Swallowing and chewing difficulties are associated with presynaptic dopaminergic dysfunction and greater non-motor symptom burden in early drug-naïve Parkinson’s patients

Background The underlying pathophysiology of swallowing and chewing difficulties is multifactorial and evidence clarifying the precise mechanisms are scarce. Dysfunction in dopamine-related and non-dopamine-related pathways, changes in cortical networks related with swallowing and peripheral mechanisms have been implicated in the pathogenesis of swallowing difficulties. We aimed at investigating whether swallowing and chewing difficulties are associated with presynaptic dopaminergic deficits, faster motor symptom progression and cognitive decline in a population of early drug-naïve patients with Parkinson’s disease. Methods By exploring the database of Parkinson’s Progression Markers Initiative we identified forty-nine early drug-naïve Parkinson’s disease patients with swallowing difficulties. Swallowing and chewing impairment was identified with SCOPA-AUT question 1 (answer regularly) and was assessed with MDS-UPDRS Part-II, Item 2.3 (Chewing and Swallowing). We compared Parkinson’s disease patients with swallowing and chewing difficulties to Parkinson’s disease patients without difficulties, and investigated differences in striatal [123I]FP-CIT single photon emission computed tomography levels. Using Cox proportional hazards analyses, we also evaluated whether swallowing impairment can predict motor deterioration and cognitive dysfunction. Results Patients with Parkinson’s disease, harbored a greater deterioration regarding motor and non-motor symptoms and decreased [123I]FP-CIT binding when compared with patients without swallowing and chewing impairment. Higher burden of swallowing and chewing dysfunction (MDS-UPDRS-II, item 2.3) was correlated with lower [123I]FP-CIT uptakes within the striatum (rs=−0.157; P=0.002) and the caudate (rs=−0.156; P=0.002). The presence of swallowing and chewing difficulties was not a predictor of motor progression (Hazard ratio [HR]: 1.143, 95% confidence interval [CI]: 0.848–1.541; P=0.379) or cognitive decline (HR: 1.294, 95% CI: 0.616–2.719; P=0.496). Conclusions Swallowing and chewing impairment is associated with decreased presynaptic dopaminergic integrity within caudate and greater motor and non-motor symptoms burden in early drug-naïve PD. Author contributions S.P. and M.P. conceived the study, conceptualized the experimental design. M.P., S.P., G.D. and G.P. gave input to experimental design. S.P. wrote the first draft and prepared the manuscript. G.P. and S.P. performed the statistical analysis. G.P., G.D., T.Y. and S.P. generated the figures. F.N., M.P., S.P., G.P., G.D. interpreted the data. All authors revised and gave input to the manuscript. Financial Disclosure Statement Data used in the preparation of this article were obtained from the Parkinson’s Progression Markers Initiative (PPMI) database (www.ppmi-info.org/data). For up-to-date information on the study, visit www.ppmi-info.org. PPMI – a public-private partnership - is sponsored by the Michael J. Fox Foundation for Parkinson’s Research (MJFF) and is co-funded by MJFF, Abbvie, Avid Radiopharmaceuticals, Biogen Idec, Bristol-Myers Squibb, Covance, Eli Lilly & Co., F. Hoffman-La Roche, Ltd., GE Healthcare, Genentech, GlaxoSmithKline, Lundbeck, Merck, MesoScale, Piramal, Pfizer and UCB.PPMI. Industry partners are contributing to PPMI through financial and in-kind donations and are playing a lead role in providing feedback on study parameters through the Industry Scientific Advisory Board (ISAB). Through close interaction with the study, the ISAB is positioned to inform the selection and review of potential progression markers that could be used in clinical testing. Mr. Polychronis, Dr. Dervenoulas, Ms Yousaf, Dr. Niccolini, Dr. Pagano and Prof. Politis report no disclosures. Potential Conflicts of Interest No potential conflict of interest relevant to this article was reported.

can also be attributed to brainstem pathology. Notably, early in the course of PD, areas that generate the central swallowing pathway in the medulla (motor nucleus of glossopharyngeal nerve, vagus nerve, reticular activating system) are exposed to the neurodegenerative processes. (10,11) In addition, the pedunculopontine tegmental nucleus, receives abnormal inhibitory through the pallidum and is thus further exposed to neurodegeneration. (12) In this study we investigated the association of swallowing and chewing impairment and dopaminergic deficits using [ 123 I]FP-CIT single photon emission computed tomography (SPECT). Finally, we explore whether swallowing and chewing difficulties were a predictor of motor symptom progression and cognitive decline.

Subjects and clinical evaluation
From the 412 PD patients included in the Parkinson's Progression Markers Initiative underwent both [ 123 I]FP-CIT SPECT assessments, and therefore were integrated in our analytical approach. Among these 398 PD patients, we identified 307 cognitively intact (MoCA≥26), with a complete 60-month follow-up and we included them for the longitudinal analysis. All PD patients were recruited between 2010-2015, diagnosed with PD less than two years prior to a screening visit, never treated with dopamine replacement therapy and presented with two among bradykinesia, resting tremor and rigidity or with asymmetric resting tremor/bradykinesia at screening. following the imaging technical operations manual (http://ppmi-info.org/). Raw SPECT data was acquired into a 128 x 128 matrix stepping each 3 degrees for a total of 120 (or 4 degrees for a total of 90) projections in a window centred on 159±10%KeV. The total scan duration was 30-45 minutes. A Chang 0 attenuation correction was applied using a customised Mu determined empirically from the anthropomorphic brain phantom acquired at each site. A standard Gaussian 3D 6.0mm filter was applied to each image volume and then normalised to standard Montreal Neurologic Institute space. Each scan was interpreted by two independent readers who were blinded to the subjects' demographics and characteristics. For quantification, SPECT image volumes were spatially normalized to an Ioflupane template. The eight most prominent axial slices containing striatum were summed and then a standardized volume of interest (VOI) template was applied to this image. VOI analyses were performed on the left and right caudate and putamen with the occipital region serving as a reference tissue. Specific binding ratios (SBR) were calculated as the ratio of the caudate or putamen VOI count density divided by count density of the occipital cortex minus 1. This measure approximates the binding potential, BP ND , when the tracer is in equilibrium at the target site and was previously reported with Ioflupane SPECT.(14)

Assessment of motor progression and cognitive decline
Motor progression was defined as a change of one point in the H&Y scale at the followup visits. Cognitive decline was defined as having a clinical deterioration of cognitive function reported by the patient or the caregiver, a MoCA score<26 and at least 2 test scores (of six neuropsychological tests indicated above; regardless of the domain tested). Scores should be above 1.5 standard deviation and below the standardized mean scores of education and age. Norms were applied according to current literature (15).
Follow-up visits took place in the outpatient unit of the reference hospitals once every 6 months and 307 early drug-naïve PD patients were followed up for an average of 60 months.

Standard protocol approvals, registrations, and patient consents.
This study is registered with ClinicalTrials.gov (No: NCT01141023). Each PPMI site has received approval from an ethical committee on human experimentation before the study's initiation. Written informed consent for research was obtained from all individuals participating in the study. The present study was performed according to the STROBE guidelines. (15)

Statistical analysis
Statistical analysis and graph illustration were performed with SPSS (version 20) and GraphPad Prism (version 6.0c) for MAC OS X, respectively. For all variables, variance homogeneity and Gaussianity were tested with Kolmogorov-Smirnov test.
Multivariate analysis of variance (MANOVA) was used to assess the main differences in clinical, imaging and non-imaging parameters between PD patients with and without swallowing and chewing difficulties. If the overall multivariate test was significant, Pvalues for each variable were calculated following Bonferroni's multiple comparisons test. Categorical variables are expressed as proportions and compared using the χ2 test.
We interrogated correlations between the swallowing and chewing scores and imaging data using Spearman's rank correlation. To explore whether swallowing and chewing impairment can predict motor disease burden and cognitive dysfunction, Cox proportional hazards analyses were carried out. The time to occurrence of the first event for a given subject was used in the Cox model. All data are presented as mean ± standard deviation (SD), and the level α was set for all comparisons at P<0.05, corrected.

Clinical characteristics
The prevalence of swallowing and chewing difficulties in the cohort of early drug-naïve

DISCUSSION
Our findings indicate that early drug-naïve PD patients with swallowing and chewing difficulties have greater presynaptic dopaminergic dysfunction in the caudate and nonmotor symptoms burden. Moreover, the loss of striatal dopaminergic function is correlated with swallowing and chewing difficulties in early drug-naïve PD patients.
Finally, swallowing and chewing difficulties are not associated with motor progression or an increased risk of cognitive dysfunction at the follow-up in early drug-naïve PD patients.
The prevalence of swallowing and chewing difficulties in the general PD patients' population ranges between 9 to 77%. (16) We found that swallowing difficulties occur in 12.3% (49/398) of early drug-naïve PD patients and are more frequent in akineticrigid phenotype. This is the first study to report the prevalence of swallowing and chewing difficulties in early drug-naïve PD.
Early drug-naïve PD patients with swallowing and chewing difficulties had increased non-motor symptom burden suggesting a close association between swallowing abnormalities and PD non-motor features. Among the non-motor symptoms, early drug-naïve PD patients with swallowing and chewing difficulties showed worse autonomic dysfunction, depressive symptoms, excessive daytime sleepiness and disordered REM sleeping behaviour. On the contrary, anxiety, cognition, olfactory dysfunction and disability did not differ between the two groups. A recent study has underlined that patients with swallowing and chewing impairment are prone to affective symptomatology such as depression and fear. (18) A recent study suggested that cognitive impairment (frontal/executive and Therefore, our findings need to be interpreted with the appropriate caution.
In conclusion, our results indicate a significant correlation between swallowing difficulties and loss of striatal dopaminergic function in early drug-naïve PD patients.
This specific subgroup of PD patients harbours a substantial burden of non-motor symptoms, without profound motor symptomatology.