Cerebrovascular reserve and white matter disease in patients with chronic anemia. (IMPROvE)

Silent white matter strokes have been recognized as a major problem in sickle
cell disease1,2. While the use of transcranial Doppler as a screening tool and
appropriate chronic transfusion therapy have reduced the risk of symptomatic
ischemic stroke in sickle cell disease (SCD) ten-fold in high risk patients,
the prevalence of silent cerebral infarct (SCI) rises 1-2% per year with no
plateau1. Sickle cell patients have a profound white matter volume loss and
abnormal white matter diffusivity, even in the absence of silent strokes3-5. As
a result, continuous neurovascular damage which has been related to impaired
neuro-cognitive function continues to be a major contributor to unemployment,
underemployment, and impaired quality of life in these patients2,6. Currently,
no test is available to measure the risk for silent stroke in SCD patients. Our
preliminary data suggests that SCIs are more common in SCD patients and occur
in specific high-risk regions7.
SCIs are caused by a mismatch in oxygen supply and demand. Although the brain
comprises only 2.5% of the body mass, it uses about 20% of the body*s oxygen at
rest8. The brain cannot survive under anaerobic conditions, so cerebral oxygen
delivery is tightly regulated. Over 98% of the oxygen that is delivered to the
brain is transported by haemoglobin. In patients with SCD, the concentration of
functional haemoglobin is chronically diminished and as a result, CBF is
increased by vasodilation to maintain oxygen delivery9,10. However, this
compensatory mechanism of cerebral vasodilation is limited. Blood vessels have
a limited capacity to dilate11. The increase in CBF under metabolic stress,
relative to the baseline flow, is known as the cerebrovascular reserve (CVR).
CVR is typically 40%-70% in healthy control subjects, but is much lower in
subjects with SCD because of their already increased resting brain blood
flow11. A decreased CVR does not directly cause damage, but it leaves the brain
vulnerable to ischemic insults. This is because the brain*s increased oxygen
extraction (under metabolic stress) can no longer compensate for decreased
oxygen delivery or increased metabolic demand.

Many groups have estimated CVR by measuring the extent to which cerebral
perfusion increases in response to vasodilation12-14. Vasodilatation can be
caused by either inhalation of carbon dioxide (CO2)-mixed air or by
administration of acetazolamide (ACZ)9. With CO2 inhalation there is an effect
of hyperventilation on CBF15. However, ACZ induces maximal cerebral
vasodilation without confounding effects of cardiovascular physiology16,17.
Therefore, using ACZ, CVR can be measured most directly and accurately. In
contrast to the CO2 inhalation, ACZ is independent of the patient's
cooperation15,18. In addition, our group has experience using ACZ in SCD
patients.

Sickle cell disease is not the only type of anaemia with white matter damage.
Thalassemia is another group with a high prevalence of silent ischemic
lesions19-21. Different studies showed white matter lesions in patients with
intermediate thalassemia that were similar to the lesions found in SCD
patients21,22. Pazgal et al. studied the presence of SCI in transfused beta
thalassemia major patients23. They assumed that transfused patients would have
less SCIs compared to patients with thalassemia intermedia. Unexpectedly,
results showed multiple SCIs in 60.7% of the patients with transfused
beta-thalassemia major. So, SCIs are observed in both nontransfused and
transfused thalassemia patients. In patients with thalassemia, SCIs are most
likely caused by thromboembolic events mostly as a consequence of
splenectomy21. However, in contrast to the SCD patients, no blockage of the
small vessels occur in patients with thalassemia.

Besides patients with SCD or thalassemia, white matter strokes also occur in
more than 30% of all healthy individuals older than 65 years of age24 and are
associated with significant neurocognitive dysfunction25, depression26,
falling27, ischemic stroke28, and all-cause mortality28. However, these
individuals do not have a hereditary hemoglobinopathy and do not experience
chronic haemolysis. In contrast to these older healthy individuals, in anaemia
patients, SCI occur in a younger age.

Our recent study (Cruise) has shown that CVR differs between healthy controls
and SCD patients. However, it is not clear from that study whether these
differences are sickle cell dependant or caused by anaemia (as in thalassemia).
To better understand the aetiology of reduced CVR in SCD, studying a group of
anaemic non-SCD patients (e.g. thalassemia patients) is required, in order to
disentangle anaemia-related impairments and anaemia plus SCD related
impairments. In addition, a group of sickle cell patients will be included that
have been selected for allogenic stem cell transplantation (SCT). By including
this group a direct comparison of the impact of SCD on the hemodynamics of the
cerebral perfusion can be obtained by comparing the parameters of the
hemodynamics of the cerebral perfusion like CVR before and after successful
SCT,

In order to understand the age depended CVR differences, inclusion of healthy
subjects is needed. By measuring the regional CVR using a magnetic resonance
imaging (MRI) technique called arterial spin labelling (ASL), we hypothesize to
have an opportunity to optimize current therapies to prevent cerebral infarct.
We will further investigate the CVR parameters such as perfusion,
vasodilatation, vascular territories and wall shear stress (WSS) before and
after vasodilatation The effect of interventions on CVR, will be investigated
by including patients who undergo blood transfusion, hydroxyurea treatment or
allogeneic SCT.

Primary Objective:
The main goal of this project is to identify CVR predictors including CBF, age,
sex and vascular stressors in anaemic and control patients using several MRI
techniques. While anaemia is correlated with other cerebrovascular risk factors
in the general population (hypertension, kidney disease, chronic inflammation,
heart failure), we assume that anaemia, by decreasing CVR, creates an increased
vulnerability to white matter damage in patients with SCD. Through the use of
simple and exchange transfusions in selected patients with SCD and thalassemia,
we will study the relative importance of haemoglobin S% and total haemoglobin
level on regional CVR. We will identify other modifiable risk factors (iron
overload, vascular inflammation) that may impair CVR. By comparing CVR and
white matter damage across a broad spectrum of SCD and thalassemia syndromes,
we will be able to separate the damaging effects of haemolytic anaemias in
general from damage specific to sickle haemoglobin.


Secondary Objective(s):
1) Whether the regions of low CVR match the distribution of white matter damage
2) Whether changes in haemoglobin level or haemoglobin S% contribute to the
global and regional CVR
3) Whether allogeneic SCT improves CBF, CVR and CMR02
4) Whether transfusion, the use of hydroxyurea or allogeneic SCT treatment
improves CVR in the white matter regions at risk for stroke.
5) Whether allogeneic SCT improved neurocognitive functioning in patients
with SCD.
6) Whether impaired CVR capacity is associated withbiomarkers of
coagulation activation, neutrophil activation, endothelial damage or oxidative
stress.

Study will be performed at two large SCD Centres of Excellence, Children*s
Hospital of Los Angeles in California, and the Academic Medical Centre of
Amsterdam. The study is a multicentre observational study with intervention
(MRI and the medicinal product). Depending upon inclusion, the project has a
duration of four years. The control group and the nontransfused patients will
undergo a single MRI-examination. Transfused patients and the patients who will
start with hydroxyurea treatment will undergo two MRI examinations (before and
after transfusion of hydroxyurea treatment). Sickle cell patients who will
undergo an allogeneic SCT will have MRI imaging prior to and 6 and 12 months
after the transplantation.

MRI is harmless, ACZ injection has proven to be safe and venapunction is
routine in patients with anaemia. Parameters obtained by blood drawn in this
study will be used clinically as well. Therefore, participation is considered
to relate with low burden and minimal risks.