Sickle cell disease: reference values and interhemispheric differences of nonimaging transcranial Doppler blood flow parameters

Rectangular tolerance regions and multivariate normal reference regions in laboratory medicine

Reference intervals are widely used in the interpretation of results of biochemical and physiological tests of patients. When there are multiple biochemical analytes measured from each subject, a multivariate reference region is needed. Because of their greater specificity against false positives, such reference regions are more desirable than separate univariate reference intervals that disregard the cross-correlations between variables. Traditionally, under multivariate normality, reference regions have been constructed as ellipsoidal regions. This approach suffers from a major drawback: it cannot detect component-wise extreme observations. In the present work, procedures are developed to construct rectangular reference regions in the multivariate normal setup. The construction is based on the criteria for tolerance intervals. The problems addressed include the computation of a rectangular tolerance region and simultaneous tolerance intervals. Also addressed is the computation of mixed reference intervals that include both two-sided and one-sided limits, simultaneously. A parametric bootstrap approach is used in the computations, and the accuracy of the proposed methodology is assessed using estimated coverage probabilities. The problem of sample size determination is also addressed, and the results are illustrated using examples that call for the computation of reference regions.

MRI detection of brain abnormality in sickle cell disease

Introduction: Over the past decades, neuroimaging studies have clarified that a significant proportion of patients with sickle cell disease (SCD) have functionally significant brain abnormalities. Clinically, structural magnetic resonance imaging (MRI) sequences (T2, FLAIR, diffusion-weighted imaging) have been used by radiologists to diagnose chronic and acute cerebral infarction (both overt and clinically silent), while magnetic resonance angiography and venography have been used to diagnose arteriopathy and venous thrombosis. In research settings, imaging scientists are increasingly applying quantitative techniques to shine further light on underlying mechanisms.Areas covered: From a June 2020 PubMed search of 'magnetic' or 'MRI' and 'sickle' over the previous 5 years, we selected manuscripts on T1-based morphometric analysis, diffusion tensor imaging, arterial spin labeling, T2-oximetry, quantitative susceptibility, and connectivity.Expert Opinion: Quantitative MRI techniques are identifying structural and hemodynamic biomarkers associated with risk of neurological and neurocognitive complications. A growing body of evidence suggests that these biomarkers are sensitive to change with treatments, such as blood transfusion and hydroxyurea, indicating that they may hold promise as endpoints in future randomized clinical trials of novel approaches including hemoglobin F upregulation, reduction of polymerization, and gene therapy. With further validation, such techniques may eventually also improve neurological and neurocognitive risk stratification in this vulnerable population.

Ultrasound Detection of Abnormal Cerebrovascular Morphology in a Mouse Model of Sickle Cell Disease Based on Wave Reflection

Sickle cell disease (SCD) is associated with a high risk of stroke, and affected individuals often have focal brain lesions termed silent cerebral infarcts. The mechanisms leading to these types of injuries are at present poorly understood. Our group has recently demonstrated a non-invasive measurement of cerebrovascular impedance and wave reflection in mice using high-frequency ultrasound in the common carotid artery. To better understand the pathophysiology in SCD, we used this approach in combination with micro-computed tomography to investigate changes in cerebrovascular morphology in the Townes mouse model of SCD. Relative to controls, the SCD mice demonstrated the following: (i) increased carotid artery diameter, blood flow and vessel wall thickness; (ii) elevated pulse wave velocity; (iii) increased reflection coefficient; and (iv) an increase in the total number of vessel segments in the brain. This study highlights the potential for wave reflection to aid the non-invasive clinical assessment of vascular pathology in SCD.

Correlating transcranial arterial Doppler velocities with haematologic parameters and haemolytic indices of Nigerian children with sickle cell anaemia

Introduction

Cerebral vasculopathy, elevated transcranial Doppler velocities and stroke are linked to excessive intravascular haemolysis in sickle cell anaemia. This study determined the prevalence and pattern of abnormal blood flow velocities in children with sickle cell anaemia from Northern Nigeria using transcranial Doppler and to correlate transcranial Doppler velocities with haematological and biochemical markers of haemolysis.

Methods

Full blood count, reticulocyte count, fetal haemoglobin and some selected biochemical markers of haemolysis of 100 children with sickle cell anaemia were determined at steady state. The time-averaged mean of maximal velocities in four major intracranial blood vessels was measured using non-imaging transcranial Doppler, which were then classified according to the stroke prevention in sickle cell disease trial protocol. A simple linear correlation between time-averaged mean of maximal velocities in the four major intracranial vessels as the dependent variable and haematological and biochemical markers of haemolysis as independent variables was also determined at the significance level of 0.05.

Results

Abnormal transcranial Doppler velocities, classified as high risk, were found in 3% of the studied patients; 11% had intermediate risk while 84% had standard risk. Most high velocities were detected in the middle cerebral artery. The time-averaged mean of maximal velocities of middle cerebral artery positively correlated with total white blood cell count, absolute neutrophil count, platelet count, reticulocyte count, serum lactate dehydrogenase and total bilirubin, while it was negatively correlated with haematocrit and fetal haemoglobin levels.

Conclusion

Our study showed low prevalence of abnormal transcranial Doppler velocities and low prevalence is unrelated to low markers of haemolysis in our study population.

Pattern of cerebral blood flow and the interrelationship of vascular parameters of transcranial Doppler imaging in children with sickle cell disease

PURPOSE

We assessed the pattern of cerebral blood flow and the relationship between the different transcranial Doppler imaging (TCDI) vascular parameters in children with sickle cell disease (SCD).

METHODS

Forty-three SCD pediatric patients with a stable condition (21 HbSS, 19 HbSβ⁰ Thal, and 3 HbSD), aged 10.1 ± 3.9 years were studied. A control group of 26 with age and sex-matched, were included for comparison. TCDI scanning was carried out using a phased array transducer of 1-3 MHz through the trans-temporal window. Peak systolic velocity (PSV), end diastolic velocity (EDV), time-averaged mean of the maximum velocity (TAMxV), pulsatility index (PI), and resistive index (RI) were in the anterior circle of Willis vessels and posterior cerebral artery.

RESULTS

The highest mean ± SD values for the middle cerebral artery PSV, EDV, TAMxV, PI, and RI were 130.30 + 26.5, 145.0 ± 21.9; 58.60 + 13.70, 67.90 ± 13.3; 84.90 + 14.50, 94.80 ± 17.9; 0.95 + 0.20, 0.80 ± 0.20; 0.58 + 0.09, 0.50 ± 0.10 in the control and SCD groups, respectively. Independent t-test showed significant difference (P < .05) for all vascular parameters in all vessels in each side between both groups. There was a positive significant correlation between TAMxV, PSV, and EDV (P = .001, r = .96), and a negative significant correlation between TAMxV and PI in the SCD group (P = .001, r = -.46) but not in the control group (P > .05, r = -.62).

CONCLUSIONS

There was a direct relationship between TAMxV and PSV and EDV, and an inverse relationship between TAMxV and PI in a group of SCD patients with normal TCDI values.

Assessment of cerebral blood flow with magnetic resonance imaging in children with sickle cell disease: A quantitative comparison with transcranial Doppler ultrasonography

INTRODUCTION

Transcranial Doppler ultrasonography (TCD) is a clinical tool for stratifying ischemic stroke risk by identifying abnormal elevations in blood flow velocity (BFV) in the middle cerebral artery (MCA). However, TCD is not effective at screening for subtle neurologic injury such as silent cerebral infarcts. To better understand this disparity, we compared TCD measures of BFV with tissue-level cerebral blood flow (CBF) using arterial spin-labeling MRI in children with and without sickle cell disease, and correlated these measurements against clinical hematologic measures of disease severity.

METHODS

TCD and MRI assessment were performed in 13 pediatric sickle cell disease patients and eight age-matched controls. Using MRI measures of MCA diameter and territory weight, TCD measures of BFV in the MCA [cm/s] were converted into units of CBF [ml min^-1100 g^-1] for comparison.

RESULTS

There was no significant association between TCD measures of BFV in the MCA and corresponding MRI measures of CBF in patients (r = .28, p = .39) or controls (r = .10, p = .81). After conversion from BFV into units of CBF, a strong association was observed between TCD and MRI measures (r = .67, p = .017 in patients, r = .86, p = .006 in controls). While BFV in the MCA showed a lack of correlation with arterial oxygen content, an inverse association was observed for CBF measurements.

CONCLUSIONS

This study demonstrates that BFV in the MCA cannot be used as a surrogate marker for tissue-level CBF in children with sickle cell disease. Therefore, TCD alone may not be sufficient for understanding and predicting subtle pathophysiology in this population, highlighting the potential clinical value of tissue-level CBF.

The significance of inadequate transcranial Doppler studies in children with sickle cell disease

Sickle cell disease (SCD) is a common cause of cerebrovascular disease in childhood. Primary stroke prevention is effective using transcranial Doppler (TCD) scans to measure intracranial blood velocities, and regular blood transfusions or hydroxycarbamide when these are abnormal. Inadequate TCD scans occur when it is not possible to measure velocities in all the main arteries. We have investigated the prevalence and significance of this in a retrospective audit of 3915 TCD scans in 1191 children, performed between 2008 and 2015. 79% scans were normal, 6.4% conditional, 2.8% abnormal and 12% inadequate. 21.6% of 1191 patients had an inadequate scan at least once. The median age of first inadequate scan was 3.3 years (0.7-19.4), with a U-shaped frequency distribution with age: 28% aged 2-3 years, 3.5% age 10 years, 25% age 16 years. In young children reduced compliance was the main reason for inadequate TCDs, whereas in older children it was due to a poor temporal ultrasound window. The prevalence of inadequate TCD was 8% in the main Vascular Laboratory at King's College Hospital and significantly higher at 16% in the outreach clinics (P<0.0001), probably due to the use of a portable ultrasound machine. Inadequate TCD scans were not associated with underlying cerebrovascular disease.

Implementation of a Process for Initial Transcranial Doppler Ultrasonography in Children With Sickle Cell Anemia

Stroke, a devastating complication of sickle cell anemia (SCA), can cause irreversible brain injury with physical and cognitive deficits. Transcranial Doppler ultrasonography (TCD) is a non-invasive tool for identifying children with SCA at highest risk of stroke. National guidelines recommend that TCD screening begin at age 2 years, yet there is research to suggest less than half of young children undergo screening. The purpose of this project was to use quality improvement methods to improve the proportion of patients aged 24-27 months who successfully completed their initial TCD from 25% to 75% by December 31, 2013. Quality improvement methods (e.g., process mapping, simplified failure mode effect analysis, and plan-do-study-act cycles) were used to develop and test processes for identifying eligible patients, scheduling TCDs, preparing children and families for the first TCD, and monitoring outcomes (i.e., TCD protocol). Progress was tracked using a report of eligible patients and a chart showing the age in months for the first successful TCD (population metric). As of December 2013, 100% of eligible patients successfully completed their initial TCD screen; this improvement was maintained for the next 20 months. In November 2014, a Welch's one-way ANOVA was conducted. Results showed a statistically significant difference between the average age of first TCD for eligible patients born in 2009 and eligible patients born during the intervention period (2010-2013; F[1,11.712]=16.03, p=0.002). Use of quality improvement methods to implement a TCD protocol was associated with improved TCD screening rates in young children with SCA.

In Vivo T1 of Blood Measurements in Children with Sickle Cell Disease Improve Cerebral Blood Flow Quantification from Arterial Spin-Labeling MRI

BACKGROUND AND PURPOSE

Children with sickle cell disease have low hematocrit and elevated CBF, the latter of which can be assessed with arterial spin-labeling MR imaging. Quantitative CBF values are obtained by using an estimation of the longitudinal relaxation time of blood (T1blood). Because T1blood depends on hematocrit in healthy individuals, we investigated the importance of measuring T1blood in vivo with MR imaging versus calculating it from hematocrit or assuming an adult fixed value recommended by the literature, hypothesizing that measured T1blood would be the most suited for CBF quantification in children with sickle cell disease.

MATERIALS AND METHODS

Four approaches for T1blood estimation were investigated in 39 patients with sickle cell disease and subsequently used in the CBF quantification from arterial spin-labeling MR imaging. First, we used 1650 ms as recommended by the literature (T1blood-fixed); second, T1blood calculated from hematocrit measured in patients (T1blood-hematocrit); third, T1blood measured in vivo with a Look-Locker MR imaging sequence (T1blood-measured); and finally, a mean value from T1blood measured in this study in children with sickle cell disease (T1blood-sickle cell disease). Quantitative flow measurements acquired with phase-contrast MR imaging served as reference values for CBF.

RESULTS

T1blood-measured (1818 ± 107 ms) was higher than the literature recommended value of 1650 ms, was significantly lower than T1blood-hematocrit (2058 ± 123 ms, P < .001), and, most interesting, did not correlate with hematocrit measurements. Use of either T1blood-measured or T1blood-sickle cell disease provided the best agreement on CBF between arterial-spin labeling and phase-contrast MR imaging reference values.

CONCLUSIONS

This work advocates the use of patient-specific measured T1blood or a standardized value (1818 ms) in the quantification of CBF from arterial spin-labeling in children with SCD.

Age-Related Changes of Normal Cerebral and Cardiac Blood Flow in Children and Adults Aged 7 Months to 61 Years

BACKGROUND

Cerebral and cardiac blood flow are important to the pathophysiology and development of cerebro- and cardiovascular diseases. The purpose of this study was to investigate the age dependence of normal cerebral and cardiac hemodynamics in children and adults over a broad range of ages.

METHODS AND RESULTS

Overall, 52 children (aged 0.6-17.2 years) and 30 adults (aged 19.2-60.7 years) without cerebro- and cardiovascular diseases were included in this study. Intracranial 4-dimensional flow and cardiac 2-dimensional phase-contrast magnetic resonance imaging were performed for all participants to measure flow parameters in the major intracranial vessels and aorta. Total cerebral blood flow (TCBF), cardiac and cerebral indexes, brain volume, and global cerebral perfusion (TCBF/brain volume) were evaluated. Flow analysis revealed that TCBF increased significantly from age 7 months to 6 years (P<0.001) and declined thereafter (P<0.001). Both cardiac and cerebral indices declined with age (P<0.001). The ratio of TCBF to ascending aortic flow declined rapidly until age 18 years (P<0.001) and remained relatively stable thereafter. Age-related changes of cerebral vascular peak velocities exhibited a trend similar to TCBF. By comparison, aortic peak velocities maintained relatively high levels in children and declined with age in adults (P<0.001). TCBF significantly correlated with brain volume in adults (P=0.005) and in 2 pediatric subgroups, aged <7 years (P<0.001) and 7 to 18 years (P=0.039).

CONCLUSIONS

Cerebral and cardiac flow parameters are highly associated with age. The findings collectively highlight the importance of age-matched control data for the characterization of intracranial and cardiac hemodynamics.

Sickle cell anemia: intracranial stenosis and silent cerebral infarcts in children with low risk of stroke

PURPOSE

Children with sickle cell anemia (SCA), who have mean blood flow velocities <170 cm/s in the terminal internal carotid (tICA) or middle cerebral (MCA) arteries on transcranial Doppler ultrasonography (TCD), are considered to be at low risk of stroke. The prevalence of intracranial stenosis, which raises the risk of stroke, is not known in these children. Here, we estimated the prevalence of stenosis and explored its association with silent cerebral infarcts determined based on Magnetic Resonance (MR) scans.

PATIENTS/METHODS

We studied prospectively a cohort of 67 children with SCA without prior clinically overt stroke or TIA (median age 8.8 years; range limits 2.3-13.1 years; 33 females) and with TCD mean velocity <170 cm/s. They underwent MR imaging of the brain and MR angiography of intracranial arteries.

RESULTS

In 7 children (10.5%, 95% CI: 4.9-20.3%) we found 10 stenoses, including 4 with isolated left tICA stenosis and 3 with multiple stenoses. We found silent infarcts in 26 children (37.7%, 95% CI: 27.2-49.5%). The median number of infarcts in an affected child was 2 (range limits: 1-9), median volume of infarcts was 171 mm(3) (range limits: 7-1060 mm(3)), and median infarct volume in relation to total brain volume was 0.020% (range limits: 0.001-0.101%). The number and volume of infarcts were significantly higher in children with arterial stenosis (both p=0.023).

CONCLUSIONS

The prevalence of intracranial arterial stenosis in children with SCA classified as at low risk of stroke by TCD mean velocity <170 cm/s is high. Children with stenosis are at higher risk of brain parenchymal injury as they have more silent cerebral infarcts.

Cerebral blood flow abnormalities in children with sickle cell disease: a systematic review

A systematic review was performed to assess whether cerebral blood flow with different imaging modalities could identify brain abnormalities in children with sickle cell disease where structural magnetic resonance imaging and transcranial Doppler velocity appeared normal. A total of 11 studies were identified which reported cerebral blood flow abnormalities alongside structural magnetic resonance imaging or transcranial Doppler velocity abnormalities in patients with sickle cell disease. Potential for bias was assessed with the quality assessment of diagnostic accuracy studies scale in addition to treatment bias. Subjects of each study were categorized into patients with and without stroke. The prevalence of abnormalities for each modality was then separately calculated in each group. The included studies had mostly moderate degrees of bias. The prevalence of blood flow abnormalities compared with structural magnetic resonance imaging abnormalities was equal to or lower in patients with stroke and equal to or greater in patients without stroke. Blood flow abnormalities were more prevalent than transcranial Doppler abnormalities in four studies of patients without stroke and in one study of patients with stroke. The studies suggest that the assessment of cerebral blood flow in sickle cell disease can be of potential value in addressing brain abnormalities at the tissue level; however, further studies are warranted.

Retinal changes in children and adolescents with sickle cell disease attending a paediatric hospital in Cairo, Egypt: risk factors and relation to ophthalmic and cerebral blood flow

BACKGROUND

Sickle cell disease (SCD) is characterised by occlusion of small blood vessels. This study aimed to assess retinal changes in patients with SCD and its correlation with time-averaged mean flow velocity (TAMV) in middle cerebral arteries (MCA) and ophthalmic arteries (OA).

METHODS

Sixty SCD patients (aged 3-18 years) attending a paediatric hospital in Cairo, Egypt, during March 2010 to November 2011, were compared with 30 healthy controls. All underwent clinical and fundus examination by indirect ophthalmoscopy, and assessment of TAMV in MCAs and OAs by transcranial Doppler, repeated 1 year later for those with conditional velocities.

RESULTS

HbS/β was diagnosed in 32 patients and HbSS in 28; 50 patients had normal fundus and 10 had bilateral non-proliferative retinopathy. Risk factors for retinopathy included HbSS, age, previous stroke, non-compliant hydroxyurea (HU) therapy, frequency of sickling crises and HbS level. TAMVs were increased in MCAs, but not in OAs, in sicklers. TAMVs in MCAs and OAs increased with non-compliant HU therapy, previous stroke, age, frequency of sickling crises and level of HbS. No significant interhemispheric difference was found.

CONCLUSION

Sickle retinopathy was correlated with TAMV in MCAs but not in OAs. A significant difference was found between initial and follow-up TAMVs in the MCAs, after 1 year of regular HU and transfusion therapy, in those with conditional velocities.

Sickle cell disease in children: accuracy of imaging transcranial Doppler ultrasonography in detection of intracranial arterial stenosis

This study aimed to determine the accuracy of imaging transcranial Doppler sonography in detection of intracranial arterial stenosis in children with sickle cell disease using three-dimensional MR angiography as a reference standard. Sixty-one children (mean age 102±39 months, 30 males), who had no history of overt stroke, and were classified as at lowest risk of stroke by mean flow velocity criterion <170 cm/s, underwent conventional and imaging transcranial Doppler ultrasonographic examinations. We employed the area under the receiver operating characteristic curve (AUC) to determine the accuracy of flow velocity measurements obtained with imaging ultrasonography with and without correction for the angle of insonation as well as with conventional ultrasonography. We also established the most efficacious velocity thresholds for detection of the stenosis. We found ten intracranial stenoses in six patients on MR angiography, but we calculated AUC only for detection of stenosis (n=6) of the left intracranial internal carotid artery. The accuracy of flow velocity with angle correction was lower than the accuracy of velocity without angle correction (AUC=0.73, 95% CI, 0.53-0.93 versus AUC=0.87, 95% CI, 0.74-1.00; p=0.017). The accuracy of flow velocity obtained with conventional ultrasonography (AUC=0.82, 95% CI, 0.67-0.97) was not different from the accuracy of flow velocities obtained with imaging ultrasonography. We found that the threshold of 165 cm/s of mean velocity without angle correction is associated with highest efficiency for imaging (92%) and conventional ultrasonography (90%). Velocity measurements without angle-correction provide good accuracy in detection of stenosis of the terminal internal carotid artery, whereas angle-corrected velocities have lower accuracy.