HHS Public Access Author manuscript Author Manuscript
Am J Hematol. Author manuscript; available in PMC 2017 September 01. Published in final edited form as: Am J Hematol. 2016 September ; 91(9): 912–917. doi:10.1002/ajh.24441.
DETERMINANTS OF RESTING CEREBRAL BLOOD FLOW IN SICKLE CELL DISEASE Adam M Bush, M.S1, Matthew T Borzage, Ph.D2, Soyoung Choi, B.S3, Lena Václavů, M.S4, Benita Tamrazi, MD5, Aart J Nederveen, Ph.D4, Thomas D Coates, M.D6, and John C Wood, M.D, Ph.D5,7 1Department
Author Manuscript
2Division
of Biomedical Engineering, University of Southern California
of Neonatology and Radiology, Children’s Hospital Los Angeles
3Neurosciences
Program, University of Southern California
4Department
of Radiology, Academic Medical Center, Amsterdam
5Department
of Radiology, Children’s Hospital Los Angeles
6Section
of Hematology, Children’s Hospital Los Angeles
7Division
of Cardiology, Children’s Hospital Los Angeles
Abstract Author Manuscript Author Manuscript
Stroke is common in children with sickle cell disease and results from an imbalance in oxygen supply and demand. Cerebral blood flow (CBF) is increased in patients with sickle cell disease to compensate for their anemia, but adequacy of their oxygen delivery has not been systematically demonstrated. This study examined the physiological determinants of CBF in 37 patients with sickle cell disease, 38 ethnicity matched control subjects and 16 patients with anemia of non-sickle origin. Cerebral blood flow was measured using phase contrast MRI of the carotid and vertebral arteries. CBF increased inversely to oxygen content (r2 = 0.69, p < 0.0001). Brain oxygen delivery, the product of CBF and oxygen content, was normal in all groups. Brain composition, specifically the relative amounts of grey and white matter, was the next strongest CBF predictor, presumably by influencing cerebral metabolic rate. Grey matter/white matter ratio and CBF declined monotonically until the age of 25 in all subjects, consistent with known maturational changes in brain composition. Further CBF reductions were observed with age in subjects older than 35 years of age, likely reflecting microvascular aging. On multivariate regression, CBF was independent of disease state, hemoglobin S, hemoglobin F, reticulocyte count and cell free hemoglobin, suggesting that it is regulated similarly in patients and control subjects. In conclusion, sickle cell disease patients had sufficient oxygen delivery at rest, but accomplish this only by marked
ADDRESS FOR CORRESPONDENCE: Dr. John C. Wood, Division of Cardiology, MS#34, Children’s Hospital of Los Angeles, 4650 Sunset Blvd, Los Angeles, CA, USA, 90027, Phone 323-361-5470, Fax: 323-361-3668. AUTHOR CONTRIBUTIONS MB, AB, JCW designed and conducted the studies, analyzed the data, and wrote the paper. SYC, AN, LV, and TDC analyzed the data and wrote the paper. CONFLICT OF INTEREST JCW receives research support in kind from Philips Healthcare. None of the others authors have conflicts relevant to the study.
Bush et al.
Page 2
Author Manuscript
increases in their resting CBF, potentially limiting their ability to further augment flow in response to stress.
Keywords Sickle Cell Disease; Cerebral Blood Flow; Stroke; Magnetic Resonance Imaging; Phase Contrast
INTRODUCTION
Author Manuscript
Stroke is one of the most devastating complications in sickle cell disease. Prior to routine transcranial Doppler screening, the risk of major stroke was 11% by the age of 18, with a peak age of incidence in the early school age years1. Aggressive transcranial Doppler (TCD) screening and chronic transfusion protocols have lowered overt stroke rates ten fold2,3, but silent strokes remain problematic4,5. A recent review by DeBaun and Kirkham suggests that silent stroke prevalence rises 1.4% per year with no plateau, even in cohorts managed with modern practices6. Chronic anemia is one of the strongest risk factors for stroke and all cause mortality in SCD6. Several studies have denoted elevations in cerebral blood flow (CBF) in SCD, presumably to preserve resting oxygen delivery7–10. However, the adequacy of that compensatory hyperemia has never been assessed or compared to patients with anemias from other etiologies8,11. In fact, there is a paucity of data exploring CBF in African Americans, in general, despite their twofold risk of cerebrovascular accidents12.
Author Manuscript
To address these shortcomings, we compared resting CBF and oxygen delivery in 36 SCD patients, 37 ethnically matched controls, and 16 patients with anemia other than SCD (Thalassemia major, congenital dyserythropoetic anemia, and spherocytosis). We also measured CBF response to 100% oxygen administration.
METHODS All studies were performed according to Good Clinical Practice and the Declaration of Helsinki. Informed consent was obtained from all patients under a protocol approved by the Committee on Clinical Investigation at the Children’s Hospital Los Angeles.
Author Manuscript
Three patient cohorts were studied. The first cohort consisted of sickle cell disease subjects 12 years old or older with SS, SB0, and SC genotypes who were free from known strokes. Patients were excluded if they had a hospitalization within the month prior to the study visit. The second cohort consisted of African and Hispanic Americans older than 12 years of age who had no prior history of neurologic insult or serious, chronic illness requiring daily medications. Most were first or second-degree relatives of the patients studied. The third cohort consisted of patients with chronic anemia, excluding sickle cell anemia, who were otherwise healthy and age-matched to the first cohort. The etiology of anemia consisted of thalassemia major (beta thalassemia or E-beta thalassemia, N=13), congenital dyserythropoietic anemia (N=1), autoimmune hemolytic anemia (N=1), and spherocytosis
Am J Hematol. Author manuscript; available in PMC 2017 September 01.
Bush et al.
Page 3
Author Manuscript
(N=2). Exclusion criteria were prior neurologic insult or major medical problems outside of their chronic anemia. At the beginning of the study visit, patients underwent measurement of vital signs and phlebotomy. Complete blood count, reticulocytes, quantitative hemoglobin electrophoresis, lactate dehydrogenase and cell free hemoglobin levels were analyzed in the clinical laboratory.
Author Manuscript
MRI examinations were performed on a Philips Achieva 3 Tesla magnet using an 8-element head coil. CBF was measured using single slice phase contrast velocity measurements with the following parameters: repetition time 12.3 ms, echo time 7.5 ms, field of view 260×260 mm, slice thickness 5 mm, signal averages 10, acquisition matrix 204 × 201, reconstruction matrix 448 × 448, bandwidth 244 Hz/pixel, and velocity encoding gradient of 200 cm/s. Details of localization and analysis have been previously published, as well as flow data from a subset of the control and anemia control cohorts13. Whole brain 3D T1 and T2 weighted images were collected for brain segmentation and volume purposes13. MR angiography was also performed to evaluate for large vessel stenoses. T1, T2 and MR angiography images were read for white matter disease and stenosis by a licensed neuroradiologist, blinded to disease status. At the end of the imaging study, CBF was reassessed after patients had been breathing 100% oxygen through a non-rebreathing mask for ten minutes. Oxygen content was estimated from hemoglobin according to the well known relationship14: (1)
Author Manuscript
where oxygen saturation was measured by pulse oximeter and pO2 is the partial pressure of oxygen, which was assumed to be 100 torr on room air. We did not estimate oxygen content on 100% oxygen because arterial blood gases were not performed. Oxygen delivery to the brain can then be calculated as the product of cerebral blood flow and oxygen content, or conversely CBF can be written as follows: (2)
Author Manuscript
Intuitively, one would then expect CBF to vary inversely with oxygen content. To control for the nonlinearity in equation [2], we log transformed all variables in our regression analyses for CBF and oxygen delivery. Log transformation improved the normality of all the predictive variables. Following log transformation, equation (2) becomes
(3)
Stepwise linear regression was performed using JMP Pro 11 (SAS, Cary, NC).
Am J Hematol. Author manuscript; available in PMC 2017 September 01.
Bush et al.
Page 4
Author Manuscript
RESULTS
Author Manuscript
Patient demographics are summarized in Table 1. The anemia control group (ACTL), when the two spherocytosis patients were excluded, had hemoglobin levels (10.3 ± 0.9 g/dl) comparable to the SCD patients. Differences in CBC parameters and reticulocytes among the groups were predictable. Cell-free hemoglobin was equally elevated in the SCD and ACTL patients, but LDH was only elevated in the SCD patients. Diastolic blood pressures were low in both SCD and ACTL patients, consistent with decreased systemic vascular resistance found in anemic patients. SCD patients were mildly desaturated compared to the other subjects, but only 3 patients had oxygen saturations below 93%. Body habitus was comparable in all three groups. The SCD patients were predominantly SS patients with 50% receiving chronic transfusion. Because many of the control subjects were family members, the population was roughly balanced between AS and AA. ACTL patients were predominantly AA hemoglobin; 3 E-beta thalassemia patients had AE electrophoresis but their native erythropoiesis was effectively suppressed. MR angiography was abnormal in only one subject, a 37 year old man with sickle cell disease who had mild narrowing of his right internal carotid artery and severe, bilateral proximal anterior cerebral artery stenosis without overt stroke. Small, white matter hyperintensities (silent strokes) greater than expected for age were observed in a total of 16 subjects, 4/38 controls, 1/14 anemia controls, and 11/37 patients with SCD (p
Am J Hematol. Author manuscript; available in PMC 2017 September 01. Published in final edited form as: Am J Hematol. 2016 September ; 91(9): 912–917. doi:10.1002/ajh.24441.
DETERMINANTS OF RESTING CEREBRAL BLOOD FLOW IN SICKLE CELL DISEASE Adam M Bush, M.S1, Matthew T Borzage, Ph.D2, Soyoung Choi, B.S3, Lena Václavů, M.S4, Benita Tamrazi, MD5, Aart J Nederveen, Ph.D4, Thomas D Coates, M.D6, and John C Wood, M.D, Ph.D5,7 1Department
Author Manuscript
2Division
of Biomedical Engineering, University of Southern California
of Neonatology and Radiology, Children’s Hospital Los Angeles
3Neurosciences
Program, University of Southern California
4Department
of Radiology, Academic Medical Center, Amsterdam
5Department
of Radiology, Children’s Hospital Los Angeles
6Section
of Hematology, Children’s Hospital Los Angeles
7Division
of Cardiology, Children’s Hospital Los Angeles
Abstract Author Manuscript Author Manuscript
Stroke is common in children with sickle cell disease and results from an imbalance in oxygen supply and demand. Cerebral blood flow (CBF) is increased in patients with sickle cell disease to compensate for their anemia, but adequacy of their oxygen delivery has not been systematically demonstrated. This study examined the physiological determinants of CBF in 37 patients with sickle cell disease, 38 ethnicity matched control subjects and 16 patients with anemia of non-sickle origin. Cerebral blood flow was measured using phase contrast MRI of the carotid and vertebral arteries. CBF increased inversely to oxygen content (r2 = 0.69, p < 0.0001). Brain oxygen delivery, the product of CBF and oxygen content, was normal in all groups. Brain composition, specifically the relative amounts of grey and white matter, was the next strongest CBF predictor, presumably by influencing cerebral metabolic rate. Grey matter/white matter ratio and CBF declined monotonically until the age of 25 in all subjects, consistent with known maturational changes in brain composition. Further CBF reductions were observed with age in subjects older than 35 years of age, likely reflecting microvascular aging. On multivariate regression, CBF was independent of disease state, hemoglobin S, hemoglobin F, reticulocyte count and cell free hemoglobin, suggesting that it is regulated similarly in patients and control subjects. In conclusion, sickle cell disease patients had sufficient oxygen delivery at rest, but accomplish this only by marked
ADDRESS FOR CORRESPONDENCE: Dr. John C. Wood, Division of Cardiology, MS#34, Children’s Hospital of Los Angeles, 4650 Sunset Blvd, Los Angeles, CA, USA, 90027, Phone 323-361-5470, Fax: 323-361-3668. AUTHOR CONTRIBUTIONS MB, AB, JCW designed and conducted the studies, analyzed the data, and wrote the paper. SYC, AN, LV, and TDC analyzed the data and wrote the paper. CONFLICT OF INTEREST JCW receives research support in kind from Philips Healthcare. None of the others authors have conflicts relevant to the study.
Bush et al.
Page 2
Author Manuscript
increases in their resting CBF, potentially limiting their ability to further augment flow in response to stress.
Keywords Sickle Cell Disease; Cerebral Blood Flow; Stroke; Magnetic Resonance Imaging; Phase Contrast
INTRODUCTION
Author Manuscript
Stroke is one of the most devastating complications in sickle cell disease. Prior to routine transcranial Doppler screening, the risk of major stroke was 11% by the age of 18, with a peak age of incidence in the early school age years1. Aggressive transcranial Doppler (TCD) screening and chronic transfusion protocols have lowered overt stroke rates ten fold2,3, but silent strokes remain problematic4,5. A recent review by DeBaun and Kirkham suggests that silent stroke prevalence rises 1.4% per year with no plateau, even in cohorts managed with modern practices6. Chronic anemia is one of the strongest risk factors for stroke and all cause mortality in SCD6. Several studies have denoted elevations in cerebral blood flow (CBF) in SCD, presumably to preserve resting oxygen delivery7–10. However, the adequacy of that compensatory hyperemia has never been assessed or compared to patients with anemias from other etiologies8,11. In fact, there is a paucity of data exploring CBF in African Americans, in general, despite their twofold risk of cerebrovascular accidents12.
Author Manuscript
To address these shortcomings, we compared resting CBF and oxygen delivery in 36 SCD patients, 37 ethnically matched controls, and 16 patients with anemia other than SCD (Thalassemia major, congenital dyserythropoetic anemia, and spherocytosis). We also measured CBF response to 100% oxygen administration.
METHODS All studies were performed according to Good Clinical Practice and the Declaration of Helsinki. Informed consent was obtained from all patients under a protocol approved by the Committee on Clinical Investigation at the Children’s Hospital Los Angeles.
Author Manuscript
Three patient cohorts were studied. The first cohort consisted of sickle cell disease subjects 12 years old or older with SS, SB0, and SC genotypes who were free from known strokes. Patients were excluded if they had a hospitalization within the month prior to the study visit. The second cohort consisted of African and Hispanic Americans older than 12 years of age who had no prior history of neurologic insult or serious, chronic illness requiring daily medications. Most were first or second-degree relatives of the patients studied. The third cohort consisted of patients with chronic anemia, excluding sickle cell anemia, who were otherwise healthy and age-matched to the first cohort. The etiology of anemia consisted of thalassemia major (beta thalassemia or E-beta thalassemia, N=13), congenital dyserythropoietic anemia (N=1), autoimmune hemolytic anemia (N=1), and spherocytosis
Am J Hematol. Author manuscript; available in PMC 2017 September 01.
Bush et al.
Page 3
Author Manuscript
(N=2). Exclusion criteria were prior neurologic insult or major medical problems outside of their chronic anemia. At the beginning of the study visit, patients underwent measurement of vital signs and phlebotomy. Complete blood count, reticulocytes, quantitative hemoglobin electrophoresis, lactate dehydrogenase and cell free hemoglobin levels were analyzed in the clinical laboratory.
Author Manuscript
MRI examinations were performed on a Philips Achieva 3 Tesla magnet using an 8-element head coil. CBF was measured using single slice phase contrast velocity measurements with the following parameters: repetition time 12.3 ms, echo time 7.5 ms, field of view 260×260 mm, slice thickness 5 mm, signal averages 10, acquisition matrix 204 × 201, reconstruction matrix 448 × 448, bandwidth 244 Hz/pixel, and velocity encoding gradient of 200 cm/s. Details of localization and analysis have been previously published, as well as flow data from a subset of the control and anemia control cohorts13. Whole brain 3D T1 and T2 weighted images were collected for brain segmentation and volume purposes13. MR angiography was also performed to evaluate for large vessel stenoses. T1, T2 and MR angiography images were read for white matter disease and stenosis by a licensed neuroradiologist, blinded to disease status. At the end of the imaging study, CBF was reassessed after patients had been breathing 100% oxygen through a non-rebreathing mask for ten minutes. Oxygen content was estimated from hemoglobin according to the well known relationship14: (1)
Author Manuscript
where oxygen saturation was measured by pulse oximeter and pO2 is the partial pressure of oxygen, which was assumed to be 100 torr on room air. We did not estimate oxygen content on 100% oxygen because arterial blood gases were not performed. Oxygen delivery to the brain can then be calculated as the product of cerebral blood flow and oxygen content, or conversely CBF can be written as follows: (2)
Author Manuscript
Intuitively, one would then expect CBF to vary inversely with oxygen content. To control for the nonlinearity in equation [2], we log transformed all variables in our regression analyses for CBF and oxygen delivery. Log transformation improved the normality of all the predictive variables. Following log transformation, equation (2) becomes
(3)
Stepwise linear regression was performed using JMP Pro 11 (SAS, Cary, NC).
Am J Hematol. Author manuscript; available in PMC 2017 September 01.
Bush et al.
Page 4
Author Manuscript
RESULTS
Author Manuscript
Patient demographics are summarized in Table 1. The anemia control group (ACTL), when the two spherocytosis patients were excluded, had hemoglobin levels (10.3 ± 0.9 g/dl) comparable to the SCD patients. Differences in CBC parameters and reticulocytes among the groups were predictable. Cell-free hemoglobin was equally elevated in the SCD and ACTL patients, but LDH was only elevated in the SCD patients. Diastolic blood pressures were low in both SCD and ACTL patients, consistent with decreased systemic vascular resistance found in anemic patients. SCD patients were mildly desaturated compared to the other subjects, but only 3 patients had oxygen saturations below 93%. Body habitus was comparable in all three groups. The SCD patients were predominantly SS patients with 50% receiving chronic transfusion. Because many of the control subjects were family members, the population was roughly balanced between AS and AA. ACTL patients were predominantly AA hemoglobin; 3 E-beta thalassemia patients had AE electrophoresis but their native erythropoiesis was effectively suppressed. MR angiography was abnormal in only one subject, a 37 year old man with sickle cell disease who had mild narrowing of his right internal carotid artery and severe, bilateral proximal anterior cerebral artery stenosis without overt stroke. Small, white matter hyperintensities (silent strokes) greater than expected for age were observed in a total of 16 subjects, 4/38 controls, 1/14 anemia controls, and 11/37 patients with SCD (p
