Current concerns in haematology. 1. Is the painful crisis of sickle cell disease a "steal" syndrome?

Introductiongenotypesofsickle cell disease arecharacterised byrecurrent attacks ofpain, whichare mostfrequent andmostsevereinhomozygous sickle celldisease andsickle cellfl° thalassaemia. The painmay belocalised to the abdomenbutthecausesofmost abdominal painful crises are unknown.Bonepainis muchmore common, andresults fromavascular necrosis ofthebonemarrow, asevidencedbytheaspiration ofnecrotic bonemarrow fromsites ofpain,`5 thebehaviour ofbone marrow on scintigraphy,26 and theclose correlation between thesites ofpainandthat oferythropoietically active bonemarrow. In children belowtheageof5years,active bone marrow occurs inthemetacarpals, metatarsals, andphalanges, andavascular necrosis results inthecharacteristic dactylitis orhandfootsyndrome. Atolder ages,active marrow disappears fromperipheral bonestobecome localised inthejuxta-articular areasoflong bones, andtheclinical picture ofdactylitis is replaced bytheclassic painful crisis. Inthis themost common sites ofpainarethelong bones, spine, andjoints, thelatter usually resulting fromthelongbonesadjacent tothe joint being affected. Ifactiveerythropoiesis withitshigh metabolic andoxygendemandsisone ofthe essential conditions ofthepainful crisis, what precipitates theavascular necrosis? An acute increase inoxygendemandmust beunlikely anditispresumed tofollow a fall inoxygen delivery. Thisreduceddelivery isusually attributed tothevaso-occlusion characteristic ofsickle celldisease, andthetermspainful crisis andvaso-occlusive crisis areoften used interchangeably. Observations which are inconsistent, ordifficult toreconcile withthis interpretation, include therole ofskincooling inprecipitating crises, thebilateral symmetry ofpain, andthefailure offactors inhibiting sickling to reducethenumberofpainful crises.