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\begin{document}

\title{Structural~ changes associated with duration of subthalamic nucleus deep
brain stimulation~ with a focus on olfactory processing}



\author[1]{John A Thompson}%
\author[2]{Jeanelle France}%
\affil[1]{University of Colorado Denver}%
\affil[2]{Affiliation not available}%


\vspace{-1em}



  
  \date{May 10, 2024}


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\textbf{ABSTRACT}

\textbf{Background:} Olfactory loss is a well-known prodromal marker in
patients with Parkinson's disease (PD). Currently, olfactory loss in PD
remains untreated; however, studies have shown that deep brain
stimulation (DBS) of the subthalamic nucleus (STN), indicated for
alleviating motor symptoms, can improve olfactory function. However, the
relationship between STN stimulation and olfactory function remains
unexamined.

\textbf{Objective:} In this study, we sought to examine whether STN
stimulation would modify olfactory bulb volume, as well as affect
changes in brain areas associated with olfactory processing including:
thalamus, hippocampus and the lateral orbitofrontal cortex.

\textbf{Methods:} We compared changes in anatomical structure between
patients that recevied STN-DBS stimulation vs patietns that were
implanted but unstimluated.

\textbf{Results:} Our results show that significant changes in olfactory
related structures occur in patients treated with STN-DBS.~

\textbf{Conclusions:} These results are the first to provide possible
anatomical explanations for olfactory improvements following STN DBS in
PD.

\textbf{Keywords:} deep brain stimulation, subthalamic nucleus,
Parkinson's disease, olfaction, olfactory deficits

\par\null\par\null\par\null

\textbf{INTRODUCTION}

Parkinson's disease (PD) has been recognized as the second most
prevalent neurodegenerative disease in the U.S., with incidence of
disease increasing with both age and life-span (Chen, 2010). PD is
characterized as a progressive and systemic disease involving several
motor and non-motor symptoms, of which include the following: tremors,
bradykinesia, rigidity, olfactory dysfunction, cognitive decline, sleep
disorders, and many others \emph{} (Jankovic and Aguilar, 2008)\emph{.}
Markers of early-stage onset in PD, that can be assessed well before the
onset of the more commonly associated motor symptoms of the disease,
could be the key to early diagnosis, treatment, and therefore, better
patient outcome. Currently, many efforts are underway to identify such
markers. Of these efforts, olfactory dysfunction, which occurs in 90\%
of PD patients before the onset of motor symptoms, has been shown to
serve as an early diagnostic marker to estimate the progression of PD,
as well as differentiate PD from other similarly presenting movement
disorders such as progressive supranuclear palsy (PSP) and essential
tremor (ET) (Haehner et al., 2009).

Despite serving as an early diagnostic marker and being present in 90\%
of the PD population, olfactory dysfunction is an overlooked symptom of
PD. The importance of this sensory system cannot be understated. It
drives both nutrient perception (flavor), avoidance of potentially
dangerous solvents, and detects human aromas, such as pheromones, aiding
social interactions (Chen, 2010). Considering its importance to survival
and quality of life, it is not surprising that olfactory dysfunction has
been linked to decreases in appetite, increases in contact with
hazardous events, disturbances in social communication, and depression
(Croy et al., 2012; Miwa et al., 2001). Therefore, recognizing and
understanding the role of olfactory dysfunction as not only a marker,
but also a symptom of PD is paramount to improving patient quality of
life and therefore patient outcome. The current treatments available for
advanced idiopathic PD patients includes dopamine pre-cursors such as
L-Dopa, and subthalamic nucleus (STN) deep brain stimulation (DBS).
Although these treatment modalities have shown improvement in managing
the motor symptoms of PD, only DBS has been shown to induce positive
observable changes in olfaction (Fabbri et al., 2015). However, little
is known about recovery of olfactory function in PD and the underlying
neuroanatomical changes that mediate recovery.

In most cases, recovery of olfactory function in PD, following DBS, is
associated with either odor discrimination (OD) or odor identification
(OI), but not odor threshold (OT) (Hummel et al., 2005; Fabbri et al.,
2015; Guo et al., 2008). A study conducted on eleven PD patients that
underwent a range of two to thirty-one months of stimulation showed
improvement in odor discrimination (OD), but no significant differences
in OT (Hummel et al., 2005). A similar study was done on forty-five PD
patients that showed significant improvement in olfaction in nine of the
patients that received bilateral STN stimulation in conjunction with
dopamine agonists, compared to no improvement in the group that received
standard medical care (Peters et al., 2010). These studies support the
hypothesis that improvements seen in OD or OI are likely due to higher
olfactory processing, rather than peripheral olfactory sensory neurons.

Across the olfactory processing network, in humans, studies have shown
that OB volume will fluctuate as a function of olfactory impairment
(Haehner et al., 2009). Specifically, OB volume was directly correlated
with duration of olfactory function loss in a cohort of non-PD patients
that suffered post-traumatic or post-infectious olfactory deficits
(Haehner et al., 2009). The ability to regain olfactory function implies
the plasticity could arise in the OB, but also further extend to
plasticity in cortical and sub-cortical structures involved in the
olfactory processing pathway, including the olfactory epithelium, and
possibly the amygdala, piriform cortex (primary olfactory cortex),
lateral orbitofrontal cortex (OFC) and hippocampus (Ham et al., 2016).
Odor-evoked memory is a critical component of higher olfactory
functioning that involves the hippocampus (Herz, 2003). On a study done
that examined functional magnetic resonance imaging (fMRI) of
participants smelling a control odorant (CO) vs the experimental odorant
(EO) of their choice, test subjects showed greater activation in brain
regions such as the amygdala and hippocampus, suggesting that odor is
strongly linked to both emotion and memory (Herz, 2003).The OFC is also
thought to be involved in both memory and decision making (Courtiol and
Wilson, 2016). The OFC serves as a sensory integration center for
several senses, and is also thought to have reciprocal communication
with structures such as the primary olfactory cortex and the mediodorsal
thalamus (Courtiol and Wilson, 2016). In addition, the convergence of
the gustatory and olfactory system is thought to take place here in the
OFC, which contributes to modern day interpretation of `flavor'
(Kringelbach, 2004). The complex and integrated circuitry of the
olfactory system could help explain why we could possibly see induced
changes in cortical structures due to the indirect effects of DBS.

Previous studies have suggested that STN-DBS in PD leads to improvements
in olfactory function; however, there is a gap in our understanding of
the underlying anatomical changes that might accompany not only
olfactory structures, but also olfactory processing brain regions
following STN-DBS. The purpose of this study was to investigate whether
duration of STN stimulation via DBS correlates with measurable changes
in OB volume in PD patients, as well as cortical structures of the
hippocampus and the lateral orbitofrontal cortex that are involved in
this sensory pathway. We hypothesize that PD patients who have undergone
longer periods of STN stimulation may exhibit greater relative change in
OB volumes compared to not only the hemisphere that received no
stimulation, but also to other PD patient cohorts who had shorter
stimulation. Additionally, we expect to see no change in OB volumes in
ET patients compared to PD patients regardless of length of STN
stimulation. Furthermore, we believe that STN-DBS might indirectly
induce changes in several other brain areas involved in olfactory
processing such as the thalamus, lateral orbitofrontal cortex, and
hippocampus. This study will complement previous studies that have
examined olfactory recovery following STN-DBS. In addition, our method
for measuring OB volume may provide a clinometric marker for olfactory
recovery following STN-DBS in PD patients.

\par\null

\textbf{METHODS}

\textbf{\emph{Experimental Design}}

We compared changes in anatomical structure between patients that
recevied STN-DBS stimulation vs patietns that were implanted but
unstimluated.

\textbf{\emph{Patient information}}

This investigation was approved by the Colorado Multiple Institution
Review Board (COMIRB). All medical imaging analyses were conducted on
de-identified data collected during the standard-of-care procedures for
routine STN-DBS implantation surgery in patients with idiopathic PD,
using a retrospective chart-review IRB-approved protocol. Patient
information was organized with respect to sex ratio, age distribution,
and length of stimulation.

a.~~~~~~~~~~~~~~~~~~ Total patients: 30 PD patients, and 3 ET patients.

b.~~~~~~~~~~~~~~~~~~ Male to Female ratio: 19:14.

c.~~~~~~~~~~~~~~~~~~ Age range: 46-80 with a mean of 63.30 (\selectlanguage{ngerman}± SD 7.76)

d.~~~~~~~~~~~~~~~~~~ Length of stimulation in days: short = 0-55; medium
= 55-306; long = \textgreater{}306.

Following approval by Colorado Multi Institutional Review Board
(COMIRB), 30 subjects with idiopathic PD and 3 subjects with ET were
recruited for the study. All subjects demonstrated typical parkinsonian
motor symptoms, including tremor, akinesia, and rigidity or classic ET
with upper limb kinetic tremors. PD inclusion criteria for the study
included the diagnosis of idiopathic PD and a positive response to
anti-Parkinson medication, amounting to a greater than 30\% improvement
on the motor subscale of the Unified Parkinson's Disease Rating Scale
(mUPDRS) in the on- vs off-state, a brain MRI scan lacking significant
abnormalities, and medical clearance for surgery. All study subjects
provided written consent for participation. Patients over the age of 75
or under the age of 18, those at increased risk of infection (e.g.,
comorbid diabetes or history of recurrent infections), and patients with
significant cognitive deficits or untreated psychiatric disorders (e.g.,
depression) detected on neuropsychological testing were excluded from
this study.

The PD population under study was broken up into two groups by
stimulation.

\textbf{\emph{MRI images used for analysis were captured with the
following settings:}}

Volume analyses were run on a T1-Weighted 3D volumetric scan (1x1x1mm3):
3000 Repetition time (TR) (ms), 90 Echo time (TE) (ms), 90 Inversion
time (TI) (ms), 320x178 Matrix, 240x180 Field of View (mm), 64.5
Bandwidth (Hz/Px), axial orientation, 32x2 Slices, and 6 min 48 sec
Acquisition time.

\textbf{\emph{NIFIT format}}

Raw DICOMS were converted to NIFTI format using Statistical Parametric
Mapping (SPM 12) in Matlab. NIFTI is a standardized format for analyzing
4D neuroimaging data.

\textbf{\emph{Three separate analyses were conducted using the following
datasets:}}

Segmented grey and white matter of each NIFTI dataset using CAT 12
Voxel-based morphometry (Figure 1). CAT 12 is a neuroimaging technique
that measures differences in brain anatomy using statistical analysis
(Ashburner and Friston, 2005). Brain area specific region of interest
segmentations were extracted using FreeSurfer (Dale et al., 1999; Fischl
et al., 1999). This process was used to generate brain area specific
estimations of volume. Using NIFTI files, I manually measured bilateral
olfactory bulb volumes using ITK-SNAP in the coronal view seen in Figure
2B (Yushkevich et al., 2006). Both left and right OB volumes were traced
to allow for an interhemispheric control, as well as compare ipsilateral
bulbs (bulb on the hemisphere that received the stimulation).

\textbf{\emph{Statistical Analysis and data organization}}

For group analyses, we used a Kruskal-Wallis non-parametric ANOVA to
compare volumes between PD stimulation groups. Significant group
differences were further tested with multiple post-hoc comparisons using
a Dunn's test. To compare between PD and ET we used a non-parametric
t-test.

\textbf{\emph{Region Specific Analysis}}

The hippocampus is important in odor-evoked memory, meaning that its
connection to olfactory structures could possibly aid or induce its
plasticity following STN-DBS. Furthermore, in PD patients, it is
olfactory identification that is usually improved with STN-DBS,
indicating the involvement of higher brain regions involved in memory
such as the hippocampus.

In PD patients, the lateral orbitofrontal cortex holds importance with
its involvement in higher olfactory processing such as odor
discrimination. Furthermore, with the convergence of gustatory and
olfaction taking place in the OFC, it would be no surprise if STN-DBS
induced volumetric changes in this structure

Olfactory bulb is not only a site of neurogenesis, but also receives
dopaminergic input; therefore, changes in its volume that have been
shown to parallel with olfactory recovery should be expected with
STN-DBS. Furthermore, with its role in being a phenotypical marker to
signify olfactory dysfunction in PD patients, we decided to track
changes in OB volume with length of stimulation to determine if this
structure can be a predictive marker of olfactory recovery following
stimulation.

\emph{\textbf{Behavioral Analysis - Olfaction}}



\textbf{RESULTS}

\emph{Hippocampus}

DBS stimulation had no significant effect on the change in volume of
hippocampus globally (p=0.91) or in the following subregions: CA1
(p=0.77), CA3 (p=0.91) or CA4 (p=0.81) (Figure 4A-D). Furthermore, there
is no trend associated with hippocampus volume and duration of
stimulation in any of the aforementioned hippocampal regions (Figure
4E).

\emph{Olfactory Cortex}

We analyzed gray matter volume in three regions of the orbitofrontal
cortex (Figure 5A-C); Kruskal-Wallis ANONVA, (\emph{df~}= 2, \selectlanguage{greek}χ\selectlanguage{english}2 =
3.44,~\emph{p} = 0.17). DBS stimulation had no significant effect on
gray matter volume in entorhinal cortex (p=0.92), laterial
oribitofrontal cortex ~(p=0.09) or medial oribitofrontal cortex
~(p=0.20). Over time, stimulation has a tendency to increase the volume
of these structures, but this trend is not significant (Figure 5E).

\emph{Olfactory Bulb}

DBS stimulation did not significantly effect olfactory bulb volume
(p=0.43). However, the absolute change in olfactory bulb volume does
increase with prolonged stimulation (p=0.01)

\par\null\par\null

\textbf{DISCUSSION}

The present study investigated the effect of varying durations of
STN-DBS on cortical and sub-cortical brain areas related to olfactory
processing, including the OB, in PD patients. The major findings of this
study were that (1) STN-DBS in PD patients is associated with a
significant impact on cortical brain areas (specifically, we observed an
increase in ipsilateral white-matter volume), (2) STN-DBS does not have
observable effects on sub-cortical brain areas (however, there were
positive trends noted in thalamic and OFC volumes with duration of
stimulation), (3) PD patients treated for longer STN-DBS exhibited
significantly larger OB volumes than those treated with short and medium
STN-DBS, and (4) the ET control group exhibited significantly larger OB
volumes following short STN-DBS compared to PD patients (likely
indicative of a lack of olfactory deficit). These data suggest a
potential mechanism that could explain recent studies suggesting that
STN-DBS may treat some of the non-motor symptoms of PD such as olfactory
dysfunction.

With loss of dopaminergic neurons in the substantia nigra, a gray matter
strucuture, along with the cognitive imparimement that has been linked
to white matter pathology in PD patients, studying global brain changes
is imperative to understanding the possible underlying effect and
possible limitations of short, medium and long term STN-DBS (Rae et al.,
2012).The current literature illustrates the regenerative capabilities
of white matter connectivity and gray matter cell body survival
associated with DBS. In a study that used a rat model of PD to
demonstrate the effects of chronic high frequency stimulation of the
STN, it was found that stimulation not only increased cell survival
rates, but also restored the dopaminergic pathways between the striatum
and subcortical regions of the brain (Khaindrava et al., 2011).
Strucutres not normally associated with PD such as the pedunculopontine
nucleus of the brainstem have also shown functional neuroplasticity
following bilateral DBS in human subjects (Schweder et al., 2010). A
subsequent study that looked at global brain matter changes using pre
and post-operative diffusion tensor imaging of PD patients that received
bilateral STN-DBS found no changes in global brain network organization,
but did find changes in local subcortical brain regions related both to
sensory, motor, and limbic functions, further supporting the unique
neuronal plasticity associated with DBS (van Hartevelt et al., 2014). In
comparison to our current study, although we similarly found no global
brain changes in both total WM and total GM volumes, there were
significant changes in IWM volume, suggesting that DBS can reestablish
structural connectivity, which could explain associated functional
connectivity responsible for the alleviation of PD symptoms extending
beyond that of motor recovery.

Olfactory information is sent from the olfactory receptor neurons to the
OB, which further gets sent to the piriform cortex of the primary
olfactory cortex, where it sends projections that interconnect with
subcortical areas such as the amygdala, hippocampus, orbitofrontal
cortex, and the thalamus (Ham et al., 2016; Wilson et al., 2004).
Current literature states that disturbances in olfaction that cause
anosmia (the absence of smell) and hyposmia (reduced ability to smell)
are correlated with decreased number of olfactory receptors, whereas
olfactory dysfunction that results in loss of odor identification,
discrimination and odor memory are linked to central rather than
peripheral mechanisms involving the OB and subcortical brain regions
mentioned (Frasnelli et al., 2010).Recovery of olfactory function would
imply neurogenesis in the OB. Studies have shown the regenerative
capabilities of the OB bulb by way of the subventricular zone, which is
embryonic tissue containing proliferative cells that remain in adulthood
located in the lateral ventricles (Lepousez et al., 2013; Lledo and
Saghatelyan, 2005; Wilson et al., 2004). Furthermore, OB volume has been
shown to relate not only to olfactory function, but also to duration of
olfactory function loss in a cohort of non-PD patients that suffered
post-traumatic or post-infectious olfactory deficits (Haehner et al.,
2009). Our study showed a considerable difference in OB volume with
length of stimulation, supporting that neurogenesis can in fact happen
at the level of the OB that could have possibly been triggered by
stimulation in the basal ganglia, which has been reported in hippocampus
following DBS in rats (Chamaa et al., 2016).

Despite the demonstration of increased IWM and olfactory bulb volume
with STN-DBS in PD patients, there are some limitations to our study.
(1) Our sample size was only 30 PD patients, only nine of which
underwent long stimulation greater than 360 days. Furthermore, our
olfactory deficit control condition (ET patients) was limited to only
short stimulation, and the number of subjects was small (n = 3). A
larger study that includes a larger sample size of long stimulation
patients would provide a clearer picture of the long term effects of
STN-DBS. (2) Pre and post smell tests using the conventional UPSIT and
Sniffin Sticks Test were not availiable to correlate with changes seen
in OB volume with DBS. Identifying functional changes and relating those
to changes in subcortical brain regions would strengthen the results
found in this study. (3) We used ET patients with no known olfactory
deficits as our control since they represented a different movement
disorder treated with DBS. This allowed for a comparison between two
neurodegenerative disorder groups; however, to normalize the
measurements of the OB, using MRI's of healthy control subjects would
have aided the results of this study.

Future directions of this study are not limited to but include: (1)
gathering a larger sample size of PD patients that have acquired more
than 360 days of stimulation, along with a larger sample of ET patients
to compare to short, medium and long PD stimulation groups in order to
strengthen the results of this study, (2) a sample size of brain MRI's
of healthy brains to compare OB volumes of both PD and ET subjects, (3)
acquiring pre and post smell tests using UPSIT and Sniffin Sticks Test
to compare volumetric changes seen in cortical and subcortical brain
regions to functional changes, (4) using olfactory training in PD
patients that do not meet the criteria for DBS surgery, and measuring
changes in OB volume pre and post training, and (5) using vertex level
analysis of subcortical brain areas to inquire if there are any
morphological changes in these structures. The most hopeful future
direction that could stem from this study is the use of non-surgical
methods to recover olfactory function in both neurodegenerative disorder
groups, as well as non-neurodegenerative disorder groups that suffer
from olfactory dysfunction. Furthermore, studying the neuroplasticity of
the subcortical brain structures both volumetrically and morphologically
could possibly open doors for future treatment methods aiding in patient
recovery.

In conclusion, the significant increase in OB volume provides an
anatomical explanation for published literature that has demonstrated
positive olfactory responses in PD patients following STN-DBS. An
increase in OB volume following STN-DBS not only supports neuronal
plasticity, but also demonstrates the effects of STN-DBS on patient
recovery that go beyond basal ganglia structures, allowing for a wider
scope of its benefits in PD associated olfactory deficits, as well as
suggesting the possible success of non-surgical alternatives such as
olfactory training in PD patients not applicable for DBS, and non-PD
related olfactory deficits. Additionally, the significant positive
changes seen in IWM volume, and the trends observed in thalamic and OFC
volumes with length of stimulation reinforce the recovery of olfactory
function through the possible reestablishment of neuronal connections
and structural changes in subcortical regions involved in higher
olfactory processing.

\par\null

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