FUNCTIONAL OLFACTORY DEFICITS IN THE OLFACTORY SYSTEM OF ALZHEIMER'S DISEASE ...

147 Pages·2014·1.11 MB·English

The Pennsylvania State University

The Graduate School

College of Medicine
FUNCTIONAL OLFACTORY DEFICITS IN THE OLFACTORY SYSTEM OF
ALZHEIMER’S DISEASE AND MILD COGNITIVE IMPAIRMENT PATIENTS AS A
POTENTIAL DIAGNOSTIC MARKER
A Dissertation in
Neuroscience
by
Megha Vasavada
 2014 Megha Vasavada
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Doctor of Philosophy

August 2014
The dissertation of Megha Vasavada was reviewed and approved* by the following:

Qing X. Yang
Professor of Radiology and Neurosurgery
Dissertation Advisor
Chair of Committee

Paul J. Eslinger
Professor of Neurology

Ralph Norgren
Professor of Neural and Behavioral Sciences

Patricia Grigson
Professor of Neural and Behavioral Sciences
Co-Chair of Neuroscience Graduate Program

Colin Barnstable
Professor and Chair of Neural and Behavioral Sciences

*Signatures are on file in the Graduate School

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ABSTRACT
Alzheimer's disease affects 5.4 million individuals in the US, causing debilitating
memory and cognitive impairment. By the time the disease has clinically manifested itself, the
pathology has progressed to the neocortex. Currently, early diagnosis and understanding of
Alzheimer’s pathology on functional deficits is the key for the development of therapy. While
volumetric measurements of the hippocampus provide excellent diagnostic assistance, post-
mortem studies have shown that the earliest pathological markers of Alzheimer’s (amyloid beta
plaques and neurofibrillary tangles) are found first in olfactory areas of the brain. Clinically,
olfaction is affected in the earliest stages of Alzheimer’s disease and mild cognitive impaired
(MCI) patients, a group considered to be at the highest risk for Alzheimer’s. Often olfactory
deficits appear prior to the manifestation of cognitive symptoms.
Magnetic resonance imaging (MRI) provides the ability to noninvasively examine the
functional and structural changes that occur prior to presentation of behavioral symptoms in
individuals with Alzheimer’s disease and MCI. Therefore, in this dissertation, MRI techniques
were utilized to investigate the involvement of the primary olfactory cortex in Alzheimer’s
disease and MCI subjects, and to determine the sensitivity of these techniques as potential
diagnostic markers of disease. The same subjects were used in each analysis, therefore the subject
information, behavioral tests, and data collection is the same for each chapter.
In chapter 2, we used both volumetric and functional MRI (fMRI) measurements to study
the diagnostic potential by investigating the primary olfactory cortex. Behavioral tests, including
the University of Pennsylvania Smell Identification Test (UPSIT), as well as cognitive tests
demonstrated olfactory and memory impairments in both Alzheimer’s and MCI patient groups
(one-way Analysis of Variance (ANOVA), P < 0.0001). The volumetric MRI of the primary
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olfactory cortex showed decreased volume in both Alzheimer’s and MCI subjects compared with
age-matched normal controls (one-way ANOVA, P < 0.001). However in terms of both volume
measurement and behavioral performance, MCI values ranged between those of Alzheimer’s and
those of normal controls.
On the other hand, the olfactory fMRI results showed that activation signal change in the
primary olfactory cortex was significantly and nearly equally decreased in the Alzheimer’s and
MCI subjects when compared with normal controls (one-way ANOVA, P < 0.0001). This
suggests that although behavioral and volumetric measurement may be variable in MCI subjects,
their activation signal in the brain is already changing. We also established that combining the
UPSIT score, hippocampal volume, and activation signal change in the primary olfactory cortex
increases the diagnostic specificity and sensitivity of Alzheimer’s and MCI.
In chapter 3, the dominant role of the central olfactory system in Alzheimer’s and MCI
was established. Whether olfactory deficits in Alzheimer’s disease and MCI are more dominantly
due to peripheral or central olfactory system deterioration is unclear. While several studies agree
with central olfactory system deterioration based on observations, olfactory fMRI and
pathological evidence are inconclusive. The olfactory paradigm used in this study had a visual
cue ―Smell?‖ accompanied by either odor presentation or no odor presentation. The presentation
with ―Smell?‖ without congruent odor presentation allowed for analysis of the primary olfactory
cortex with an afferent stimulus that was perceived as equal to the subjects. The visual and motor
systems were not impaired in AD and MCI subjects therefore all no stimulus provided could be
perceived as unequal. We hypothesized that if the dysfunction is outside the brain similar
activation signal change would be observed in all three groups when the visual cue ―Smell?‖ was
presented without congruent odor and group differences would only be found when the visual cue
and odor were presented congruently (this is when stimulus is perceived differently between the
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groups since both MCI and AD subjects have trouble with olfactory function). Without an
odorant; however, the normal controls exhibited greater activation signal change compared with
both the Alzheimer’s and MCI subjects (one-way ANOVA, P < 0.05). This suggested central
olfactory system dominance; however, our study was not able to disprove concomitant
dysfunction of the peripheral olfactory system in Alzheimer’s and MCI.
In chapter 4, functional connectivity analysis was performed on our olfactory fMRI data
to learn further about the olfactory network. Functional connectivity is defined as the correlation
of interregional neural interactions during particular tasks or from spontaneous activity during
rest. We observed functional connectivity of the piriform was decreased to the striatum, thalamus,
and anterior cingulate cortex for both Alzheimer’s and MCI subjects (ANOVA, P < 0.001). The
Alzheimer’s group trended toward greater disconnection of the olfactory network compared with
MCI subjects, although the difference did not achieve statistical significance. The trend toward
preservation of connectivity in MCI subjects may explain their observed higher behavioral
function.
Therefore, we conclude that the central olfactory system is the dominant system involved
in Alzheimer’s and MCI patients, and is causing olfactory deficits. We demonstrated that fMRI
showed decreased activation in the primary olfactory cortex of MCI subjects, and was in fact
similar to the decreased activation of Alzheimer’s disease subjects. This indicates consistent early
functional changes in the brains of MCI subjects despite variability in their behavioral and
volumetric measurements. fMRI, thus has great potential to be used as an early diagnostic marker
in Alzheimer’s disease and MCI, and may also be used to study the progression of disease.

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TABLE OF CONTENTS
List of Figures .......................................................................................................................... ix
List of Tables ........................................................................................................................... x
List of Abbreviations ............................................................................................................... xi
Acknowledgements .................................................................................................................. xiii

Chapter 1 Introduction ............................................................................................................. 1
1.1 Alzheimer’s Disease .................................................................................................. 1
1.2 Mild Cognitive Impairment........................................................................................ 5
1.3 Anatomy of the Olfactory System .............................................................................. 5
1.4 Olfactory Deficits in Alzheimer’s Disease................................................................. 8
1.5 Pathological Changes in Olfactory Areas .................................................................. 11
1.5.1 Neurofibrillary Tangles ...................................................................................... 11
1.5.2 Amyloid Beta Plaques ....................................................................................... 12
1.5.3 Atrophy .............................................................................................................. 12
1.6 Neuroimaging in Alzheimer’s Disease ...................................................................... 13
1.5.1 Volumetric Studies ............................................................................................ 15
1.5.2 Functional MRI Studies ..................................................................................... 16
1.7 Central versus Peripheral Olfactory dysfucntion in Alzheimer’s Disease ................. 18
1.8 Rationale .................................................................................................................... 19
1.9 References .................................................................................................................. 22

Chapter 2 Functional and structural degeneration of the primary olfactory cortex in AD
and MCI ........................................................................................................................... 33
2.1 Abstract .................................................................................................................... 33
2.2 Introduction ................................................................................................................ 34
2.3 Methods ...................................................................................................................... 36
2.3.1 Study Cohort ...................................................................................................... 36
2.3.2 Behavioral Tests ................................................................................................ 38
2.3.3 Olfactory Stimulation Paradigm ........................................................................ 38
2.3.4 Imaging Protocol ................................................................................................ 41
2.3.5 fMRI Data Processing and Analysis .................................................................. 41
2.3.6 Region of Interest Analysis of the Primary Olfactory Cortex and
Hippocampus .................................................................................................... 42
2.4 Results ........................................................................................................................ 44
2.4.1 Demographics and Behavioral Results .............................................................. 44
2.4.2 Aging Effect ....................................................................................................... 46
2.4.3 Olfactory fMRI .................................................................................................. 46
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2.4.4 Relations of Brain Volume and Activation Volume in the Primary
Olfactory Cortex and Hippocampus ............................................................... 49
2.4.5 Correlation Between the Behavioral and MRI Results ...................................... 51
2.4.6 Logistic Regression Analysis ............................................................................. 53
2.5 Discussion .................................................................................................................. 55
2.6 References .................................................................................................................. 60

Chapter 3 Functional Connectivity of the Piriform is Disrupted in AD and MCI ................... 65
3.1 Abstract .................................................................................................................... 65
3.2 Introduction ................................................................................................................ 66
3.3 Methods ...................................................................................................................... 68
3.3.1 Study Cohort ...................................................................................................... 68
3.3.2 Behavioral Tests ................................................................................................ 68
3.3.3 Olfactory Stimulation Paradigm ........................................................................ 69
3.3.4 Imaging Protocol ................................................................................................ 69
3.3.5 fMRI Data Processing and Analysis .................................................................. 69
3.3.6 Region of Interest Analysis of the Primary Olfactory Cortex and
Hippocampus .................................................................................................... 70
3.4 Results ........................................................................................................................ 70
3.4.1 Demographics and Behavioral Results .............................................................. 70
3.4.2 Aging Effect ....................................................................................................... 71
3.4.3 Olfactory fMRI .................................................................................................. 71
3.4.4 Correlation Between the Behavioral and MRI Results ...................................... 75
3.4.5 Four Lavender Concentrations ........................................................................... 75
3.5 Discussion .................................................................................................................. 78
3.6 References .................................................................................................................. 86

Chapter 4 Central Olfactory Dysfunction is the Dominant Cause of Olfactory Deficits in
AD and MCI .......................................................................................................... 90
4.1 Abstract .................................................................................................................... 90
4.2 Introduction ................................................................................................................ 91
4.3 Methods ...................................................................................................................... 93
4.3.1 Study Cohort ...................................................................................................... 93
4.3.2 Behavioral Tests ................................................................................................ 93
4.3.3 Olfactory Stimulation Paradigm ........................................................................ 93
4.3.4 Imaging Protocol ................................................................................................ 94
4.3.5 Functional Connectivity Analysis ...................................................................... 94
4.3.6 Statistical Analysis ............................................................................................. 97
4.4 Results ........................................................................................................................ 97
4.4.1 Demographics and Behavioral Results .............................................................. 97
4.4.2 Functional Connectivity of the Piriform ............................................................ 97
4.4.3 Lateralization of Connectivity ........................................................................... 101
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4.4.4 Correlation of Functional Connectivity to the University of Pennsylvania
Smell Identification Test and Cognitive Tests .................................................... 105
4.5 Discussion .................................................................................................................. 107
4.6 References .................................................................................................................. 112

Chapter 5 Conclusion ............................................................................................................... 121
5.1 Olfactory System in Alzheimer’s Disease ............................................................... 121
5.2 Olfactory fMRI Paradigm ........................................................................................ 122
5.3 Central Olfactory System Dysfunction Causes Olfactory Symptoms ...................... 123
5.4 Volumetric Measurements ....................................................................................... 124
5.5 Olfactory fMRI ........................................................................................................ 125
5.6 Future Studies .......................................................................................................... 126
5.7 Summary .................................................................................................................. 127
5.8 References ................................................................................................................ 129

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LIST OF FIGURES
Figure 1-1. Pathological stages. ............................................................................................... 4
Figure 1-2. Human olfactory system........................................................................................ 7
Figure 2-1. Olfactory fMRI paradigm...................................................................................... 40
Figure 2-2. 3D display of the primary olfactory cortex. .......................................................... 43
Figure 2-3. Olfaction and cognitive tests. ................................................................................ 45
Figure 2-4. Activation in the primary olfactory cortex and hippocampus. .............................. 47
Figure 2-5. Activation volume in AD and MCI. ...................................................................... 48
Figure 2-6 Structural and functional changes .......................................................................... 50
Figure 2-7. Receiver operating characteristic (ROC) curves. .................................................. 54
Figure 3-1. Olfactory activation maps. .................................................................................... 73
Figure 3-2. Activated volume. ................................................................................................. 74
Figure 3-3. Four concentrations ............................................................................................... 77
Figure 3-4. Olfactory fMRI paradigm with and without olfactory stimulation ....................... 81
Figure 3-5. Hemodynamic response function (HRF).. ............................................................. 83
Figure 4-1. Functional connectivity of the piriform. ............................................................... 99
Figure 4-2. Functional connectivity disruption ........................................................................ 100
Figure 4-3. Olfactory network matrix. ..................................................................................... 102
Figure 4-4. Lateralization of olfactory network ....................................................................... 104
Figure 4-5. Correlations between smell and functional connectivity. ..................................... 106

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LIST OF TABLES
Table 1-1 Clinical stages of Alzheimer’s disease .................................................................... 3
Table 2-1 Demographic and behavioral data of the study cohort ............................................ 37
Table 2-2. Correlations between behavioral and MRI measurements of all subjects.. ............ 52
Table 3-1. Correlations between behavioral and imaging measurements of all subjects.. ...... 76
Table 4-1. Anatomically defined regions of interest. ............................................................... 96

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