Evidence of transfusion-induced reductions in cerebral capillary shunting in sickle cell disease

Cerebral hemodynamic changes after haploidentical hematopoietic stem cell transplant in adults with sickle cell disease

ABSTRACT

Preliminary evidence from a series of 4 adults with sickle cell disease (SCD) suggests that hematopoietic stem cell transplant (HSCT) improves cerebral hemodynamics. HSCT largely normalizes cerebral hemodynamics in children with SCD. We tested the hypothesis in adults with SCD that cerebral blood flow (CBF), oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen (CMRO2) measured using magnetic resonance imaging, normalized to healthy values, comparing measurements from ∼1 month before to 12 to 24 months after HSCT (n = 11; age, 33.3 ± 8.9 years; 389 ± 150 days after HSCT) with age-, race- and sex-matched values from healthy adults without sickle trait (n = 28; age, 30.2 ± 5.6 years). Before transplant, 7 patients had neurological indications for transplant (eg, overt stroke) and 4 had nonneurological reasons for haploidentical bone marrow transplant (haplo-BMT). All received haplo-BMT from first-degree relatives (parent, sibling, or child donor) with reduced-intensity preparation and maintained engraftment. Before transplant, CBF was elevated (CBF, 69.11 ± 24.7 mL/100 g/min) compared with that of controls (P = .004). Mean CBF declined significantly after haplo-BMT (posttransplant CBF, 48.2 ± 13.9 mL/100 g/min; P = .003). OEF was not different from that of controls at baseline and did not change significantly after haplo-BMT (pretransplant, 43.1 ± 6.7%; posttransplant, 39.6 ± 7.0%; P = .34). After transplant, CBF and OEF were not significantly different from controls (CBF, 48.2 ± 13.4 mL/100 g/min; P = .78; and OEF, 39.6 ± 7.0%; P > .99). CMRO2 did not change significantly after haplo-BMT (pretransplant, 3.18 ± 0.87 mL O2/100 g/min; posttransplant, 2.95 ± 0.83; P = .56). Major complications of haplo-BMT included 1 infection-related death and 1 severe chronic graft-versus-host disease. Haplo-BMT in adults with SCD reduces CBF to that of control values and maintains OEF and CMRO2 on average at levels observed in healthy adult controls. The trial was registered at www.clinicaltrials.gov as #NCT01850108.

Red cell exchange transfusions increase cerebral capillary transit times and may alter oxygen extraction in sickle cell disease

Persons with sickle cell disease (SCD) suffer from chronic hemolytic anemia, reduced blood oxygen content, and lifelong risk of silent and overt stroke. Major conventional stroke risk factors are absent in most individuals with SCD, yet nearly 50% have evidence of brain infarcts by the age of 30 years, indicating alternative etiologies for ischemia. We investigated whether radiological evidence of accelerated blood water transit through capillaries, visible on arterial spin labeling (ASL) magnetic resonance imaging, reduces following transfusion-induced increases in hemoglobin and relates to oxygen extraction fraction (OEF). Neurological evaluation along with anatomical and hemodynamic imaging with cerebral blood flow (CBF)-weighted pseudocontinuous ASL and OEF imaging with T2 -relaxation-under-spin-tagging were applied in sequence before and after blood transfusion therapy (n = 32) and in a comparator cohort of nontransfused SCD participants on hydroxyurea therapy scanned at two time points to assess stability without interim intervention (n = 13). OEF was calculated separately using models derived from human hemoglobin-F, hemoglobin-A, and hemoglobin-S. Gray matter CBF and dural sinus signal, indicative of rapid blood transit, were evaluated at each time point and compared with OEF using paired statistical tests (significance: two-sided p < 0.05). No significant change in sinus signal was observed in nontransfused participants (p = 0.650), but a reduction was observed in transfused participants (p = 0.034), consistent with slower red cell transit following transfusion. The dural sinus signal intensity was inversely associated with OEF pretransfusion (p = 0.011), but not posttransfusion. Study findings suggest that transfusion-induced increases in total hemoglobin may lengthen blood transit times through cerebral capillaries and alter cerebral OEF in SCD.

Venous cerebral blood flow quantification and cognition in patients with sickle cell anemia

Prior studies have described high venous signal qualitatively using arterial spin labelling (ASL) in patients with sickle cell anemia (SCA), consistent with arteriovenous shunting. We aimed to quantify the effect and explored cross-sectional associations with arterial oxygen content (CaO2), disease-modifying treatments, silent cerebral infarction (SCI), and cognitive performance. 94 patients with SCA and 42 controls underwent cognitive assessment and MRI with single- and multi- inflow time (TI) ASL sequences. Cerebral blood flow (CBF) and bolus arrival time (BAT) were examined across gray and white matter and high-signal regions of the sagittal sinus. Across gray and white matter, increases in CBF and reductions in BAT were observed in association with reduced CaO2 in patients, irrespective of sequence. Across high-signal sagittal sinus regions, CBF was also increased in association with reduced CaO2 using both sequences. However, BAT was increased rather than reduced in patients across these regions, with no association with CaO2. Using the multiTI sequence in patients, increases in CBF across white matter and high-signal sagittal sinus regions were associated with poorer cognitive performance. These novel findings highlight the utility of multiTI ASL in illuminating, and identifying objectively quantifiable and functionally significant markers of, regional hemodynamic stress in patients with SCA.

Assessment of functional shunting in patients with sickle cell disease

Silent cerebral infarcts (SCI) are common in patients with sickle cell disease (SCD) and are thought to be caused by a mismatch between oxygen delivery and consumption. Functional cerebrovascular shunting is defined as reduced oxygen offloading due to the rapid transit of blood through the capillaries caused by increased flow and has been suggested as a potential mechanism underlying reduced oxygenation and SCI. We investigated the venous arterial spin labeling signal (VS) in the sagittal sinus as a proxy biomarker of cerebral functional shunting, and its association with hemodynamic imaging and hematological laboratory parameters. We included 28 children and 38 adults with SCD, and ten healthy racematched adult controls. VS, cerebral blood flow (CBF), velocity in the brain feeding arteries, oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO2) were measured before and after acetazolamide administration. VS was higher in patients with SCD compared to controls (P<0.01) and was increased after acetazolamide administration in all groups (P<0.01). VS was primarily predicted by CBF (P<0.01), but CBF-corrected VS was also associated with decreased CMRO2 (P<0.01). Additionally, higher disease severity defined by low hemoglobin and increased hemolysis was associated with higher CBF-corrected VS. Finally, CMRO2 was negatively correlated with fetal hemoglobin, and positively correlated with lactate dehydrogenase, which could be explained by changes in oxygen affinity. These findings provide evidence for cerebral functional shunting and encourage future studies investigating the potential link to aberrant capillary exchange in SCD.

Advances in neuroimaging to improve care in sickle cell disease

Sickle cell disease is associated with progressive and increased neurological morbidity throughout the lifespan. In people with sickle cell anaemia (the most common and severe type of sickle cell disease), silent cerebral infarcts are found in more than a third of adolescents by age 18 years and roughly half of young adults by age 30 years, many of whom have cognitive impairment despite having few or no conventional stroke risk factors. Common anatomical neuroimaging in individuals with sickle disease can assess structural brain injury, such as stroke and silent cerebral infarcts; however, emerging advanced neuroimaging methods can provide novel insights into the pathophysiology of sickle cell disease, including insights into the cerebral haemodynamic and metabolic contributors of neurological injury. Advanced neuroimaging methods, particularly methods that report on aberrant cerebral blood flow and oxygen delivery, have potential for triaging patients for appropriate disease-modifying or curative therapies before they have irreversible neurological injury, and for confirming the benefit of new therapies on brain health in clinical trials.