- Email: [email protected]
Diffusion tensor imaging of the hippocampus in chronic cigarette smokers
Accepted Manuscript Title: Diffusion tensor imaging of the hippocampus in chronic cigarette smokers Author: Ihsan Yuce Mecit Kantarci Papatya Keles Hakki Yesilyurt Hayri Ogul Hatice Yuce Suat Eren PII: DOI: Reference:
S0720-048X(16)30175-9 http://dx.doi.org/doi:10.1016/j.ejrad.2016.06.003 EURR 7489
To appear in:
European Journal of Radiology
Received date: Revised date: Accepted date:
6-3-2016 29-5-2016 5-6-2016
Please cite this article as: Yuce Ihsan, Kantarci Mecit, Keles Papatya, Yesilyurt Hakki, Ogul Hayri, Yuce Hatice, Eren Suat.Diffusion tensor imaging of the hippocampus in chronic cigarette smokers.European Journal of Radiology http://dx.doi.org/10.1016/j.ejrad.2016.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Diffusion Tensor Imaging of the Hippocampus in Chronic Cigarette Smokers
Ihsan Yuce1 (MD), Mecit Kantarci1 (MD, PhD), Papatya Keles2 (MD), Hakki Yesilyurt2 (MD), Hayri Ogul (MD) 1, Hatice Yuce3 (MD), Suat Eren1 (MD)
1Ataturk
University, School of Medicine, Department of Radiology, Erzurum, Turkey
2Ataturk
University, School of Medicine, Department of Anatomy, Erzurum, Turkey
3Ataturk
University, School of Medicine, Department of Psychiatry, Erzurum, Turkey
Corresponding author
: Ihsan Yuce M.D.
Address
: Ataturk University School of Medicine, Department of Radiology, Erzurum, Turkey
Telephone
: +90 507 036 6910 (GSM) : +90 442 2361212-6960 (work)
Fax
: +90 (442) 2361301
E-mail
: [email protected]
2
Abstract Purpose: Previous studies have demonstrated neuronal dis-integrity in chronic smokers using diffusion tensor imaging (DTI). However, assessment of hippocampal DTI has not been performed in this group. The purpose of this study was to investigate the hippocampal integrity in chronic smokers and non-smokers and to compare this to memory performance. Methods: We used in vivo DTI to measure the differences in hippocampal integrity between 88 chronic smoker and 88 non-smoking subjects. DTI was performed on a 3 tesla scanner. We administered a verbal learning test to assess new verbal learning capabilities. The immediate recall (IR) was administered immediately after test procedure and delayed recall (DR) after 15 minutes. Results: Mean values of fractional anisotropy (FA) for non-smokers and smokers were 0.46 and 0.40, respectively (p<0.05). Mean IR word number for non-smokers and smokers was 11.6, 9.04, respectively (p<0.05). The mean word number of DR for non-smokers and smokers was 10.2, 7.2, respectively (p<0.05). Conclusions: This is the first study of hippocampal DTI assessment in the chronic smokers. We found that decreased hippocampal FA associated with neuronal disintegrity and worse memory performance in chronic smokers than non-smokers. We also found a low positive correlation hippocampal FA values with memory performance in nonsmoking group
Keywords: Hippocampus, diffusion tensor, fractional anisotropy, memory, smoking
Introduction Chronic cigarette smoking negatively affects the central nervous system (CNS) despite the acute effects on nicotine on neurons [1]. The adverse effects of chronic cigarette smoking on the CNS have been evaluated by several authors and smoking has been shown to adversely affect cognitive performance [2-5]. The most of previous studies with healthy, young-to-middle-aged adults found no significant differences between smokers and non-smokers on total hippocampal volume [2,6,7]. On the other hand, many studies have shown that chronic cigarette smoking is related to worse cognitive outcome [3-5]. Therefore, it is clear that the
3
need for studies using aside from hippocampal volume measurement for assessment of worse cognitive outcomes in chronic smokers. DTI is a magnetic resonance imaging (MRI) technique that acquires information about tissue structure by random water diffusion. DTI enables investigation of the white matter integrity. Two major values have been used for DTI data including the mean diffusivity (MD) and FA. The MD is associated with the size of self-diffusion of water while FA is related to the directional dependence of water diffusion. MD has closely related a destructive process in the gray matter while FA indicates microstructural integrity and organization of neurons within the brain regions [8]. Decreased hippocampal FA of the has been related with poor navigational skills [9,10]. The purpose of our study was to compare DTI measurements of the hippocampus (as a structure responsible for episodic memory) and memory performance in chronic smokers and non-smokers.
Materials and Methods Study Design This was a prospective cohort study. Data were obtained from 176 individuals including 88 smokers and 88 non-smokers between 20-45 from January 2015 to October 2015. All participants provided written informed consent and the study protocol was approved by the institutional Medical Ethics Committee. The inclusion criteria were healthy subjects between 20-45 years of age; in the smoking cohort 10 years of smoking (at least one packet per day) was required. Individuals were excluded if they reported any history of mental retardation, a medical (including diabetes mellitus, hypertension and other cardiovascular diseases) or neurological disorder (including stroke and transient ischemic attack), drug abuse or dependence, psychiatric illness, claustrophobia or other MRI contraindications. Memory Testing To avoid the potential acute effects of nicotine stimulation, smoking participants were asked to not smoke just before the procedure. We performed a 20-word verbal learning test for memory performance based on Rey's Auditory Verbal Learning Test [11]. It was used to assess the ability to learn new verbal content. Persons were presented a sequence of 20 Turkish words in triplicate and then tested for IR in which
4
they were asked to repeat as many of these words as possible. Free DR was tested 15 min later. A Scale of IR was the sum of the number of words remembered immediately after memory testing. The number of words remembered after 15 minutes was the DR. Diffusion Tensor Imaging All MRI studies were acquired a 3 tesla MRI scanner (Magnetom Skyra; Siemens Healthcare, Berlin, Germany). A three-dimensional T1 weighted magnetisationprepared rapid gradient-echo (MP-RAGE) sequence was used for conventional MRI and a sensitivity-encoded single-shot echo-planar imaging (SENSE- EPI) sequence was used for DTI. Sequence parameters for conventional MRI included: TR/TE = 1420/2.98 ms; flip angle 9; voxel size 1x1x1 mm, matrix 186x256, slice thickness 1.15 mm, FOV 256x90. The DTI protocol parameters as follows: TR/TE = 3700/95 ms, voxel size 1.9x1.9x4 mm, matrix 128x128, slice thickness 4 mm, FOV 240x100. We used 20 diffusion-weighted scans with a b-value of 1000 s/mm2. Commonly using measures derived from DTI include FA, MD, axial diffusivity (AD), and radial diffusivity (RD). FA defines the degree of directional coherence of diffusion and is related to axonal diameter, density, and myelination. MD is the mean value of all the eigenvalues. Other metrics including AD and RD mean the magnitude of diffusion parallel and perpendicular to the axonal bundle, respectively. Decrease in FA can be caused by increase in RD, typically related to demyelination, or by decrease in AD, which is thought to exhibit axonal injury. Post processing of DTI data The DTI data was evaluated by the same physician (IY) who had six years of experience. The resulting MR images were assessed to a specific workstation (Syngo via Neuro 3D, Siemens Medical Imaging, Erlangen, Germany). Circular voxels of interest (VOI, its diameter was 1 cm) were placed on the bilateral hippocampal head to calculate and analyze the DTI values (Figure 1, 2). Mean values of the FA, MD, AD and RD of bilateral hippocampal head were automatically calculated by the software. In addition, bilateral hippocampal volumes were calculated by manual tracing method using software (Vitrea, Toshiba, Japan) (Figure 3).
Statistical Analysis
5
All statistical analysis was performed using MedCalc software (version 12.2.1.0, MedCalc, Mariakerke, Belgium). In both groups (smoking and non- smoking), the mean values of FA, MD, AD and RD were assessed by Student’s t-test in terms of statistical significance. In addition, immediate recall and delayed recall values (number of words remembered) were evaluated by Student’s t-test in both groups. Pearson correlation coefficients were used to measure the impact of age, hippocampal volume and DTI measurements. The correlation of memory performance with DTI values of hippocampal head was evaluated via Pearson correlation coefficients. The strongest relation is indicated by a correlation coefficient of -1 or 1. A p value of less than 0.05 showed a statistically significant difference. Results The smoking group included 88 persons (44 male and 44 female), and the nonsmoking group includes 88 persons (44 male and 44 female). The mean age of the non-smokers was 28.7 ± 7.54 years (age range: 22 - 44 years). The mean age of smokers was: 32.4 ± 5.47 years (age range: 24 - 43 years). Characteristics of the study sample are given in Table 1. Mean values of FA for non-smokers and smokers were 0.46 and 0.40, respectively (p<0,0001) (Figure 4, Table 2). Mean values of MD for non-smokers and smokers were 0.0026 and 0.0027, respectively (p=0,0383) (Figure 5, Table 2). In addition, mean values of AD for non-smokers and smokers were 0.0013 and 0.0014, respectively (p<0,001). The mean values of RD for nonsmokers and smokers were 0.0020 and 0.0022, respectively (p=0,0014). Mean word numbers of IR for non-smokers and smokers were 11.6 and 9.04, respectively (p<0,0001) (Figure 6, Table 2). The mean word numbers of DR for non-smokers and smokers were 10.2 and 7.2, respectively (P<0,0001) (Figure 7, Table 2). Mean memory performance were average of the IR and DR values for each group. There was statistically significant low positive correlation about mean memory performance with FA values in nonsmoking group while there was no statistically significant correlation about mean memory performance with FA values in smoking group (Table 3, Figure 8). On the other hand, there was no statistically significant positive or negative correlation about remaining values (MD, RD and AD) with memory performance in both groups. Correlations between age, hippocampal volumes and DTI measures are given in Table 4. We did not find any statistically significant differences between smokers and non-smokers in terms of education level, age, gender and hippocampal volume (p>0.05).
6
Discussion The present study shows that the mean value of hippocampal FA is higher and mean value of hippocampal MD is lower in non-smokers than chronic smokers (p<0.05). In addition, memory performance of chronic smokers is less than non-smokers (p<0.05) and our study shows statistically significant low correlation about memory performance with FA values in nonsmoking group. Although many studies have been performed regarding hippocampal DTI measurements in Alzheimer disease, vascular dementia, epilepsy, age-related changes and cerebral small vessel disease, to the best of our knowledge, our study is the first report that showed hippocampal DTI measurements with memory performance in chronic smokers [12-17]. We hypothesized that chronic smoking would adversely affect the hippocampal structural integrity and memory performance. Acute nicotine administration provides stimulates cognitive abilities including attention and memory due to multiple interactions on the CNS. However, the effect of chronic cigarette smoking on CNS function is different than the acute effects of smoking. Many studies have shown that chronic cigarette smoking is related to worse cognitive outcome [3-5]. DTI data including MD and FA have generally been used to assess neuronal integrity [18]. Previous studies found that lower FA and higher MD values are associated with neuronal dis-integrity [13,19]. One of them, Heijer et al also found that a higher MD of the hippocampus is associated with poor memory performance [13]. In another study, Norden et al found reverse relationships between verbal memory performance and hippocampal MD [17]. Our results did not show negative correlation between MD and memory performance. However, we found that lower FA values and higher MD values in the smoking group compared with nonsmoking group. The hippocampus is located in the medial temporal lobe. This is the structure responsible for episodic memory function. Many studies have assessed the relationship between the memory performance and hippocampal volume. They have found that hippocampal volumes positively correlate with memory function [20-22]. On the other hand, hippocampal dis-integrity caused by decreased FA has been correlated with poor navigational skills with a slowed response time on a visual task [9,10]. Our study also showed decreased hippocampal FA and worse memory performance in chronic smokers than non-smokers. In addition we did not find any significant correlation between FA and hippocampal volume in both groups (Table 3).
7
These findings may explain decreased cognitive function in chronic smokers despite normal hippocampal volume in healthy young-to-middle-aged adults. Higher levels of FA are typically considered to indicate better integrity of the white matter. Previous studies of drug abusers showed that a decrease in FA in the brain white matter was associated with lower neuronal integrity [22]. Our results also showed that FA values in the hippocampal head were significantly higher in the nonsmoking group versus the smoking group (p<0.05). However, Paul et al. found significantly high FA in the corpus callosum of smokers versus non-smokers [1]. In addition, they show that microstructural integrity associated with elevated FA may not be beneficial due to abnormal angiogenesis and arteriogenesis because of nicotinic stimulation of the cholinergic system. Similarly, Liao et al. found increased FA in chronic smokers, and they suggested that increased FA in the fronto-parietal white matter might be associated with inflammatory changes and axonal damage in chronic cigarette smokers [23]. The final conclusion is that we need more studies on this topic. Our study has a few limitations. First, our results will need to be confirmed in a larger series because our study population was relatively small. This may be the reason for no significant correlation among values outside of FA (very small values such as MD, AD, RD). Second, there are several memory centers in the brain including the hippocampus, prefrontal cortex, amygdala, basal ganglia, cerebellum, etc. We assessed only the hippocampal head by DTI to assess the memory-associated brain region. However, the FA is more difficult to interpret due to hippocampal gray matter density. Future studies should investigate other centers of the memory. Third, we used a manual tracing system on the hippocampal head. It may not have been fully covered because it has an atypical morphology. Fourth, our results may be affected partial volume effects because of hippocampus near the lateral ventricles. Conclusions In conclusion, this study is the first experience about hippocampal DTI measurements in chronic smokers. We found decreased hippocampal FA and worse memory performance in chronic smokers versus non-smokers and we found a low positive correlation hippocampal FA values with memory performance in nonsmoking group. However, additional studies are needed regarding hippocampal neuronal integrity using DTI. Conflict of Interest
8
We declare that we have no conflict of interest.
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Taylor G, Clark RC, Pogun S, Gordon E, Chronic cigarette smoking and the microstructural integrity of white matter in healthy adults: a diffusion tensor imaging study, Nicotine Tob Res. 10 (2008) 137-147 2.
Brody AL, Mandelkern MA, Jarvik ME, Lee GS, Smith EC, Huang JC, Bota
RG, Bartzokis G, London ED, Differences between smokers and nonsmokers in regional gray matter volumes and densities, Biological Psychiatry. 55 (2004) 77–84 3.
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cognitive performance in the community elderly: A longitudinal study, Journal of Geriatric Psychiatry and Neurology. 16 (2003) 18– 22 4.
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cognitive function in the elderly without dementia, Neurology. 65 (2005) 870–875 5.
Paul R, Brickman AM, Cohen R, Williams LM, Niaura R, Pogun S, Clark CR,
Gunstad J, Gordon E, Cognitive status of young and older cigarette smokers: Data from the International Brain Database, Journal of Clinical Neuroscience. 13 (2006) 457–465 6.
Gallinat J, Meisenzahl E, Jacobsen LK, Kalus P, Bierbrauer J, Kienast T,
Witthaus H, Leopold K, Seifert F, Schubert F, Staedtgen M. Smoking and structural brain deficits: a volumetric MR investigation. Eur J Neurosci. 2006; 24:1744–1750 7.
Yu R, Zhao L, Lu L. Regional grey and white matter changes in heavy male
smokers. PLoS One. 2011; 6:e27440 8.
Basser PJ, Mattiello J, LeBihan D, MR diffusion tensor spectroscopy and
imaging, Biophys J. 66 (1994) 259–267 9.
Laria G, Lanyon LJ, Fox CJ, Giaschi D, Barton JS, Navigational skills correlate
with hippocampal fractional anisotropy in humans, Hippocampus. 18 (2008) 335– 339
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Lebel C, Gee M, Camicioli R, Wieler M, Martin W, Beaulieu C,
Diffusion
tensor imaging of white matter tract evolution over the lifespan, NeuroImage. 60 (2012) 340–352 11.
Brand N, Jolles J, Learning and retrieval rate of words presented auditorily and
visually, J Gen Psychol. 112 (1985) 201–210 12.
Fellgiebel A, Yakushev I, Diffusion tensor imaging of the hippocampus in MCI
and early Alzheimer's disease, J Alzheimers Dis. 26 (2011) 257-262 13.
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Av, Krestin GP, Hofman A, Niessen WJ, Breteler MM, Structural and diffusion MRI measures of the hippocampus and memory performance, Neuroimage. 63 (2012) 1782-1789 14.
Ostojic J, Kozic D, Pavlovic A, Semnic M, Todorovic A, Petrovic K, Covickovic-
Sternic N, Hippocampal diffusion tensor imaging microstructural changes in vascular dementia. Acta Neurol Belg J. (2015) Jan 3 [Epub ahead of print] 15.
Oguz KK, Tezer I, Sanverdi E, Has AC, Bilginer B, Dolgun A, Saygi S, Effect
of patient sex on white matter alterations in unilateral medial temporal lobe epilepsy with hippocampal sclerosis assessed by diffusion tensor imaging, AJNR Am J Neuroradiol. 34 (2013) 1010-1015 16.
Rogalski E, Stebbins GT, Barnes CA, Murphy CM, Stoub TR, George S,
Ferrari C, Shah RC, deToledo-Morrell L, Age-related changes in parahippocampal white matter integrity: a diffusion tensor imaging study, Neuropsychologia. 50 (2012) 1759-1765 17.
van Norden AG, de Laat KF, Fick I, van Uden IW, van Oudheusden LJ, Gons
RA, Norris DG, Zwiers MP, Kessels RP, de Leeuw FE, Diffusion tensor imaging of the hippocampus and verbal memory performance: the RUN DMC study, Hum Brain Mapp. 33 (2012) 542-551 18.
Alexander AL, Lee JE, Lazar M, Field AS, Diffusion tensor imaging of the
brain, Neurotherapeutics. 4 (2007) 316-329 19.
Pfefferbaum A, Sullivan EV, Hedehus M, Adalsteinsson E, Lim KO, Moseley In
vivo detection and functional correlates of white matter microstructural disruption in chronic alcoholism. Alcoholism. Clinical and Experimental Research 24 (2000) 1214–1221
10
20.
Hackert VH, den Heijer T, Oudkerk M, Koudstaal PJ, Hofman A,Breteler MM,
Hippocampal
head
size
associated
with
verbal
memory performance
in
nondemented elderly, NeuroImage. 17 (2002) 1365–1372 21.
Lye TC, Piguet O, Grayson DA, Creasey H, Ridley LJ, Bennett HP, Broe GA,
Hippocampal size and memory function in the ninth and tenth decades of life: the Sydney Older Persons Study, J Neurol Neurosurg Psychiatry. 75 (2004) 548–554 22.
Petersen RC, Jack Jr, Xu YC, Waring SC, O'Brien PC, Smith GE, Ivnik RJ,
Tangalos EG, Boeve BF, Kokmen E, Memory and MRI-based hippocampal volumes in aging and AD, Neurology. 54 (2000) 581–587 23.
Liao Y, Tang J, Deng Q, Deng Y, Luo T, Wang X, Chen H, Liu T, Chen X,
Brody AL, Hao W Bilateral fronto-parietal integrity in young chronic cigarette smokers: a diffusion tensor imaging study. PLoS One (2011): 6:e26460.
11
Figure Legends Figure 1: Three-dimensional (3-D) image shows bilateral hippocampal head (white arrows) in whole brain white matter tracts (A). In addition, another 3-D image shows white matter tracts of hippocampal head (dashed arrows) (B).
12
Figure 2: Coronal T1 weighted image shows VOI’s on hippocampal heads. VOI’s involve of both gray and white matter areas. VOI: Voxel of interest.
13
Figure 3: Sagittal T1 weighted image (upper) and 3-D image (below) show hippocampal volume measurement using software (Vitrea, Toshiba, Japan).
14
Figure 4: The comparative graphic of mean values of FA for non-smokers and smokers.
15
Figure 5: The comparative graphic of mean values of MD for non-smokers and smokers.
16
Figure 6: The comparative graphic of mean word number of IR for non-smokers and smokers.
17
Figure 7: The comparative graphic of mean word number of DR for non-smokers and smokers
18
Figure 8: A correlation graphic of FA values with mean memory performance in nonsmoking group.
19
Table 1: Characteristics of the study sample
Patient characteristics
n=176
Mean age, years (SD)
30.55 (6.5)
Gender (%men)
88 (50)
Education (%) Low education
0 (0)
Middle education
121 (68)
High education
55 (31)
Absolute hippocampal volume, ml Right
3.3 (0.5)
Left
3.4 (0.6)
FA of hippocampus
0.4363 (0.06)
MD of hippocampus
0.0026 (0.00029)
Memory Performance Immediate recall
10.34 (2.98)
Delayed Recall
8.77 (3.82)
Values are mean or number (%)
20
Table 2: A detailed table of non-smoking and smoking groups Non-smoking Group
FA
Smoking group Cl
Mean
SD
Mean
SD
0.4671
0.0445 0.4577 to 0.47 0.4055 0.0037 0.3925 to 0.41 p 65
MD
0.0026
0.0013
28
IR
0.0020
11.6364
2.58
=
0.0463
0.0002 0.002 to 0.002 0.0022 0.0003 0.0022 to 0.00 p
2.8047 11.04 to 12.23 9.04
=
0.0239
0.0001 0.0013 to 0.00 0.0014 0.0003 0.0013 to 0.00 p 14
<
0.0001
0.0003 0.0025 to 0.00 0.0027 0.0002 0.0025 to 0.00 p
13 RD
CL
85
26 AD
P value
22
0.0014
7.0 to 10.0
p
=
<
0.0001 DR
10.2727
4.0930 9.40 to 11.13
7.27
2.84
6.0 to 8.0
p 0.0001
<
21
Table 3: Correlation scores of memory performances with FA values in both groups.
Group
FA-Mean
r score
Memory
95%
Confidence P value
interval for r
Performance Non-smoking
Low
positive +0.2480
0,04072 to 0,4349
p<0.05
-0,2283 to 0,1904
p>0.05
correlation Smoking
No correlation
-0,01979
22
Table 4: Correlations between age, hippocampal volumes and DTI measures for
Smoking group
Age
Volume (mean)
FA (mean)
MD (mean)
Age
-
-0.25
-0.401
0.381
-
-0.01
-0.01
-
-0.13
Volume (mean) FA (mean) MD (mean)
-
both groups. Non-smoking group
Age
Volume (mean)
FA (mean)
MD (mean)
Age
-
-0.231
-0.381
0.361
-
-0.03
-0.03
-
-0.14
Volume (mean) FA (mean) MD (mean) 1
p<0.05
1
p<0.05
-
S0720-048X(16)30175-9 http://dx.doi.org/doi:10.1016/j.ejrad.2016.06.003 EURR 7489
To appear in:
European Journal of Radiology
Received date: Revised date: Accepted date:
6-3-2016 29-5-2016 5-6-2016
Please cite this article as: Yuce Ihsan, Kantarci Mecit, Keles Papatya, Yesilyurt Hakki, Ogul Hayri, Yuce Hatice, Eren Suat.Diffusion tensor imaging of the hippocampus in chronic cigarette smokers.European Journal of Radiology http://dx.doi.org/10.1016/j.ejrad.2016.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Diffusion Tensor Imaging of the Hippocampus in Chronic Cigarette Smokers
Ihsan Yuce1 (MD), Mecit Kantarci1 (MD, PhD), Papatya Keles2 (MD), Hakki Yesilyurt2 (MD), Hayri Ogul (MD) 1, Hatice Yuce3 (MD), Suat Eren1 (MD)
1Ataturk
University, School of Medicine, Department of Radiology, Erzurum, Turkey
2Ataturk
University, School of Medicine, Department of Anatomy, Erzurum, Turkey
3Ataturk
University, School of Medicine, Department of Psychiatry, Erzurum, Turkey
Corresponding author
: Ihsan Yuce M.D.
Address
: Ataturk University School of Medicine, Department of Radiology, Erzurum, Turkey
Telephone
: +90 507 036 6910 (GSM) : +90 442 2361212-6960 (work)
Fax
: +90 (442) 2361301
: [email protected]
2
Abstract Purpose: Previous studies have demonstrated neuronal dis-integrity in chronic smokers using diffusion tensor imaging (DTI). However, assessment of hippocampal DTI has not been performed in this group. The purpose of this study was to investigate the hippocampal integrity in chronic smokers and non-smokers and to compare this to memory performance. Methods: We used in vivo DTI to measure the differences in hippocampal integrity between 88 chronic smoker and 88 non-smoking subjects. DTI was performed on a 3 tesla scanner. We administered a verbal learning test to assess new verbal learning capabilities. The immediate recall (IR) was administered immediately after test procedure and delayed recall (DR) after 15 minutes. Results: Mean values of fractional anisotropy (FA) for non-smokers and smokers were 0.46 and 0.40, respectively (p<0.05). Mean IR word number for non-smokers and smokers was 11.6, 9.04, respectively (p<0.05). The mean word number of DR for non-smokers and smokers was 10.2, 7.2, respectively (p<0.05). Conclusions: This is the first study of hippocampal DTI assessment in the chronic smokers. We found that decreased hippocampal FA associated with neuronal disintegrity and worse memory performance in chronic smokers than non-smokers. We also found a low positive correlation hippocampal FA values with memory performance in nonsmoking group
Keywords: Hippocampus, diffusion tensor, fractional anisotropy, memory, smoking
Introduction Chronic cigarette smoking negatively affects the central nervous system (CNS) despite the acute effects on nicotine on neurons [1]. The adverse effects of chronic cigarette smoking on the CNS have been evaluated by several authors and smoking has been shown to adversely affect cognitive performance [2-5]. The most of previous studies with healthy, young-to-middle-aged adults found no significant differences between smokers and non-smokers on total hippocampal volume [2,6,7]. On the other hand, many studies have shown that chronic cigarette smoking is related to worse cognitive outcome [3-5]. Therefore, it is clear that the
3
need for studies using aside from hippocampal volume measurement for assessment of worse cognitive outcomes in chronic smokers. DTI is a magnetic resonance imaging (MRI) technique that acquires information about tissue structure by random water diffusion. DTI enables investigation of the white matter integrity. Two major values have been used for DTI data including the mean diffusivity (MD) and FA. The MD is associated with the size of self-diffusion of water while FA is related to the directional dependence of water diffusion. MD has closely related a destructive process in the gray matter while FA indicates microstructural integrity and organization of neurons within the brain regions [8]. Decreased hippocampal FA of the has been related with poor navigational skills [9,10]. The purpose of our study was to compare DTI measurements of the hippocampus (as a structure responsible for episodic memory) and memory performance in chronic smokers and non-smokers.
Materials and Methods Study Design This was a prospective cohort study. Data were obtained from 176 individuals including 88 smokers and 88 non-smokers between 20-45 from January 2015 to October 2015. All participants provided written informed consent and the study protocol was approved by the institutional Medical Ethics Committee. The inclusion criteria were healthy subjects between 20-45 years of age; in the smoking cohort 10 years of smoking (at least one packet per day) was required. Individuals were excluded if they reported any history of mental retardation, a medical (including diabetes mellitus, hypertension and other cardiovascular diseases) or neurological disorder (including stroke and transient ischemic attack), drug abuse or dependence, psychiatric illness, claustrophobia or other MRI contraindications. Memory Testing To avoid the potential acute effects of nicotine stimulation, smoking participants were asked to not smoke just before the procedure. We performed a 20-word verbal learning test for memory performance based on Rey's Auditory Verbal Learning Test [11]. It was used to assess the ability to learn new verbal content. Persons were presented a sequence of 20 Turkish words in triplicate and then tested for IR in which
4
they were asked to repeat as many of these words as possible. Free DR was tested 15 min later. A Scale of IR was the sum of the number of words remembered immediately after memory testing. The number of words remembered after 15 minutes was the DR. Diffusion Tensor Imaging All MRI studies were acquired a 3 tesla MRI scanner (Magnetom Skyra; Siemens Healthcare, Berlin, Germany). A three-dimensional T1 weighted magnetisationprepared rapid gradient-echo (MP-RAGE) sequence was used for conventional MRI and a sensitivity-encoded single-shot echo-planar imaging (SENSE- EPI) sequence was used for DTI. Sequence parameters for conventional MRI included: TR/TE = 1420/2.98 ms; flip angle 9; voxel size 1x1x1 mm, matrix 186x256, slice thickness 1.15 mm, FOV 256x90. The DTI protocol parameters as follows: TR/TE = 3700/95 ms, voxel size 1.9x1.9x4 mm, matrix 128x128, slice thickness 4 mm, FOV 240x100. We used 20 diffusion-weighted scans with a b-value of 1000 s/mm2. Commonly using measures derived from DTI include FA, MD, axial diffusivity (AD), and radial diffusivity (RD). FA defines the degree of directional coherence of diffusion and is related to axonal diameter, density, and myelination. MD is the mean value of all the eigenvalues. Other metrics including AD and RD mean the magnitude of diffusion parallel and perpendicular to the axonal bundle, respectively. Decrease in FA can be caused by increase in RD, typically related to demyelination, or by decrease in AD, which is thought to exhibit axonal injury. Post processing of DTI data The DTI data was evaluated by the same physician (IY) who had six years of experience. The resulting MR images were assessed to a specific workstation (Syngo via Neuro 3D, Siemens Medical Imaging, Erlangen, Germany). Circular voxels of interest (VOI, its diameter was 1 cm) were placed on the bilateral hippocampal head to calculate and analyze the DTI values (Figure 1, 2). Mean values of the FA, MD, AD and RD of bilateral hippocampal head were automatically calculated by the software. In addition, bilateral hippocampal volumes were calculated by manual tracing method using software (Vitrea, Toshiba, Japan) (Figure 3).
Statistical Analysis
5
All statistical analysis was performed using MedCalc software (version 12.2.1.0, MedCalc, Mariakerke, Belgium). In both groups (smoking and non- smoking), the mean values of FA, MD, AD and RD were assessed by Student’s t-test in terms of statistical significance. In addition, immediate recall and delayed recall values (number of words remembered) were evaluated by Student’s t-test in both groups. Pearson correlation coefficients were used to measure the impact of age, hippocampal volume and DTI measurements. The correlation of memory performance with DTI values of hippocampal head was evaluated via Pearson correlation coefficients. The strongest relation is indicated by a correlation coefficient of -1 or 1. A p value of less than 0.05 showed a statistically significant difference. Results The smoking group included 88 persons (44 male and 44 female), and the nonsmoking group includes 88 persons (44 male and 44 female). The mean age of the non-smokers was 28.7 ± 7.54 years (age range: 22 - 44 years). The mean age of smokers was: 32.4 ± 5.47 years (age range: 24 - 43 years). Characteristics of the study sample are given in Table 1. Mean values of FA for non-smokers and smokers were 0.46 and 0.40, respectively (p<0,0001) (Figure 4, Table 2). Mean values of MD for non-smokers and smokers were 0.0026 and 0.0027, respectively (p=0,0383) (Figure 5, Table 2). In addition, mean values of AD for non-smokers and smokers were 0.0013 and 0.0014, respectively (p<0,001). The mean values of RD for nonsmokers and smokers were 0.0020 and 0.0022, respectively (p=0,0014). Mean word numbers of IR for non-smokers and smokers were 11.6 and 9.04, respectively (p<0,0001) (Figure 6, Table 2). The mean word numbers of DR for non-smokers and smokers were 10.2 and 7.2, respectively (P<0,0001) (Figure 7, Table 2). Mean memory performance were average of the IR and DR values for each group. There was statistically significant low positive correlation about mean memory performance with FA values in nonsmoking group while there was no statistically significant correlation about mean memory performance with FA values in smoking group (Table 3, Figure 8). On the other hand, there was no statistically significant positive or negative correlation about remaining values (MD, RD and AD) with memory performance in both groups. Correlations between age, hippocampal volumes and DTI measures are given in Table 4. We did not find any statistically significant differences between smokers and non-smokers in terms of education level, age, gender and hippocampal volume (p>0.05).
6
Discussion The present study shows that the mean value of hippocampal FA is higher and mean value of hippocampal MD is lower in non-smokers than chronic smokers (p<0.05). In addition, memory performance of chronic smokers is less than non-smokers (p<0.05) and our study shows statistically significant low correlation about memory performance with FA values in nonsmoking group. Although many studies have been performed regarding hippocampal DTI measurements in Alzheimer disease, vascular dementia, epilepsy, age-related changes and cerebral small vessel disease, to the best of our knowledge, our study is the first report that showed hippocampal DTI measurements with memory performance in chronic smokers [12-17]. We hypothesized that chronic smoking would adversely affect the hippocampal structural integrity and memory performance. Acute nicotine administration provides stimulates cognitive abilities including attention and memory due to multiple interactions on the CNS. However, the effect of chronic cigarette smoking on CNS function is different than the acute effects of smoking. Many studies have shown that chronic cigarette smoking is related to worse cognitive outcome [3-5]. DTI data including MD and FA have generally been used to assess neuronal integrity [18]. Previous studies found that lower FA and higher MD values are associated with neuronal dis-integrity [13,19]. One of them, Heijer et al also found that a higher MD of the hippocampus is associated with poor memory performance [13]. In another study, Norden et al found reverse relationships between verbal memory performance and hippocampal MD [17]. Our results did not show negative correlation between MD and memory performance. However, we found that lower FA values and higher MD values in the smoking group compared with nonsmoking group. The hippocampus is located in the medial temporal lobe. This is the structure responsible for episodic memory function. Many studies have assessed the relationship between the memory performance and hippocampal volume. They have found that hippocampal volumes positively correlate with memory function [20-22]. On the other hand, hippocampal dis-integrity caused by decreased FA has been correlated with poor navigational skills with a slowed response time on a visual task [9,10]. Our study also showed decreased hippocampal FA and worse memory performance in chronic smokers than non-smokers. In addition we did not find any significant correlation between FA and hippocampal volume in both groups (Table 3).
7
These findings may explain decreased cognitive function in chronic smokers despite normal hippocampal volume in healthy young-to-middle-aged adults. Higher levels of FA are typically considered to indicate better integrity of the white matter. Previous studies of drug abusers showed that a decrease in FA in the brain white matter was associated with lower neuronal integrity [22]. Our results also showed that FA values in the hippocampal head were significantly higher in the nonsmoking group versus the smoking group (p<0.05). However, Paul et al. found significantly high FA in the corpus callosum of smokers versus non-smokers [1]. In addition, they show that microstructural integrity associated with elevated FA may not be beneficial due to abnormal angiogenesis and arteriogenesis because of nicotinic stimulation of the cholinergic system. Similarly, Liao et al. found increased FA in chronic smokers, and they suggested that increased FA in the fronto-parietal white matter might be associated with inflammatory changes and axonal damage in chronic cigarette smokers [23]. The final conclusion is that we need more studies on this topic. Our study has a few limitations. First, our results will need to be confirmed in a larger series because our study population was relatively small. This may be the reason for no significant correlation among values outside of FA (very small values such as MD, AD, RD). Second, there are several memory centers in the brain including the hippocampus, prefrontal cortex, amygdala, basal ganglia, cerebellum, etc. We assessed only the hippocampal head by DTI to assess the memory-associated brain region. However, the FA is more difficult to interpret due to hippocampal gray matter density. Future studies should investigate other centers of the memory. Third, we used a manual tracing system on the hippocampal head. It may not have been fully covered because it has an atypical morphology. Fourth, our results may be affected partial volume effects because of hippocampus near the lateral ventricles. Conclusions In conclusion, this study is the first experience about hippocampal DTI measurements in chronic smokers. We found decreased hippocampal FA and worse memory performance in chronic smokers versus non-smokers and we found a low positive correlation hippocampal FA values with memory performance in nonsmoking group. However, additional studies are needed regarding hippocampal neuronal integrity using DTI. Conflict of Interest
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We declare that we have no conflict of interest.
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Figure Legends Figure 1: Three-dimensional (3-D) image shows bilateral hippocampal head (white arrows) in whole brain white matter tracts (A). In addition, another 3-D image shows white matter tracts of hippocampal head (dashed arrows) (B).
12
Figure 2: Coronal T1 weighted image shows VOI’s on hippocampal heads. VOI’s involve of both gray and white matter areas. VOI: Voxel of interest.
13
Figure 3: Sagittal T1 weighted image (upper) and 3-D image (below) show hippocampal volume measurement using software (Vitrea, Toshiba, Japan).
14
Figure 4: The comparative graphic of mean values of FA for non-smokers and smokers.
15
Figure 5: The comparative graphic of mean values of MD for non-smokers and smokers.
16
Figure 6: The comparative graphic of mean word number of IR for non-smokers and smokers.
17
Figure 7: The comparative graphic of mean word number of DR for non-smokers and smokers
18
Figure 8: A correlation graphic of FA values with mean memory performance in nonsmoking group.
19
Table 1: Characteristics of the study sample
Patient characteristics
n=176
Mean age, years (SD)
30.55 (6.5)
Gender (%men)
88 (50)
Education (%) Low education
0 (0)
Middle education
121 (68)
High education
55 (31)
Absolute hippocampal volume, ml Right
3.3 (0.5)
Left
3.4 (0.6)
FA of hippocampus
0.4363 (0.06)
MD of hippocampus
0.0026 (0.00029)
Memory Performance Immediate recall
10.34 (2.98)
Delayed Recall
8.77 (3.82)
Values are mean or number (%)
20
Table 2: A detailed table of non-smoking and smoking groups Non-smoking Group
FA
Smoking group Cl
Mean
SD
Mean
SD
0.4671
0.0445 0.4577 to 0.47 0.4055 0.0037 0.3925 to 0.41 p 65
MD
0.0026
0.0013
28
IR
0.0020
11.6364
2.58
=
0.0463
0.0002 0.002 to 0.002 0.0022 0.0003 0.0022 to 0.00 p
2.8047 11.04 to 12.23 9.04
=
0.0239
0.0001 0.0013 to 0.00 0.0014 0.0003 0.0013 to 0.00 p 14
<
0.0001
0.0003 0.0025 to 0.00 0.0027 0.0002 0.0025 to 0.00 p
13 RD
CL
85
26 AD
P value
22
0.0014
7.0 to 10.0
p
=
<
0.0001 DR
10.2727
4.0930 9.40 to 11.13
7.27
2.84
6.0 to 8.0
p 0.0001
<
21
Table 3: Correlation scores of memory performances with FA values in both groups.
Group
FA-Mean
r score
Memory
95%
Confidence P value
interval for r
Performance Non-smoking
Low
positive +0.2480
0,04072 to 0,4349
p<0.05
-0,2283 to 0,1904
p>0.05
correlation Smoking
No correlation
-0,01979
22
Table 4: Correlations between age, hippocampal volumes and DTI measures for
Smoking group
Age
Volume (mean)
FA (mean)
MD (mean)
Age
-
-0.25
-0.401
0.381
-
-0.01
-0.01
-
-0.13
Volume (mean) FA (mean) MD (mean)
-
both groups. Non-smoking group
Age
Volume (mean)
FA (mean)
MD (mean)
Age
-
-0.231
-0.381
0.361
-
-0.03
-0.03
-
-0.14
Volume (mean) FA (mean) MD (mean) 1
p<0.05
1
p<0.05
-