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LETTER TO THE EDITOR |
Monica L. Hulbert, MD
Division of Pediatric Hematology/Oncology
Riley Hospital for Children
Indianapolis, IN 46202
Michael R. DeBaun, MD, MPH
Department of Pediatrics
Washington University School of Medicine
St Louis, MO 63110
Lori C. Jordan, MD
Department of Neurology
James F. Casella, MD
Division of Pediatric Hematology
Department of Pediatrics
Johns Hopkins University School of Medicine
Baltimore, MD 21205
The observation of Ross et al provides a serendipitous insight into the possible effects of transfusion and subarachnoid hemorrhage (SAH) on cerebral blood flow velocities (CBFVs) in children with sickle cell disease (SCD). Their patient, an 8-year-old girl with hemoglobin SS, had abnormally high transcranial Doppler (TCD) velocities of the bilateral middle cerebral arteries (204 cm/second on the left and 220 cm/second on the right) before blood transfusion for acute chest syndrome. CBFV is inversely proportional to hematocrit,1 so worsening anemia may have been partially responsible for the elevation. Increases in metabolic requirements of the brain, such as with a fever or hypoxia, also increase CBFV as the brain increases blood flow to meet metabolic demands.2 However, although not seen on angiography, the degree of elevation suggests that other processes, such as fixed stenosis or vasoconstriction, may have also contributed. Elevated velocity was associated with hemorrhagic stroke in 9 children screened for or participating in the Stroke Prevention Trial in Sickle Cell Anemia (STOP) study, but this association was not as strong as that seen for ischemic stoke.3 Because anemia and other factors are important contributors to increases in CBFV, TCD has only been validated as a screening study for stroke when the patient is not acutely ill. This is the most widely accepted indication for TCD in children, but it is also used to evaluate for cardiac or pulmonary shunts, to document absent cerebral blood flow in brain death, and to evaluate for vasospasm after SAH.4
The patient had a profound global decrease in CBFV at least 6 hours before her seizure. The decrease in velocity was likely multifactorial, with some attributable to the 65% increase in hematocrit and the decrease in the percentage of hemoglobin S5 and the rest possibly attributable to the SAH. Acutely, CBFV is decreased globally in SAH, with the greatest decreases in the most severe hemorrhages.6 Subsequently, CBFV increases, typically between 4 and 12 days after the initial hemorrhage, and is usually secondary to vasospasm or proliferative arteriopathy.7 Existing cutoffs for vasospasm (>120 cm/second for mild and >180 cm/second for severe) have only been validated for adults without significant anemia.
Of the general population with spontaneous SAH, 15% to 20% will not have an etiology identified.6 The exact mechanism of SAH in children with SCD is unknown. It may result from an aneurysm or arteriovenous malformation, as in other children. Patients with preexisting cerebrovascular disease may have bleeding from the leptomeningeal collateral vessels that develop in patients with multiple distal cerebral vessel branch occlusions.8 However, this child had an evaluation with magnetic resonance and conventional angiography, which did not demonstrate any evidence of aneurysm, obstructive vasculopathy, or previous ischemic stroke. As we discussed in our article, SAH in children with SCD commonly occurs after recent blood transfusion, as in this child's case.9 This may be a result of decreased ability to autoregulate cerebral blood flow after blood transfusion, perhaps related to changes in blood volume, the effects of vasoactive substances in stored blood, or other causes.
With no etiology identified for the SAH, Ross et al face difficult decisions in treating their patient. There are no firm data regarding acute management of SAH in children with SCD, but the management tools applied to patients with SAH but without SCD seem reasonable: stabilization in a neurologic or pediatric ICU, consideration of nimodipine to decrease the risk of vasospasm, and a search for and occlusion of potential aneurysm or arteriovenous malformation. Most hematologists would recommend exchange blood transfusion to decrease the hemoglobin S level to <30% to maximize tissue oxygenation during the acute period of recovery from the SAH.10 Existing data are inadequate to guide long-term therapy aimed at reducing the risk of recurrent neurologic events. Certainly the patient's CBFV should be ascertained again once she has fully recovered from the acute event; if her CBFV is still elevated, a chronic blood-transfusion regimen would be recommended on the basis of the results of the STOP study. However, in patients without chronically elevated CBFV or history of ischemic stroke, the optimal management is not known. Usual care or chronic blood-transfusion therapy has been used in children with SCD and SAH.9 Ultimately, the decision on treatment approach must then be based on professional judgment and discussion with the patient and family. We appreciate the authors’ thoughtful comments and concur that the use of TCD in children with SAH requires additional study.
REFERENCES
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