Objective

To explore the influence of the various pain subtypes experienced by PD patients on sleep.

Methods

Three hundred consecutive PD patients were assessed with the PD Sleep Scale-Version 2 (PDSS-2), King's PD Pain Scale (KPPS), King's PD Pain Questionnaire (KPPQ), Visual Analog Scales for Pain (VAS-Pain), and Hospital Anxiety and Depression Scale.

Results

According to the PDSS-2, 99.3% of our sample suffered from at least one sleep issue. Those who reported experiencing any modality of pain suffered significantly more from sleep disorders than those who did not (all, P < 0.003). The PDSS-2 showed moderate-to-high correlations with the KPPS (r_S = 0.57), KPPQ (0.57), and VAS-Pain (0.35). When PDSS-2 items 10 to 12 (pain-related) were excluded, the correlation values decreased to 0.50, 0.51, and 0.28, respectively, while these items showed moderate-to-high correlations with KPPS (0.56), KPPQ (0.54), and VAS-Pain (0.42). Among the variables analyzed, multiple linear regression models suggested that KPPS and KPPQ were the most relevant predictors of sleep disorders (as per the PDSS-2), although following exclusion of PDSS-2 pain items, depression was the relevant predictor. Depression and anxiety were the most relevant predictors in the analysis involving the VAS-Pain. Regression analysis, considering only the KPPS domains, showed that nocturnal and musculoskeletal pains were the best predictors of overall nocturnal sleep disorder.

Conclusions

Pain showed a moderate association with nocturnal sleep dysfunction in PD. Some pain subtypes had a greater effect on sleep than others.

A detailed clinical study of pain in 1957 participants with early/moderate Parkinson's disease

INTRODUCTION

The causes of pain in early/moderate Parkinson's disease (PD) are not well understood. Although peripheral factors such as rigidity, reduced joint movements and poor posture may contribute towards the development of pain, central mechanisms including altered nociceptive processing may also be involved.

METHODS

We performed a large clinical study to investigate potential factors contributing towards pain in early/moderate PD. We recruited 1957 PD participants who had detailed assessments of pain, motor and non-motor symptoms. The King's Parkinson's Pain scale was used to quantify different subtypes of pain.

RESULTS

85% of participants reported pain (42% with moderate to severe pain). Pain influenced quality of life more than motor symptoms in a multiple regression model. Factors predicting overall pain severity included affective symptoms, autonomic symptoms, motor complications, female gender and younger age, but not motor impairment or disease duration. There was negligible correlation between the severity of motor impairment and the severity of musculoskeletal or dystonic pain as well as between the severity of OFF period motor problems and the severity of OFF period pain or OFF period dystonic pain. Features of central sensitization, including allodynia and altered pain sensation were common in this population. The use of drugs targeting central pain was very low.

CONCLUSIONS

Pain in early/moderate PD cannot be explained by peripheral factors. Central causes may play a much more important role than previously considered. These results should lead to a major shift in the investigation and management of this common and disabling symptom.

Parkinson disease: A systemic review of pain sensitivities and its association with clinical pain and response to dopaminergic stimulation

Patients with Parkinson disease (PD) experience hyperalgesia on evoked pain sensitivity testing, although the relationship of this with persistent pain in PD is less certain. Studies examining this have generated contradictory findings. Further, the role of dopaminergic deficiency as an underlying substrate for hyperalgesia is controversial. We report the results of meta-analyses of the PD pain sensitivity literature in an attempt to answer these questions. We identified 429 records, of which ten articles compared pain sensitivity between PD patients that experienced clinical pain (PDP) to those who did not (PDNP), and twenty studies that examined the effect of dopaminergic medications on pain sensitivity in PD patients. PDP patients experienced a moderate increase in pain sensitivity, had more severe disease, required higher dosages of medication, and were more likely to be female when compared to those PDNP patients. PD patients also had reduced pain sensitivity when tested on dopaminergic medications compared to when they were not on medications. Overall, the results of this systematic review and meta-analysis supports the hypothesis that hyperalgesia contributes to clinical pain in PD, and that the underlying mechanism may be dopaminergically driven.

Association of Parkinson's disease-related pain with plasma interleukin-1, interleukin-6, interleukin-10, and tumour necrosis factor-α

OBJECTIVE

To study the association between Parkinson's disease (PD)-related pain and plasma interleukin (IL)‑1, IL‑6, IL‑10, and tumour necrosis factor (TNF)‑α levels.

METHODS

Sixty-seven participants were enrolled. Plasma inflammatory cytokine levels of IL-1, IL-6, IL-10, and TNF-α were measured with enzyme-linked immunosorbent assay. We additionally administered the third part of the Unified Parkinson's Disease Rating Scale (UPDRS III) and Hoehn and Yahr (H-Y) scale stage and recorded the course of the disease, the type and location of the pain, and the use of drugs.

RESULTS

The level of IL-1 was significantly higher in the PD-with-pain than in the healthy-control group (P < 0.05). There was no significant difference among groups in the other examined cytokine levels. There was a statistically significant difference between the PD-with-pain and the PD-without-pain groups in UPDRS III and H-Y stage. Additionally, the IL-1 level was significantly higher in patients who received a levodopa dosage of >250 mg than in their counterparts who received ≤250 mg, and the IL-1 level was higher in patients with an H-Y stage of >2 and UPDRS III of >27 than in their counterparts with an H-Y stage of ≤2 and UPDRS III of ≤27. The expression of TNF-α was higher in patients aged ≥70 years than in their counterparts aged <70 years. The level of IL-10 was significantly lower in the patients with an H-Y stage of >2 than in their counterparts with an H-Y stage of ≤2.

CONCLUSION

The elevated level of IL-1 and the depressed level of IL-10 in the peripheral blood of patients with PD-related pain suggests that certain inflammatory cytokines may be implicated in the occurrence and clinical symptoms of PD-related pain.

Decreased sleep quality in Parkinson's patients is associated with higher anxiety and depression prevalence and severity, and correlates with pain intensity and quality

OBJECTIVE

Pain, poor sleep quality, restless legs syndrome (RLS), and other symptoms are frequently reported by patients with Parkinson's disease (PD). However, the impact that pain severity and interference has on non-motor symptoms (NMS) has not been extensively studied. The objective of the present study is to explore the relationship between sleep quality in PD to pain and other NMS that affect quality of life.

METHODS

The study included 100 PD patients and 100 age and gender-matched controls assessed for pain severity and pain interference using the Brief Pain Inventory and sleep quality using the Pittsburgh Sleep Quality Index. Participants were also evaluated for their subjective levels of anxiety and depression using the Hospital Anxiety and Depression Scale.

RESULTS

PD patients with poor sleep quality had greater pain severity and pain interference than controls and PD patients with good or borderline sleep quality. PD patients with poor sleep quality also had the greatest case-ness and severity for depression and anxiety. However, RLS was not significantly correlated with depression, anxiety or pain.

DISCUSSION

Poor sleep quality in PD patients is related to greater pain severity, pain interference, and more radiating and paresthestic pain that is independent of RLS. There is a higher prevalence of depression and anxiety in PD patients compared to controls, especially in PD patients with poor sleep quality. Our findings suggest a relationship between poor sleep quality in PD with pain, anxiety and depression. Prospective studies are warranted to investigate the causal relationship.

King's Parkinson's Disease Pain Scale for Assessment of Pain Relief Following Deep Brain Stimulation for Parkinson's Disease

OBJECTIVE

Pain is a prevalent and debilitating nonmotor symptom of Parkinson's disease (PD) that is often inadequately managed. Deep brain stimulation (DBS) has been shown to relieve pain in PD but an effective method of identifying which types of PD pain respond to DBS has not been established. We examine the effects of DBS on different types of PD pain using the King's Parkinson's disease pain scale (KPDPS), the only validated scale of PD pain.

METHODS

We prospectively followed 18 PD patients undergoing subthalamic nucleus (STN) or Globus pallidus interna (GPi) DBS. Subjects completed the KPDPS, low back disability index (LBDI), and McGill pain questionnaire (MPQ), preoperatively and at six months postoperatively. Subjects underwent the unified Parkinson's disease rating scale-III (UPDRS-III) with preoperative scores ON medication and postoperative scores ON medication/DBS stimulation.

RESULTS

Of the 18 patients, a total of 12 subjects had STN DBS and 6 had GPi DBS. As a group, subjects showed improvement in total KPDPS score at six-month postoperative follow-up (p = 0.004). Fluctuation and nocturnal pain were most significantly improved (p = 0.006, 0.01, respectively). Significant improvements were found in fluctuation-related pain domain following GPi DBS.

CONCLUSIONS

In this pilot study, we are the first group to employ KPDPS to monitor pain relief following DBS in PD patients. We demonstrate that fluctuation-related pain and nocturnal pain significantly improve with DBS. Use of the KPDPS in the future will allow better understanding of how STN and GPi DBS treat PD pain over time.

Long-term Efficacy of Safinamide on Parkinson's Disease Chronic Pain

Introduction

Chronic pain is an important yet overlooked non-motor symptom of Parkinson’s disease (PD), caused by an imbalance of the dopaminergic and glutamatergic systems. Safinamide has a multimodal mechanism of action, dopaminergic (reversible MAO-B inhibition) and non-dopaminergic (modulation of the abnormal glutamate release), that might be beneficial for both motor and non-motor symptoms.

Objectives

To investigate the long-term (2-year) efficacy of safinamide on PD chronic pain and to confirm the positive effects observed after 6 months of treatment.

Methods

This is a post hoc analysis of the data from the 2-year study 018, focused on the reduction of concomitant pain treatments and on the scores of pain-related items of the Parkinson’s disease quality of life questionnaire (PDQ-39).

Results

Safinamide, compared with placebo, significantly improved the PDQ-39 items 37 (“painful cramps or spasm,” p = 0.0074) and 39 (“unpleasantly hot or cold,” p = 0.0209) and significantly reduced the number of concomitant pain treatments by 26.2% (p = 0.005). A significantly greater proportion of patients in the safinamide group was not using pain drugs after 2 years of treatment (p = 0.0478).

Conclusions

The positive effects of safinamide on PD chronic pain were maintained in the long term. Further investigations are desirable to confirm their clinical relevance.

Funding

Zambon SpA.

Structural and Functional Brain Patterns of Non-Motor Syndromes in Parkinson's Disease

Parkinson’s disease (PD) is a common, progressive and multisystem neurodegenerative disorder characterized by motor and non-motor symptoms. Advanced magnetic resonance imaging, positron emission tomography, and functional magnetic resonance imaging can render the view toward understanding the neural basis of these non-motor syndromes, as they help to understand the underlying pathophysiological abnormalities. This review provides an up-to-date description of structural and functional brain alterations in patients with PD with cognitive deficits, visual hallucinations, fatigue, impulsive behavior disorders, sleep disorders, and pain.

Central pain processing in "drug-naïve" pain-free patients with Parkinson's disease

BACKGROUND

Despite its clinical relevance, the pathophysiology of pain in Parkinson's disease (PD) is still largely unknown, and both central and peripheral mechanisms have been invoked.

OBJECTIVES

To investigate whether central pain processing is altered in "drug-naive" pain-free PD (dnPD) patients.

METHODS

Using event-related functional MRI (fMRI), functional response to forearm heat stimulation (FHS) at two different intensities (41°C and 53°C) was investigated in 20 pain-free dnPD patients, compared with 18 healthy controls (HCs). Secondary analyses were performed to evaluate associations between BOLD signal changes and PD clinical features and behavioral responses.

RESULTS

During low-innocuous FHS (41°C), no activation differences were found between dnPD patients and HCs. During high-noxious FHS (53°C) a significantly increased activation in the left somatosensory cortex, left cerebellum, and right low pons was observed in dnPD patients compared to HCs. In the latter experimental condition, fMRI BOLD signal changes in the right low pons (p < .0001; R = -0.8) and in the cerebellum (p = .004; R = -0.7) were negatively correlated with pain intensity ratings only in dnPD patients. No statistically significant difference in experimental pain perception was detected between dnPD patients and HCs.

CONCLUSIONS

Our findings suggest that a functional remodulation of pain processing pathways occurs even in the absence of clinically overt pain symptoms in dnPD patients. These mechanisms may eventually become dysfunctional over time, contributing to the emergence of pain symptoms in more advanced PD stages. The comprehension of pain-related mechanisms may improve the clinical approach and therapeutic management of this disabling nonmotor symptom.

Somatosensory function is impaired in patients with idiopathic REM sleep behaviour disorder

BACKGROUND

Idiopathic REM sleep behaviour disorder (iRBD) has been recognised as a significant biomarker for developing a neurodegenerative alpha-synucleinopathy, which is why iRBD is considered to be a prodromal state for alpha-synucleinopathies including Parkinson's disease (PD). Many patients with PD suffer from complaints of pain and present impaired somatosensory function. We hypothesized that pain perception and somatosensory function could be altered already in a preclinical stage of PD including iRBD. Hence, the objective of this study was to investigate pain perception and somatosensory function in patients with iRBD.

METHODS

Quantitative sensory testing (QST), laser evoked potentials (LEPs), and conditioned pain modulation (CPM) testing were performed in 13 iRBD patients without any clinical signs of PD or narcolepsy (11 males, 2 females, mean age 65.2 years) and 15 gender- and age-matched healthy control subjects (12 males, 3 females, mean age 65.8 years).

RESULTS

Thermal detection thresholds were higher in the iRBD group compared with the control group (cold detection threshold (CDT) p = 0.020, thermal sensory limen (TSL) p = 0.001), indicating an impaired temperature sensation in iRBD patients. The N2/P2 LEPs amplitude was smaller in iRBD patients than controls, but not statistically significant (p = 0.053).

CONCLUSIONS

This study found an impaired somatosensory function in iRBD patients, suggesting that somatosensory impairment might be an early feature in the neurodegenerative process of PD.

A Review of Pain Prevalence in Alzheimer's, Vascular, Frontotemporal and Lewy Body Dementias

BACKGROUND

Numerous studies have reported on pain in dementia. It has been hypothesized that pain perception differs between dementia subtypes, and therefore, the prevalence of pain differs between dementia subtypes. However, there remains a paucity of evidence on the differences in the prevalence of pain in different dementia subtypes. This review aimed to determine the prevalence of pain for the major dementia subtypes: Alzheimer's disease (AD), vascular dementia (VaD), frontotemporal dementia (FTD) and dementia with Lewy bodies (DLB).

SUMMARY

We found 10 studies that met our inclusion criteria. Most of these studies reported on AD; studies reporting the prevalence of pain in people with DLB were scarce, and for FTD, we found no studies. The sample-weighted prevalence of pain could only be calculated for AD, VaD and mixed dementia: AD 45.8% (95% confidence interval, CI: 33.4-58.5%), VaD 56.2% (95% CI: 47.7-64.4%) and mixed dementia 53.9% (95% CI: 37.4-70.1%).

KEY MESSAGES

Studies investigating the prevalence of pain in dementia subtypes were scarce; however, we found a high prevalence of pain in dementia without significant differences between the dementia subtypes. More studies are required to draw firm conclusions on the differences in the prevalence of pain between dementia subtypes.

Pain processing in atypical Parkinsonisms and Parkinson disease: A comparative neurophysiological study

OBJECTIVE

Pain is a frequent non-motor feature in Parkinsonism but mechanistic data on the alteration of pain processing are insufficient to understand the possible causes and to define specifically-targeted treatments.

METHODS

we investigated spinal nociception through the neurophysiological measure of the threshold (TR) of nociceptive withdrawal reflex (NWR) and its temporal summation threshold (TST) comparatively in 12 Progressive Supranuclear Palsy (PSP) subjects, 11 Multiple System Atrophy (MSA) patients, 15 Parkinson's disease (PD) subjects and 24 healthy controls (HC). We also investigated the modulatory effect of L-Dopa in these three parkinsonian groups.

RESULTS

We found a significant reduction in the TR of NWR and in the TST of NWR in PSP, MSA and PD patients compared with HC. L-Dopa induced an increase in the TR of NWR in the PSP group while TST of NWR increased in both PSP and PD.

CONCLUSIONS

Our neurophysiological findings identify a facilitation of nociceptive processing in PSP that is broadly similar to that observed in MSA and PD. Specific peculiarities have emerged for PSP.

SIGNIFICANCE

Our data advance the knowledge of the neurophysiology of nociception in the advanced phases of parkinsonian syndromes and on the role of dopaminergic pathways in the control on pain processing.