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. Author manuscript; available in PMC: 2023 Aug 1.
Published in final edited form as: J Neural Transm (Vienna). 2022 Jun 26;129(8):1001–1009. doi: 10.1007/s00702-022-02523-3

Cholinergic brain network deficits associated with vestibular sensory conflict deficits in Parkinson’s disease: correlation with postural and gait deficits

Nicolaas I Bohnen 1,2,3,4,5,*, Prabesh Kanel 1,4,5, Sitven Roytman 1, Peter JH Scott 1, Robert Koeppe 1, Roger L Albin 2,3,4,5, Kevin A Kerber 2,3, Martijn LTM Müller 1,4
PMCID: PMC9308723  NIHMSID: NIHMS1819720  PMID: 35753016

Abstract

Background:

To examine regional cerebral vesicular acetylcholine transporter (VAChT) ligand [18F]fluoroethoxybenzovesamicol ([18F]-FEOBV) PET binding in Parkinson’s disease (PD) patients with and without vestibular sensory conflict deficits (VSCD). To examine associations between VSCD-associated cholinergic brain deficits and postural instability and gait difficulties (PIGD).

Methods:

PD persons (M70/F22; mean age 67.6±7.4 years) completed clinical assessments for imbalance, falls, freezing of gait (FoG), modified Romberg sensory conflict testing, and underwent VAChT PET. Volumes-of-interest (VOI)-based analyses included detailed thalamic and cerebellar parcellations. VSCD-associated VAChT VOI selection used stepwise logistic regression analysis. Vesicular monoamine transporter type 2 (VMAT2) [11C]dihydrotetrabenazine (DTBZ) PET imaging was available in 54 patients. Analyses of covariance were performed to compare VSCD-associated cholinergic deficits between patients with and without PIGD motor features while accounting for confounders.

Results:

PET sampling passed acceptance criteria in 73 patients. This data-driven analysis identified cholinergic deficits in five brain VOIs associating with the presence of VSCD: Medial geniculate nucleus (MGN) (P<0.0001), parahippocampal gyrus (P=0.0043), inferior nucleus of the pulvinar (P=0.047), fusiform gyrus (P=0.035) and the amygdala (P=0.019). Composite VSCD-associated [18F]FEOBV binding deficits in these 5 regions was significantly lower in patients with imbalance (−8.3%, F=6.5, P=0.015; total model: F=5.1, P=0.0008), falls (−6.9%, F=4.9, P=0.03; total model F=4.7, P=0.0015), and FoG (−14.2%, F=9.0, P=0.0043; total model F=5.8, P=0.0003), independent of age, duration of disease, gender and nigrostriatal dopaminergic losses. Post-hoc analysis using MGN VAChT binding as the single cholinergic VOI demonstrated similar significant associations with imbalance, falls and FoG.

Conclusion:

VSCD-associated cholinergic network changes localize to distinct structures involved in multi-sensory, in particular vestibular, and multimodal cognitive and motor integration brain regions. Relative clinical effects of VSCD associated cholinergic network deficits were largest for FoG followed by postural imbalance and falls. The MGN was the most significant region identified.

Keywords: Parkinson’s disease, medial geniculate nucleus, vestibular, falls, freezing of gait, postural control, vestibular sensory conflict

INTRODUCTION

Advancing Parkinson’s disease (PD) is associated with disabling postural instability and gait difficulties (PIGD), including postural imbalance, falls, and freezing of gait (FoG). These motor features are progressive and become increasingly refractory to dopaminergic treatment (Lopez et al. 2010; Vu et al. 2012). The development of these motor and associated cognitive comorbidities likely reflects accumulating non-dopaminergic systems pathologies, such as cholinergic system changes (Bohnen et al. 2009; Muller et al. 2013; Bohnen et al. 2019). We recently showed that cholinergic terminal deficits in the medial geniculate nucleus (MGN), an important multisensory (in particular, auditory and vestibular) processing metathalamic relay station, associate robustly with clinical ratings of PIGD severity (Bohnen et al. 2021). PD is a disorder of aging and VSCD was shown to be an expected common co-morbidity in PD (Venhovens et al. 2016; van Wensen et al. 2013; Becker-Bense et al. 2017). There are no prior studies examining cholinergic brain system changes that specifically assess possible mechanisms of VSCD in PD. The primary goal of this study is to examine regional cerebral vesicular acetylcholine transporter (VAChT) ligand [18F]fluoroethoxybenzovesamicol ([18F]-FEOBV) PET binding in Parkinson’s disease PD) patients with and without VSCD. The secondary goal is to examine associations between VSCD-associated brain cholinergic deficits and postural instability and gait difficulties (PIGD). We hypothesized a distinct topography of subcortical (basal ganglia, thalamus, cerebellum, limbic cortex) and cortical cholinergic terminal changes would associate with the presence of VSCD. We also hypothesized that VSCD-cholinergic network deficits have specific associations with PIGD motor changes (postural imbalance, falls and freezing of gait, FoG) in PD.

MATERIALS AND METHODS

SUBJECTS

This cross-sectional study involved 92 patients with PD (M70/F22; mean age 67.6±7.4 years, mean motor disease duration 6.0±4.6 years. Parkinson’s disease subjects met the UK Parkinson’s Disease Society Brain Bank clinical diagnostic criteria (Hughes et al. 1992). Most subjects had moderate severity of disease: 6 subjects in Hoehn & Yahr (HY) stage 1, 3 in HY stage 1.5, 20 in HY stage 2, 20 in HY stage 2.5, 40 in HY stage 3, and 3 in NY stage 4 and overall a mean HY stage of 2.5±0. Thirty-one subjects were taking a combination of dopamine agonist and carbidopa-levodopa medications, 45 were using carbidopa-levodopa alone, 10 were taking dopamine agonists alone, and 6 were not receiving dopaminergic drugs. A healthy control (HC) group (n=21, M8/F11) with a mean age of 67.8 ± 7.8 years was included for normative PET imaging data. No subjects were treated with anti-cholinergic or cholinesterase inhibitor drugs. Subjects with evidence of large vessel stroke or other intracranial lesions on anatomic imaging were excluded. Findings of cholinergic correlates of total summed PIGD motor ratings obtained in an overlapping cohort of subjects as in this study were published previously (Bohnen et al. 2021). The prior paper included 108 patients of whom 88 passed the PET quality control acceptance regions for small regions (see below). The present study consists of a subset of 92 patients for whom data for the modified Romberg test were available and of whom 73 passed the PET quality control criteria.

CLINICAL ASSESSMENT

The Movement Disorder Society-revised Unified Parkinson’s Disease Rating Scale (MDS-UPDRS) motor examination was performed in the morning in the dopaminergic medication ‘off’ state. Subjects completed the Montreal Cognitive Assessment (MoCa; Nasreddine et al. 2005). Mean MoCa scores were 26.2 ± 3.3. The mean motor examination score on the MDS-UPDRSIII was 35.5 ± 14.2 (range 2–74) (Goetz et al. 2007). Imbalance was defined as Hoehn and Yahr stage 2.5 or higher as a score >0 on MDS-UPDRS item 3.12 is defined as evidence of postural imbalance (Goetz et al. 2004; Lee et al. 2012). Participants were asked about fall history. A fall was defined as an unexpected event during which a person falls to the ground. FoG status was defined by direct observation of motor freezing behavior by an experienced neurologist during the MDS-UPDRS assessment (score on item 3.11 > 0).

VESTIBULAR CONFLICT DEFICIT ASSESSMENT

The modified Romberg tests of Standing Balance on Firm and Compliant Support Surfaces was used to identify the presence of VSCD (Shumway-Cook and Horak 1986; Agrawal et al. 2013). The modified Romberg Test examines ability to stand unassisted using 4 test conditions designed specifically to evaluate the different sensory inputs that contribute to balance—vestibular system input, vision, and proprioception (Agrawal et al. 2011). The different conditions are 1: standing on firm surface with eyes open (depends on visual, proprioceptive and vestibular input); 2: standing on firm surface with eyes closed (depends on proprioceptive and vestibular input); 3: standing on compliant surface with eyes open (depends on visual and vestibular input), and 4: designed to focus specifically on vestibular function - participants have to maintain balance on a foam-padded surface (to obscure proprioceptive input) with eyes closed (to eliminate visual input). A time to fall < 20 seconds on the Romberg 4 test (standing on foam surface with eyes closed) in the absence of falling during conditions 1–3 is defined as evidence of VSCD (Agrawal et al. 2013); see table 1. This criterion was validated in study of patients with vestibular disorders (Cohen et al. 1993). The modified Romberg test is similar to a component of the Sensory Organization Test (SOT) with the Romberg 4 subtest corresponding to falls on SOT subtest 5, which has specificity of 80% and sensitivity of 80% to identify vestibular dysfunctions (Cohen and Kimball 2008).

Table 1.

The modified Romberg tests of Standing Balance on Firm and Compliant Support Surfaces was used to identify the presence of VSCD (Shumway-Cook and Horak 1986; Agrawal et al. 2013). The modified Romberg Test examines ability to stand unassisted using 4 test conditions designed specifically to evaluate the different sensory inputs that contribute to balance—vestibular system input, vision, and proprioception (Agrawal et al. 2011). A time to fall < 20 seconds on the Romberg 4 test (standing on foam surface with eyes closed) in the absence of falling during conditions 1–3 is defined as evidence of VSCD.

Romberg condition 1
Stand with eyes open on firm surface		 2
Stand with eyes closed on firm surface		 3
Stand on foam mat with eyes open		 4
Stand on foam mat with eyes closed		 Diagnosis of VSCD:
Failing the Romberg 4 condition in the absence of failing conditions 1–3
Sensory input Visual, propioceptive & vestibular Propioceptive & vestibular mainly Visual & vestibular Vestibular only
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STANDARD PROTOCOL APPROVAL, REGISTRATION, AND SUBJECTS CONSENT

This study (ClinicalTrials.gov Identifiers: NCT02458430 & NCT01754168) was approved by the Institutional Review Boards of the University of Michigan and Ann Arbor Veterans Affairs Healthcare System. All participants provided written informed consent.

IMAGING AND ANALYSIS

Brain MRI was performed on a 3 Tesla Philips Achieva system (Philips, Best, The Netherlands) and PET imaging was performed in 3D imaging mode with a Biograph 6 TruPoint PET/CT scanner (Siemens Molecular Imaging, Inc., Knoxville, TN) as previously reported (Bohnen et al. 2021). Images were corrected for scatter and motion. [18F]FEOBV was prepared as described previously (Shao et al. 2011b; Shao et al. 2011a). [18F]-FEOBV delayed dynamic imaging was performed over 30 minutes (in six 5-minute frames) starting 3 hours after an intravenous bolus dose injection of 8 mCi [18F]-FEOBV (Petrou et al. 2014). [11C]DTBZ was performed in a subset of 54 patients and prepared as previously reported (Shao et al. 2011a). A 60-minute bolus/infusion protocol was used for [11C]DTBZ PET imaging (15 mCi; (Koeppe et al. 1997). [11C]DTBZ PET imaging was performed in the dopaminergic medication ‘off’ state in the morning. MRI-PET registration statistical parametric mapping (SPM) software (SPM12; Wellcome Trust Centre for Neuroimaging, University College, London, England [https://www.fil.ion.ucl.ac.uk/spm/software/spm12/]), motion, scatter and partial volume correction was performed as previously reported (Bohnen et al. 2021). FEOBV PET images were analyzed non-invasively using a supratentorial white matter reference tissue approach as previously reported (Aghourian et al. 2017; Nejad-Davarani et al. 2018). Distribution volume ratios (DVR) were calculated from ratio of summed six delayed imaging frames (3 hours after injection) for gray matter target and white matter reference tissues (Nejad-Davarani et al. 2018). [11C]DTBZ DVR of the bilaterally averaged striatum was determined based on the Logan plot graphical analysis method with the supratentorial cortex as reference region (Logan et al. 2001). Frontal, temporal, parietal and occipital cortical volume-of-interests (VOIs) were computed as the average of neocortical regions with the exception of the pre-, para-, and post-central cortical regions, which were defined separately using Freesurfer software (http://surfer.nmr.mgh.harvard.edu) based on labels from the Mindboggle-101 dataset (Klein and Tourville 2012). Neocortical regions not belonging to a specific brain lobe, such as the fusiform gyrus or insula, were analyzed separately. Limbic cortical areas were defined for the hippocampus, parahippocampal gyrus, entorhinal cortex, amygdala, parahippocampal gyrus, anterior and posterior cingulum. Striatal regions were also defined. A 2004 study using a high resolution PET instrument demonstrated feasibility of quantitative PET assessment of small brain regions, including the geniculate regions, because of decreased partial volume effects (Heiss et al. 2004). Small-sized detailed parcellations for the cerebellum (total of 21 regions) and thalamic complex nuclei (total of 26 regions) were defined using cerebellar SUIT toolbox (version 3.2; (Diedrichsen 2006) and the Iglesias atlas, respectively (Iglesias et al. 2018). VOIs with a minimum acceptance criterion of at least five surviving voxels (3.8 mm3 per voxel) were included in our study to overcome the partial volume effects due to possible spillover from adjacent regions as previously reported (Bohnen et al. 2021). A total of 20 cerebellar and 17 bilaterally averaged thalamic complex nuclei met acceptance criteria for analysis. Globus pallidus pars interna (GPi) and externa (GPe) were defined as previously reported (Bohnen et al. 2021). Collectively for all three atlases a total number of 57 VOIs were used for the analysis.

STATISTICAL ANALYSIS

VSCD-associated VAChT VOI selection was based on a data-driven approach - stepwise logistic regression analysis with forward selection. Analyses of covariance were performed to compare VSCD-associated [18F]FEOBV binding between subjects with and without PIGD motor features (postural imbalance, falls and freezing of gait, FoG) while accounting for confounders (age, duration of disease, gender, and nigrostriatal dopaminergic denervation). Statistical analyses were performed in SAS version 9.4, SAS institute, Cary, NC.

RESULTS

Demographic and clinical information:

A total of 92 Parkinson’s disease subjects were included in the study cohort. The number of subjects with incomplete (not meeting acceptance criteria) PET volume-of-interest (VOI) sampling data was 19, leaving 73 subjects available for analysis. Demographic and clinical variables of these 73 subjects (M56/F17) were similar to the original study population: mean age 68.0 ± 7.6 years, duration of disease 6.0 ± 3.8 years, Hoehn & Yahr stage 2.6 ± 0.6, MDS-UPDRSIII total motor scores 36.5 ± 14.2, and MoCa scores 26.1 ± 3.4. VSCD was present in 25 subjects (34.3%). Postural imbalance, defined as modified HY stage ≥2.5 (Goetz et al. 2004) was present in 63 (69.9%) subjects. Twenty-seven subjects (37%) reported history of falls. FoG was observed in 10 patients (13.7%).

Data-driven VOI selection approach of regional [18F]FEOBV binding associating with VSCD:

The data-driven approach (stepwise logistic regression using forward selection) identified five (bilaterally averaged) brain VOIs that met model entry and retention criteria: MGN (χ2=; P<0.0001), parahippocampal gyrus (χ2=8.2; P=0.0043), inferior nucleus of the pulvinar (χ2=3.9; P=0.047), fusiform gyrus (χ2=4.5; P=0.035) and the amygdala (χ2=5.5; P=0.019). (Table 2).

Table 2.

Results of a data-driven approach to identify cholinergic brain regions associating with VSCD status in persons with PD. Bilaterally averaged VOI regions are presented.

Results of stepwise logistic regression with forward selection method
Model-selected VAChT VOIs Step χ2 P-value
Medial geniculate nucleus (MGN) 1 15.7 <0.0001
Parahippocampal gyrus 2 8.2 0.0043
Inferior nucleus of the pulvinar 3 3.9 0.047
Fusiform gyrus 4 4.5 0.035
Amygdala 5 5.5 0.019
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Relationship between FEOBV binding in the MGN and a composite topographic measure of VSCD-cholinergic system changes and the presence of postural imbalance, falls and FoG in persons with PD:

We examined clinical imbalance correlates of cholinergic MGN and system changes in 54 patients (F12/M42) with [18F]FEOBV PET scans and who also had completed dopaminergic brain PET imaging ([11C]DTBZ PET) while adjusting covariate effects of striatal dopamine binding, age, duration of disease and gender. We used a composite measure of the 5 bilaterally averaged VSCD model-selected central cholinergic system changes (see Table 3). Given binding magnitude differences between different subcortical and cortical regions, percentage binding differences between a PD subject and normal control subject data, a mean FEOBV PET percentage binding in each of the 5 VOIs identified in the data-driven approach were computed.

Table 3.

Relationship between composite topographic measure of VSCD-cholinergic network changes and the presence of imbalance, falls and FoG in persons with PD (n=54).

VAChT region PIGD status subgroup model Age Duration Gender Striatal dopamine
[11C]DTBZ PET		 Total model
Composite of 5 VSCD model-selected regions Imbalance status
F=6.5, P=0.015
(8.3%* lower in HY ≥2.5 stage)		 F=6.2, P=0.017 F=0.0, P=0.96 F=6.6, P=0.014 F=2.2, P=0.15 R2=0.35, F=5.1, P=0.0008
Composite of 5 VSCD model-selected regions Fall status
F=4.9, P=0.03
(6.9%* lower in fallers)		 F=9.2, P=0.004 F=0.2, P=0.63 F=3.5, P=0.07 F=1.0, P=0.33 R2=0.33, F=4.7, P=0.0015
Composite of 5 VSCD model-selected regions FoG status
F=9.0, P=0.0043
(14.2%* lower in freezers)		 F=6.3, P=0.016 F=0.6, P=0.45 F=3.7, P=0.06 F=0.1, P=0.76 R2=0.39, F=5.8, P=0.0003
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ANCOVA F-values (with levels of significance) are listed for the group effects and age, duration of disease, gender, striatal dopamine [11C]DTBZ PET binding and total model effects. FoG = freezing of gait.

*

covariate adjusted

Composite VSCD-associated [18F]FEOBV binding in these 5 regions was significantly lower in PD subjects with imbalance (−8.3%, F=6.5, P=0.015; total model: F=5.1, P=0.0008), falls (−6.9%, F=4.9, P=0.03; total model F=4.7, P=0.0015) and FoG (−14.2%, F=9.0, P=0.0043; total model F=5.8, P=0.0003), independent of age, duration of disease, gender and nigrostriatal dopaminergic deficits (Table 3).

Post-hoc analysis of the relationship between MGN FEOBV binding and postural imbalance, falls, and FoG:

The MGN was the most significant VOI associated with VSCD (Table 3). Therefore, we performed a post-hoc analysis to examine specific associations between MGN FEOBV binding and the three PIGD measures while accounting for clinical and disease-specific confounders in the subset of 54 patients who completed [11C]DTBZ PET. This analysis is analogous to the ANCOVA analysis performed for the 5-region composite VSCD binding measure described above.

Post-hoc analysis using the MGN FEOBV binding as the single VOI demonstrated similar significant associations with imbalance, falls, and FoG: MGN [18F]FEOBV binding was significantly lower in patients with imbalance (−9.6%, F=5.2, P=0.027; total model: F=3.6, P=0.008), falls (−9.9%, F=5.9, P=0.019; total model F=3.8, P=0.006) and FoG (−22%, F=13.5, P=0.0006; total model F=5.7, P=0.0004), independent of age, duration of disease, gender and nigrostriatal dopaminergic losses (Table 4).

Table 4:

Relationship between MGN [18F]FEOBV binding in the medial geniculate nucleus (MGN) and the presence of imbalance, falls and FoG in persons with PD (n=54).

VAChT region PIGD status subgroup model Age Duration Gender Striatal dopamine
[11C]DTBZ PET		 Total model
MGN Imbalance status
F=5.20, P=0.027
(9.6%* lower in HY ≥2.5 stage stage)		 F=3.9, P=0.054 F=0.01, P=0.91 F=2.2, P=0.14 F=3.2, P=0.08 R2=0.28, F=3.6, P=0.008
MGN Fall status
F=5.9, P=0.019
(9.9%* lower in fallers)		 F=6.2, P=0.017 F=0.2, P=0.69 F=0.8, P=0.39 F=1.8, P=0.18 R2=0.29, F=3.8, P=0.006
MGN FoG status
F=13.5, P=0.0006
(22%* lower in freezers)		 F=3.6, P=0.07 F=0.7, P=0.41 F=0.8, P=0.37 F=0.04, P=0.84 R2=0.38, F=5.7, P=0.0004
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ANCOVA F-values (with levels of significance) are listed for the group effects and age, duration of disease, gender, striatal dopamine [11C]DTBZ PET binding and total model effects. FoG = freezing of gait.

*

covariate adjusted

DISCUSSION

Our results indicate that in PD subjects, regions with VSCD-associated cholinergic terminal deficits include the MGN, parahippocampal gyrus, inferior nucleus of the pulvinar, fusiform gyrus, and the amygdala. The MGN was the region with the highest level of significance in the data-driven selection model. All these structures play roles in multisensory processing and the MGN and inferior nucleus of the pulvinar likely play particularly important roles in vestibular sensory processing. Hodological and functional studies indicate an important role for the MGN in vestibular sensory processing and multisensory integration relevant to gait and balance disorders in PD. The MGN receives inputs from medial vestibular, dorsal vestibular, and superior vestibular nuclei (Mergner et al. 1981; Kotchabhakdi et al. 1980). A multisensory processing role of the MGN, including downstream processing of vestibular sensory information, may depend on the medial region of the MGN. The medial (magnocellular) subdivision of the MGN is the multisensory division of the auditory thalamus and receives non-auditory inputs including vestibular, visual, somatosensory, and nociceptive stimuli (Jones 2007). Cholinergic MGN afferents arise from the pedunculopontine-laterodorsal tegmental complex (PPN-LDT) ((Motts and Schofield 2010), a cholinergic projection system involved in postural control functions (Muller et al. 2013). PPN-LDT cholinergic afferents are highly collateralized and PPD-LDT neurons projecting to the MGN may innervate neurons of the vestibular complex, including medial vestibular neurons providing cholinergic innervation of the midline cerebellum (Zhang et al. 2016). Degeneration of PPN-LDT cholinergic efferents is common in PD and strongly linked to PIGD deficits (Bohnen et al. 2009; Muller et al. 2013; Bohnen et al. 2019).The MGN may also act as a relay station for vestibular input to the striatum (Potegal et al. 1971). Furthermore, retrograde tracer injections in the caudate, putamen and amygdala labeled the medial division of the MGN (LeDoux et al. 1985; LeDoux et al. 1990). The inferior nucleus of the pulvinar plays an important role in processing retinal information (via the superior colliculus; another structure receiving PPN-LDT cholinergic afferents) for object motion detection, identification of stimulus salience, and modulation of saccades as part of a parallel input to visual association cortices bypassing the lateral geniculate nucleus (LGN) classical retino-geniculo-calcarine pathway (Benarroch 2015; Berman and Wurtz 2010; Lyon et al. 2010). The parahippocampal gyrus surrounds the hippocampus and plays an important role in both spatial memory (Squire and Zola-Morgan 1991) and spatial navigation (Aguirre et al. 1996); both functions that are essential functions to find your way in a complex environment (Maguire et al. 1996). There is evidence for a role of the amygdala for processing vestibular information (Best et al. 2014). In addition to high-level visual processing, the fusiform gyrus is involved in memory, multisensory integration and perception (Borra and Luppino 2017). These previously identified multisensory and multimodal processing functions imply that cholinergic deficits within these five model-selected regions play an important role in multisensory (including vestibular), cognitive, and motor integration functions needed for effective postural control during stance and gait. The cholinergic deficits identified in mediotemporal structures may also have a specific role in spatial orientation and navigation deficits often found in PD.

Comparison of the present results with our prior analyses indicate that this pattern of regional cholinergic deficits is specifically associated with VSCD in PD. The five VSCD-associated regions with cholinergic terminal deficits (MGN, parahippocampal gyrus, inferior nucleus of the pulvinar, fusiform gyrus and the amygdala) are largely distinct from the pattern of regional cholinergic terminal deficits previously identified as associated with total PIGD motor ratings: entorhinal cortex, globus pallidus interna, lateral genicular nucleus (LGN), lateral posterior nucleus of the thalamus, caudate nucleus, and anterior cingulum, the sole exception being the MGN. LGN plays a primary role in processing of visual information and modulation of visual attention (Halassa and Kastner 2017). Our previous study found that LGN cholinergic terminal deficits associate with both falls and FoG in PD (Bohnen et al. 2019). More recently, we showed that [18F]FEOBV binding in the LGN and other regions, such as the caudate nucleus and lateral posterior nucleus of the thalamus associated with non-episodic PIGD features (Bohnen et al. 2021). Cholinergic denervation in these regions likely play contributing roles to PIGD features distinct from vestibular dysfunction or the multi-sensory processing implied by VSCD-associated regional cholinergic deficits. It is also possible that these structures may play more downstream role in multisensory processing related to PIGD motor features in PD. Other notable absent structures in VSCD-associated regional cholinergic deficits include the cerebellum, the hippocampus, and entorhinal cortex. Our cumulative results suggest that multiple circuit dysfunctions contribute to PIGD features of PD.

The analyses of covariance showed that both the 5-region composite and MGN [18F]FEOBV binding deficits were significantly associated with postural imbalance, fall history, and FoG status. These results were independent of age, duration of disease, gender, and nigrostriatal dopaminergic losses. Relative effects were largest for freezing of gait (FoG), which is typically caused by turning movements, walking in confined spaces, and trying to avoid obstacles. These are ambulation situations placing heavy demands on multisensory, cognitive, and motor integration capacities. Our previous studies failed to show a significant covariate nigrostriatal dopaminergic deficit effect for falls in PD (Bohnen et al. 2009; Bohnen et al. 2012). We previously reported that PD subjects with FoG had more severe striatal dopaminergic losses compared to non-freezers (Bohnen et al. 2014). Our current findings do not demonstrate significant covariate effects of nigrostriatal dopaminergic deficits in the relationship between FoG and VSCD-associated regional cholinergic deficits.

MGN cholinergic terminal deficits had the most robust associations with each of the three PIGD motor features. We previously reported that cholinergic MGN deficits were the best predictor of PIGD motor ratings (Bohnen et al. 2021). There is also evidence for a role of MGN dysfunction in PD from both pharmacological and functional connectivity MRI studies (Taguchi et al. 2019; Wang et al. 2021). A resting state functional MRI analysis found evidence of reduced connectivity between the left MGN and left postcentral gyrus in subjects with PD and FoG as compared to PD subjects without FoG and normal controls (Wang et al. 2021). A pharmacological study found that anti-parkinsonian medications improve regional cerebral blood flow in the MGN, LGN and the substantia nigra in patients with PD (Taguchi et al. 2019).

Our findings support novel approaches for the management of VSCD and PIGD features in PD. Portable in-home caloric vestibular stimulation (so-called thermoneuromodulation, TNM) will be a prime candidate for future studies in patients with VSCD and PIGD motor features (Wilkinson et al. 2019). TNM has been shown to induce neural activation in dorsal brainstem/thalamic regions that may overlap with VSCD-associated regional cholinergic deficits (Black et al. 2016). Anti-cholinergic drugs are commonly used for treatment of vestibular hyper-activity (i.e., vertigo). Most patients with VSCD, however, do not have vertigo but rather a sense of imbalance. Given our preliminary findings of cholinergic deficits in PD subjects with VSCD, we suggest that cholinergic augmentation drugs may deserved further attention in persons with VSCD. If confirmed, this would be an entirely novel therapeutic approach to treat VSCD in patients with PD. Based on animal studies, acoustic tone or medial geniculate stimulation cue training may be associated with neocortical neuroplasticity and reduced akinesia under haloperidol challenge, suggesting another therapeutic strategy in patients with PD and troublesome PIGD motor features (Brown et al. 2010).

There are several limitations of this study. VSCD status was determined based on clinical assessment only and no specific vestibular testing was performed. The modified Romberg sensory conflict test, however, was specifically designed to isolate vestibular dysfunction associated with postural imbalance. Future studies may perform specific vestibular function testing in addition to clinical assessments. We assessed dopaminergic losses using the vesicular monoamine transporter type 2 (VMAT2) [11C]DTBZ PET ligand, which allows accurate and specific assessment of dopamine binding in the striatum but not in extra-striatal regions (e.g., thalamic nuclei or mediotemporal structures).

CONCLUSION

In PD subjects, VSCD-associated regional cholinergic terminal deficits were found in the MGN, parahippocampal gyrus, inferior nucleus of the pulvinar, fusiform gyrus, and amygdala. With the exception of the MGN, these regions are distinct from previously identified regions with cholinergic deficits associated with PIGD motor ratings. The VSCD-associated regions with cholinergic deficits localize to brain regions involved in multi-sensory, in particular vestibular, and multimodal cognitive and motor integrative functions essential for effective postural control during stance or gait. Relative clinical effects of VSCD-associated regional cholinergic deficits were largest for FoG, followed by postural imbalance and falls. MGN cholinergic terminal deficits alone demonstrated similar, and even more robust, associations with the presence of postural imbalance, falls, and FoG in PD. Modulation of vestibular function may be a viable approach to mitigate PIGD features of PD.

Acknowledgments

The authors thank Christine Minderovic, Cyrus Sarosh, the PET technologists, cyclotron operators, and chemists, for their assistance. We are indebted to the subjects who participated in this study.

Funding

This study was funded by National Institutes of Health (R01 AG073100, P01 NS015655, RO1 NS070856, P50 NS091856, P50 NS123067), Department of Veterans Affairs grant (I01 RX001631), the Michael J. Fox Foundation, and the Parkinson’s Foundation. None of the funding agencies had a role in the design and conduct of the study, in the collection, management, analysis and interpretation of the data, in the preparation, review or approval of the manuscript, nor in the decision to submit the manuscript for publication.

Footnotes

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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