383 research outputs found
Cross-over behaviour in a communication network
We address the problem of message transfer in a communication network. The
network consists of nodes and links, with the nodes lying on a two dimensional
lattice. Each node has connections with its nearest neighbours, whereas some
special nodes, which are designated as hubs, have connections to all the sites
within a certain area of influence. The degree distribution for this network is
bimodal in nature and has finite variance. The distribution of travel times
between two sites situated at a fixed distance on this lattice shows fat
fractal behaviour as a function of hub-density. If extra assortative
connections are now introduced between the hubs so that each hub is connected
to two or three other hubs, the distribution crosses over to power-law
behaviour. Cross-over behaviour is also seen if end-to-end short cuts are
introduced between hubs whose areas of influence overlap, but this is much
milder in nature. In yet another information transmission process, namely, the
spread of infection on the network with assortative connections, we again
observed cross-over behaviour of another type, viz. from one power-law to
another for the threshold values of disease transmission probability. Our
results are relevant for the understanding of the role of network topology in
information spread processes.Comment: 12 figure
Atherosclerosis, C-reactive protein, and risk for open-angle glaucoma: The Rotterdam Study
PURPOSE. To test the hypotheses that atherosclerosis and elevated serum C-reactive protein (CRP) levels are risk factors for open-angle glaucoma (OAG). METHODS. In a prospective, population-based cohort study, all participants 55 years and older and at risk for incident OAG underwent, at baseline (1990-1993) and at follow-up (1997-1999), the same ophthalmic examination, including visual field testing and optic disc photography. Baseline atherosclerosis was assessed by means of echography of the carotid arteries, abdominal x-ray examination, and ankle-arm index; baseline serum CRP levels were used in the analyses. The diagnosis of OAG was based on an algorithm using optic disc measures and visual field loss. Odds ratios of OAG were computed with logistic regression analyses. Risk factors were categorized in tenues and according to standard deviation. RESULTS. After a mean follow-up of 6.5 years, incident OAG was diagnosed in 87 of 3842 (2.3%) participants at risk for OAG. Carotid artery plaques, carotid intima-media thickness, aortic calcifications, ankle-arm index, and CRP levels were not significant risk factors for OAG. The odds ratio, given for the highest and lowest tertiles, for carotid plaques was 1.43 (95% confidence interval [CI], 0.68-2.99), for carotid intima-media thickness 0.86 (95% CI, 0.47-1.57), for aortic calcifications 1.02 (95% CI, 0.60-1.75), for ankle-arm index 0.69 (95% CI, 0.38-1.25), and for CRP 1.19 (95% CI, 0.68-2.07). CONCLUSIONS. In this prospective, population-based study, neither atherosclerosis nor serum CRP level was an important risk factor for OAG.
Incidence of glaucomatous visual field loss after two decades of follow-up: the Rotterdam Study
To determine the incidence of glaucomatous visual field loss (GVFL) two decades after the start of the Rotterdam Study, and to compare known risk factors for open-angle glaucoma (OAG) between different clinical manifestations of OAG. Of 6806 participants aged 55 years and older from the population-based Rotterdam Study, 3939 underwent visual field testing at baseline and at least one follow-up round. The ophthalmic examinations included optic disc assessment and measurements of intraocular pressure (IOP), refractive error, diastolic blood pressure (DBP), and height and weight. The incidence rate of GVFL was calculated. Associations with the risk factors age, gender, baseline IOP, family history, myopia, DBP, and body-mass index [BMI] were assessed using Cox regression, with different clinical manifestations of OAG as outcome measure (glaucomatous optic neuropathy (GON), GVFL, GVFL and GON, GVFL without GON, and GON without GVFL). Median follow-up was 11.1 (IQR 6.8–17.2; range 5.0–20.3) years. The incidence rate of GVFL was 2.9 (95% confidence interval 2.4–3.4) per 1000 person years (140 cas
Menopausal status and distensibility of the common carotid artery
Although several studies have shown that exogenous estrogens have
beneficial effects on arterial characteristics, the effect of endogenous
estrogen on the vascular system is still unknown. In this study,
distensibility, an indicator of arterial elasticity, of the common carotid
artery was compared in pre- and postmenopausal women. The study comprised
93 premenopausal and 93 postmenopausal women of similar age (range, 43 to
55 years). Women were selected from respondents to a mailed questionnaire
about the menopause, which was sent to all women aged 40 to 60 years in
the Dutch town of Zoetermeer (n=12 675). Postmenopausal women who were at
least 3 years past natural menopause or whose menses had stopped naturally
before age 48, were age-matched with premenopausal women with regular
menses and without menopausal complaints. The selection aimed at
maximizing the contrast in estrogen status between pre- and postmenopausal
women of the same age. Distensibility of the carotid artery was measured
noninvasively with B-mode ultrasound and a vessel wall movement detector
system. Arterial distensibility is expressed as the change in arterial
diameter (distension, DeltaD) with the cardiac cycle, adjusted for lumen
diameter, pulse pressure, and mean arterial blood pressure. Compared with
premenopausal women, postmenopausal women had significantly lower arterial
distension (DeltaD 370.5 microm [SE 9.5] versus 397.3 microm [SE 9.6]).
These results suggest that the distensibility of the common carotid artery
is negatively affected by natural menopause in presumed healthy women
Arterial stiffness as underlying mechanism of disagreement between an oscillometric blood pressure monitor and a sphygmomanometer
Oscillometric blood pressure devices tend to overestimate systolic blood
pressure and underestimate diastolic blood pressure compared with
sphygmomanometers. Recent studies indicate that discrepancies in
performance between these devices may differ between healthy and diabetic
subjects. Arterial stiffness in diabetics could be the underlying factor
explaining these differences. We studied differences between a Dinamap
oscillometric blood pressure monitor and a random-zero sphygmomanometer in
relation to arterial stiffness in 1808 healthy elderly subjects. The study
was conducted within the Rotterdam Study, a population-based cohort study
of subjects aged 55 years and older. Systolic and diastolic blood pressure
differences between a Dinamap and a random-zero sphygmomanometer were
related to arterial stiffness, as measured by carotid-femoral pulse wave
velocity. Increased arterial stiffness was associated with higher systolic
and diastolic blood pressure readings by the Dinamap compared with the
random-zero sphygmomanometer, independent of age, gender, and average mean
blood pressure level of both devices. The beta-coefficient (95% CI) was
0.25 (0.00 to 0.50) mm Hg/(m/s) for the systolic blood pressure difference
and 0.35 (0.20 to 0.50) mm Hg/(m/s) for the diastolic blood pressure
difference. The results indicate that a Dinamap oscillometric blood
pressure device, in comparison to a random-zero sphygmomanometer,
overestimates systolic and diastolic blood pressure readings in subjects
with stiff arteries
The role of caretakers in disease dynamics
One of the key challenges in modeling the dynamics of contagion phenomena is
to understand how the structure of social interactions shapes the time course
of a disease. Complex network theory has provided significant advances in this
context. However, awareness of an epidemic in a population typically yields
behavioral changes that correspond to changes in the network structure on which
the disease evolves. This feedback mechanism has not been investigated in
depth. For example, one would intuitively expect susceptible individuals to
avoid other infecteds. However, doctors treating patients or parents tending
sick children may also increase the amount of contact made with an infecteds,
in an effort to speed up recovery but also exposing themselves to higher risks
of infection. We study the role of these caretaker links in an adaptive network
models where individuals react to a disease by increasing or decreasing the
amount of contact they make with infected individuals. We find that pure
avoidance, with only few caretaker links, is the best strategy for curtailing
an SIS disease in networks that possess a large topological variability. In
more homogeneous networks, disease prevalence is decreased for low
concentrations of caretakers whereas a high prevalence emerges if caretaker
concentration passes a well defined critical value.Comment: 8 pages, 9 figure
Association between arterial stiffness and atherosclerosis: the Rotterdam Study
BACKGROUND AND PURPOSE: Studies of the association between arterial
stiffness and atherosclerosis are contradictory. We studied stiffness of
the aorta and the common carotid artery in relation to several indicators
of atherosclerosis. METHODS: This study was conducted within the Rotterdam
Study in >3000 elderly subjects aged 60 to 101 years. Aortic stiffness was
assessed by measuring carotid-femoral pulse wave velocity, and common
carotid artery stiffness was assessed by measuring common carotid
distensibility. Atherosclerosis was assessed by common carotid
intima-media thickness, plaques in the carotid artery and in the aorta,
and the presence of peripheral arterial disease. Data were analyzed by
ANCOVA with adjustment for age, sex, mean arterial pressure, and heart
rate. RESULTS: Both aortic and common carotid artery stiffness were found
to have a strong positive association with common carotid intima-media
thickness, severity of plaques in the carotid artery, and severity of
plaques in the aorta (P: for trend <0.01 for all associations). Subjects
with peripheral arterial disease had significantly increased aortic
stiffness (P:=0.001) and borderline significantly increased common carotid
artery stiffness (P:=0.08) compared with subjects without peripheral
arterial disease. Results were similar after additional adjustment for
cardiovascular risk factors and after exclusion of subjects with prevalent
cardiovascular disease. CONCLUSIONS: This population-based study shows
that arterial stiffness is strongly associated with atherosclerosis at
various sites in the vascular tree
Microglial activation in Alzheimer's disease: an (R)-[11C]PK11195 positron emission tomography study
AbstractInflammatory mechanisms, like microglial activation, could be involved in the pathogenesis of Alzheimer's disease (AD). (R)-[11C]PK11195 (1-(2-chlorophenyl)-N-methyl-N-1(1-methylpropyl)-3-isoquinolinecarboxamide), a positron emission tomography (PET) ligand, can be used to quantify microglial activation in vivo. The purpose of this study was to assess whether increased (R)-[11C]PK11195 binding is present in AD and mild cognitive impairment (MCI), currently also known as “prodromal AD.”MethodsNineteen patients with probable AD, 10 patients with prodromal AD (MCI), and 21 healthy control subjects were analyzed. Parametric images of binding potential (BPND) of (R)-[11C]PK11195 scans were generated using receptor parametric mapping (RPM) with supervised cluster analysis. Differences between subject groups were tested using mixed model analysis, and associations between BPND and cognition were evaluated using Pearson correlation coefficients.ResultsVoxel-wise statistical parametric mapping (SPM) analysis showed small clusters of significantly increased (R)-[11C]PK11195 BPND in occipital lobe in AD dementia patients compared with healthy control subjects. Regions of interest (ROI)-based analyses showed no differences, with large overlap between groups. There were no differences in (R)-[11C]PK11195 BPND between clinically stable prodromal AD patients and those who progressed to dementia, and BPND did not correlate with cognitive function.ConclusionMicroglial activation is a subtle phenomenon occurring in AD
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