Quantitative MR imaging of children with sickle cell disease: striking T1 elevation in the thalamus

Social adjustment in children diagnosed with sickle cell disease: A retrospective study

BACKGROUND

Children with sickle cell disease (SCD) are at risk for physical, psychological, and social adjustment challenges. This study sought to investigate social adjustment and related factors in children living with SCD.

METHODS

Data from 32 children (50% male, mean age = 10.32 years, SD = 3.27) were retrospectively collected from a neuropsychology clinic at a tertiary care pediatric hospital. Social adjustment was measured using the Behavior Assessment System for Children (BASC-3) parent-proxy, withdrawal subscale, and the Pediatric Quality of Life Inventory (PedsQL) Generic Module Social Functioning self- and parent-proxy subscales. Other measures captured executive functioning (i.e., Behavior Rating Inventory of Executive Function, Second Edition (BRIEF-2) Parent Form) and non-disease-related associations with social adjustment, including number of years in Canada and family functioning (i.e., PedsQL Family Impact Module).

RESULTS

Sixteen percent of patients reported elevated social adjustment difficulties. Multiple linear regression found better family functioning [B = .48, t = 2.65, p = .016], and higher executive functioning [B = -.43, t = -2.39, p = .028] were related to higher scores on the PedsQL parent-proxy ratings of social adjustment [F(4,18) = 5.88, p = .003]. Male sex [B = .54, t = 3.08, p = .005], and having lived more years in Canada [B = .55, t = 2.81, p = .009], were related to higher PedsQL self-reported social adjustment [F(4,23) = 3.75, p = .017]. The model examining the BASC-3 withdrawal subscale was not statistically significant [F(4,16) = 1.63, p = .22].

IMPLICATIONS

Social adjustment in children diagnosed with SCD warrants future research to understand the influence of executive function, and non-disease-related factors, particularly focusing on sociocultural factors.

Physiological correlates of intellectual function in children with sickle cell disease: hypoxaemia, hyperaemia and brain infarction

Lowered intelligence relative to controls is evident by mid-childhood in children with sickle cell disease. There is consensus that brain infarct contributes to this deficit, but the subtle lowering of IQ in children with normal MRI scans might be accounted for by chronic systemic complications leading to insufficient oxygen delivery to the brain. We investigated the relationship between daytime oxyhaemoglobin saturation (SpO2), cerebral blood flow velocity (CBFV) and intellectual function (IQ) using path-analysis in 30 adolescents with sickle cell disease (mean age 17.4 years, SD 4.2). Initial analyses revealed that the association between SpO2 and Full Scale IQ (FSIQ) was fully mediated by increased CBFV, whereby SpO2 was negatively correlated with CBFV and CBFV was negatively correlated with FSIQ, i.e. decreases in oxygen saturation are associated with increases in velocity, and increased velocity is associated with lowered IQ scores. The mediated relationship suggests that lowered IQ may be a function of abnormal oxygen delivery to the brain. Further analyses showed that the association between CBFV and IQ was significant for verbal but not for performance IQ. The pathophysiology characteristic of SCD can interfere with brain function and constrain intellectual development, even in the absence of an infarct. This supports the hypothesis that lowered intellectual function is partly explained by chronic hypoxia, and has wider implications for our understanding of SCD pathophysiology.

Hypoxic adaptation during development: relation to pattern of neurological presentation and cognitive disability

Children with acute hypoxic-ischaemic events (e.g. stroke) and chronic neurological conditions associated with hypoxia frequently present to paediatric neurologists. Failure to adapt to hypoxia may be a common pathophysiological pathway linking a number of other conditions of childhood with cognitive deficit. There is evidence that congenital cardiac disease, asthma and sleep disordered breathing, for example, are associated with cognitive deficit, but little is known about the mechanism and whether there is any structural change. This review describes what is known about how the brain reacts and adapts to hypoxia, focusing on epilepsy and sickle cell disease (SCD). We prospectively recorded overnight oxyhaemoglobin saturation (SpO2) in 18 children with intractable epilepsy, six of whom were currently or recently in minor status (MS). Children with MS were more likely to have an abnormal sleep study defined as either mean baseline SpO2 <94% or >4 dips of >4% in SpO2/hour (p = .04). In our series of prospectively followed patients with SCD who subsequently developed acute neurological symptoms and signs, mean overnight SpO2 was lower in those with cerebrovascular disease on magnetic resonance angiography (Mann-Whitney, p = .01). Acute, intermittent and chronic hypoxia may have detrimental effects on the brain, the clinical manifestations perhaps depending on rapidity of presentation and prior exposure.

Sickle cell disease: the neurological complications

The genetic cause of sickle cell disease has been known for decades, yet the reasons for its clinical variability are not fully understood. The neurological complications result from one point mutation that causes vasculopathy of both large and small vessels. Anemia and the resultant cerebral hyperemia produce conditions of hemodynamic insufficiency. Sickled cells adhere to the endothelium, contributing to a cascade of activated inflammatory cells and clotting factors, which result in a nidus for thrombus formation. Because the cerebrovascular reserve becomes exhausted, the capacity for compensatory cerebral mechanisms is severely limited. There is evidence of small-vessel sludging, and a relative deficiency of nitric oxide in these vessels further reduces compensatory vasodilatation. Both clinical strokes and silent infarcts occur, affecting motor and cognitive function. New data suggest that, in addition to sickle cell disease, other factors, both environmental (eg, hypoxia and inflammation) and genetic (eg, mutations resulting in thrombogenesis), may contribute to a patient's stroke risk. The stroke risk is polygenic, and sickle cell disease can be considered a model for all cerebrovascular disease. This complex disease underscores the potential intellectual and practical distance between the determination of molecular genetics and effective clinical application and therapeutics.

More than meets the eye: significant regional heterogeneity in human cortical T1

Segmented k-space acquisition of data was used to decrease the acquisition time and to increase the imaging resolution of the precise and accurate inversion recovery (PAIR) method of measuring T(1). We validated the new TurboPAIR method by measuring T(1) in 158 regions of interest in 12 volunteers, using both PAIR and TurboPAIR. We found a 3% difference between methods, which could be corrected by linear regression. After validation, the TurboPAIR method was used to test a hypothesis that there is significant regional heterogeneity in cortical T(1). We measured cortical gray matter T(1) in 11 right-handed volunteers, in 48 regions of interest scattered over frontal and parietal cortex, and in 46 ROIs along the central sulcus (CS). We found that T(1) in the CS is less than T(1) elsewhere in the cortex (p<0.001), and that there is considerable hemispheric asymmetry in T(1) in gray matter, but not in white matter. In central gray structures (caudate, thalamus, nucleus pulvinarus), and in the posterior CS (sensory cortex), right hemisphere T(1) was significantly greater than left hemisphere T(1) (p< or =0.004). In cortical gray matter of the frontal lobe and anterior CS (motor cortex), left hemisphere T(1) was significantly greater than right hemisphere T(1) (p< or =0.003). These findings demonstrate that there is considerable regional heterogeneity in human cortical T(1) that is unexplained by differences in tissue iron content, but may be evidence of an inherent anatomic asymmetry of the brain.

Diffuse T1 reduction in gray matter of sickle cell disease patients: evidence of selective vulnerability to damage?

The objective of our study was to test the hypothesis that subtle brain abnormality can be present in pediatric sickle cell disease (SCD) patients normal by conventional MR imaging (cMRI). We examined 50 SCD patients to identify those patients who were normal by cMRI. Quantitative MR imaging (qMRI) was then used to map spin-lattice relaxation time (T1) in a single slice in brain tissue of all 50 patients and in 52 healthy age-similar controls. We also used a radiofrequency (RF) pulse to saturate blood spins flowing into the T1 map slice, to characterize the effect of blood flow on brain T1. Abnormalities were noted by cMRI in 42% (21/50) of patients, with lacunae in 32%, and encephalo malacia in 20%. Brain T1 in patients normal by cMRI was significantly lower than controls, in caudate, thalamus, and cortex (p < or =0.007), and regression showed that gray matter T1 abnormality was present in caudate and cortex by age 4 (p < or =0.002). In patients abnormal by cMRI, T1 reductions in gray matter were larger and more significant. White matter T1 was not significantly increased except in patients abnormal by cMRI. RF saturation in a slab below the T1 map produced no significant change in T1, compared to RF saturation in a slab above the T1 map, suggesting that inflow of untipped spins in blood does not cause an artifactual shortening of T1. Gray matter T1 abnormality was present in patients normal by cMRI, while white matter T1 abnormality was present only in patients also abnormal by cMRI. These findings suggest that gray matter is selectively vulnerable to damage in pediatric SCD patients and that white matter damage occurs later in the disease process. Our inability to find an effect from saturation of inflowing blood implies that rapid perfusion cannot account for T1 reduction in gray matter.

Role of positron emission tomography in determining the extent of CNS ischemia in patients with sickle cell disease

Nearly 25% of patients with sickle cell disease (SCD) experience central nervous system morbidity involving both large and small vessel disease. Optimal imaging methods for determining the extent of ischemia are not known. Positron emission tomography (PET) has the unique ability to show tissue function as well as structure. Reports concerning patients with non-SCD neurodegenerative disorders suggest PET may be useful in determining prognosis. We compared magnetic resonance imaging, magnetic resonance angiography, and neuropsychological testing with PET prospectively. Six patients with SCD and a history of stroke, aged 10 to 28, were enrolled. PET studies were performed on an ECAT HR 47 scanner (Siemens/CTI, Knoxville, TN) using 18-F-fluorodeoxyglucose as a tracer. PET interpretations were conducted in blinded fashion. MRI studies found two patients with only small vessel disease and four with both large and small vessel disease. In two of four subjects with large vessel disease, PET showed a corresponding metabolic abnormality and also identified an area of hypometabolism extending beyond the anatomical lesion as shown by MRI. PET did not demonstrate an abnormality corresponding with small vessel disease. Detailed neuropsychological testing demonstrated cognitive dysfunction in all cases. For some patients, PET may add sensitivity in detecting impaired metabolism in the area surrounding a major vessel infarct. However, the technique does not appear to be generally useful in characterizing small watershed or deep white matter infarcts. Larger studies, to include control subjects and carefully selected untransfused SCD patients, are needed. A combination of conventional imaging and neuropsychological testing remains the preferred evaluation for most SCD patients with neurologic symptoms.

Effect of a gadodiamide contrast agent on the reliability of brain tissue T1 measurements

To determine whether brain spin-lattice relaxation time (T1) can routinely be measured after contrast-agent injection, we measured T1 by a precise and accurate inversion-recovery (PAIR) method in five brain tumor patients, before and again after contrast-agent injection. The T1 in at least 20 regions of interest (ROIs) was measured in each patient, avoiding areas of contrast enhancement visible by conventional MR imaging. Contrast-agent injection reduced T1 in 51 regions of interest in white matter by less than 1% (not significant), and in 50 regions of interest in gray matter by less than 2% (p = 0.001). Pixel-by-pixel plots demonstrate that T1 is reduced substantially in extra-parenchymal tissues, but not in brain tissues. Therefore, T1 mapping with the precise and accurate inversion-recovery method can routinely be done after contrast injection. Our results suggest that the precise and accurate inversion-recovery method is not sensitive to the T1 of blood in the presence of an intact blood-brain barrier, although a substantial T1 reduction does occur in the absence of a blood-brain barrier.

Quantitative MRI of the brain in children with sickle cell disease reveals abnormalities unseen by conventional MRI

Conventional MRI (cMRI) has shown that brain abnormalities without clinical stroke can manifest in patients with sickle cell disease (SCD). We used quantitative MRI (qMRI) and psychometric testing to determine whether brain abnormalities can also be present in patients with SCD who appear normal on cMRI. Patients 4 years of age and older with no clinical evidence of stroke were stratified by cMRI as normal (n = 17) or abnormal (n = 13). Spin-lattice relaxation time (T1) of gray and white matter structures was measured by the precise and accurate inversion recovery (PAIR) qMRI method. Patient cognitive ability was assessed with a standard psychometric instrument (WISC-III or WISC-R). In all 30 patients with SCD, qMRI T1 was lower than in 24 age- and race-matched controls, in cortical gray matter (P < .0006) and caudate (P < .0009), as well as in the ratio of gray-to-white matter T1 (P < .008). In the 17 patients who were shown to be normal by cMRI, qMRI T1 was still lower than in controls, in both cortical gray matter (P < .02) and caudate (P < .004). Histograms of voxel T1 show that the proportion of voxels with T1 values intermediate between gray and white matter (ie, consistent with encephalomalacia) was 9% higher than controls in patients shown to be normal by cMRI (P < .05) and 15% higher than controls in patients shown to be abnormal by cMRI (P < .0005). The full scale intelligence quotient (FSIQ) of all patients with SCD was 75, compared to the FSIQ of 88 in a historical control group of patient siblings (P < .001). The FSIQ of patients shown to be normal by cMRI was 79, significantly lower than the FSIQ of patient siblings (P < .04). The FSIQ of 71 in patients shown to be abnormal by cMRI was significantly lower than both the patient siblings (P < .005) and the patients shown to be normal by cMRI (P < .04). Patients shown to be abnormal by cMRI scored lower than patients shown to be normal by cMRI, specifically on the subtests of vocabulary (P = .003) and information (P = .03). Cognitive impairment is thus significant, even in patients with SCD who were shown to be normal by cMRI, suggesting that cMRI may be insensitive to subtle neurologic damage that can be detected by qMRI. Because cognitive impairment can occur in children normal by cMRI, our findings imply that prophylactic therapy may be needed earlier in the course of SCD to mitigate neurologic damage.