Pathophysiology of sickle cell disease

INTRODUCTION — Sickle hemoglobin (Hb S, alpha2:beta^S2) is a result of a specific point mutation in the gene HBB, which encodes hemoglobin beta chains. It substitutes valine for the normal glutamic acid at the seventh amino acid (HBB p.glu7val). (See 'Sickle hemoglobin' below.)

Homozygosity for Hb S or compound heterozygosity for Hb S and one of certain other HBB variants (including beta thalassemia) causes the phenotype of sickle cell disease (SCD), characterized by vaso-occlusive pain, organ damage, hemolytic anemia, countless disease complications, and shortened survival.

This topic discusses the pathophysiology of SCD, including the mechanisms of Hb S polymerization, vaso-occlusion, and hemolysis.

Separate topics discuss the clinical consequences of these changes in different organ systems:

●General overview – (See "Overview of the clinical manifestations of sickle cell disease".)

●Pain – (See "Evaluation of acute pain in sickle cell disease".)

●Kidney – (See "Sickle cell disease effects on the kidney".)

●Splenic infarction – (See "Splenomegaly and other splenic disorders in adults", section on 'Focal splenic lesions'.)

●Functional asplenia – (See "Evaluation and management of fever in children and adults with sickle cell disease", section on 'Risk of life-threatening infection'.)

●Pulmonary complications – (See "Acute chest syndrome (ACS) in sickle cell disease (adults and children)" and "Overview of the pulmonary complications of sickle cell disease" and "Pulmonary hypertension associated with sickle cell disease".)

●Stroke – (See "Prevention of stroke (initial or recurrent) in sickle cell disease" and "Acute stroke (ischemic and hemorrhagic) in children and adults with sickle cell disease".)

●Priapism – (See "Priapism and erectile dysfunction in sickle cell disease".)

GENETICS

Sickle hemoglobin — SCD is a monogenic (single gene) disorder due to a specific variant in the beta globin gene, HBB, which encodes the beta globin chain of hemoglobin (Hb). HBB is located on chromosome 11 (11p 15.4) in the beta globin gene cluster (figure 1). (See "Structure and function of normal hemoglobins", section on 'Hemoglobin structure'.)

The sickle cell variant that produces Hb S is a single point mutation that creates an amino acid substitution (valine instead of the normal glutamic acid) at amino acid 7, referred to as p.glu7val (figure 2). The position of the variant was previously referred to as p.glu6val; this was changed to p.glu7val in the revised convention for amino acid numbering that includes the cleaved initial methionine residue as residue number 1. The p.glu7val mutation is the only point mutation that produces Hb S.

Phylogenetic analysis using data from various genome databases determined that Hb S arose over 7000 years ago [1]. The investigation suggested a single origin of the mutation in West or Central Africa, perhaps in the Green Sahara, which was wet and rainy at the time, or in the equatorial rainforest. Malaria was endemic in both areas during the Holocene period. The nearly exclusive presence of the original haplotype in certain regions of Africa suggests a single origin. This likely preceded a population split, possibly in the area of Cameroon, which led to the Bantu expansions approximately 2400 years later (approximately 5000 years ago).

SCD is an autosomal recessive trait. For the disease to manifest, there must be homozygosity for Hb S or compound heterozygosity for Hb S and beta thalassemia or a different HBB variant that interacts with Hb S. (See 'Compound heterozygous syndromes' below.)

Heterozygosity for Hb S (without a pathogenic variant at the other HBB allele) causes sickle cell trait, a usually benign carrier state with approximately 40 percent Hb S, without vaso-occlusion or hemolysis. Carriers have a normal life expectancy and slightly increased risk for thromboembolic disease and impaired kidney function. (See "Sickle cell trait".)

Rare variants exist in which two mutations are present on the same HBB allele, one of them coding for Hb S and the other for a distinct mutation at another location in the same gene.

●The first described was a variant called Hb C-Harlem, which had the p.glu7val mutation plus p.asp74asn; together this pair of mutations caused the hemoglobin to comigrate with Hb C on gel electrophoresis. (See "Hemoglobin variants including Hb C, Hb D, and Hb E", section on 'Hb C-Harlem'.)

●Approximately 15 other variants have been described with Hb S plus another mutation in the same HBB allele [2]. Their phenotypes vary in the heterozygous state or as compound heterozygotes with Hb S. Hb S-Antilles (p.glu7val plus p.val24ile) can cause SCD in heterozygotes because this variant produces a Hb that polymerizes more readily than Hb S [2].

Compound heterozygous syndromes — Compound heterozygous SCD syndromes include coinheritance of Hb S with another HBB variant that interacts with Hb S or with beta thalassemia. These compound heterozygous syndromes generally are associated with a Hb S percentage of ≥50 percent. Common examples of the second mutation include:

●A beta thalassemia mutation (beta⁰ or beta⁺)

●Hb C

●Hb D Los Angeles (Punjab)

●Hb E

●Hb O-Arab

These and other less common genotypes that also can cause SCD are discussed separately. (See "Hemoglobin variants including Hb C, Hb D, and Hb E" and "Overview of compound sickle cell syndromes".)

Most other HBB variants that have been described in compound heterozygosity with Hb S do not interact with the Hb S mutation, either directly by increasing polymerization or indirectly by enhancing cell density, akin the effects of Hb C. The variants that do not produce the SCD phenotype are generally associated with the largely benign phenotype of sickle cell trait.

The phenotype of compound heterozygosity with beta thalassemia depends on the specific beta thalassemia variant. (See "Molecular genetics of the thalassemia syndromes".)

●Beta thalassemia variants that permit some normal Hb A expression (referred to as beta⁺) in combination with Hb S have a higher Hb S level than sickle cell trait. Depending on the Hb A level (which can range from <5 percent to >45 percent), vaso-occlusion and hemolysis can be severe or mild.

●Beta thalassemia variants that produce no hemoglobin (referred to as beta⁰) in combination with Hb S have a phenotype much like that of homozygosity for Hb S. They are characterized by an absence of normal Hb A, with only Hbs S and F, and increased levels of Hb A2.

Modifier genes (Hb F, alpha thalassemia, PKLR) — SCD is clinically heterogeneous. Individuals with identical Hb genotypes can have different frequencies of vaso-occlusive pain, variable rates of hemolysis, and different spectra of chronic complications. These differences are due partly to the modulating effects of modifier genes. The two most important genetic modifiers of the phenotype of SCD are levels of fetal hemoglobin (Hb F) and the presence of coincident alpha thalassemia. Variants in PKLR, the gene for pyruvate kinase (PK) deficiency, are also under investigation as disease modifiers. (See "Pyruvate kinase deficiency".)

●Hb F – Fetal hemoglobin (Hb F; alpha2:gamma2) and Hb A2 (alpha2:delta2) directly inhibit Hb S polymerization because these hemoglobins and their hybrid tetramers (alpha2:gamma,beta^S; alpha2:delta,beta^S) cannot be incorporated into the Hb S polymer [3]. Both the gamma chain of Hb F and the delta chain of Hb A2 have a glutamine residue at position 87 that accounts for a major portion of their anti-polymerization effects [4]. Gene therapy constructs that take advantage of this property are discussed separately. (See "Investigational therapies for sickle cell disease", section on 'Anti-sickling beta globin gene' and "Hematopoietic stem cell transplantation in sickle cell disease".)

●Alpha thalassemia – Alpha thalassemia, discussed below, indirectly inhibits polymerization by decreasing alpha globin chain synthesis, thereby reducing mean corpuscular hemoglobin concentration (MCHC). (See 'Effects of concomitant alpha thalassemia' below.)

●PKLR – PKLR variants are common in people of African descent.

•One study found a group of PKLR variants was associated with increased risk of hospitalization for acute sickle cell pain, although no information was provided about 2,3 BPG levels or hemoglobin-oxygen affinity, the most likely effectors of PK deficiency on the phenotype of SCD [5].

•In an informative case report, a genetic variant of PKLR was associated with acute painful episodes in sickle cell trait, which is usually an asymptomatic carrier state [6]. In this case, severe PK deficiency was present and was associated with 2,3 BPG levels nearly twice normal and P50 of 41.5 mm Hg (versus 25 mm Hg in controls), hemoglobin 8.5 g/dL, and frequent episodes of acute sickle cell pain. The presence of variant PKLR alleles in SCD noted above was associated with heterozygosity for intronic variants of unknown pathophysiologic significance.

•PK activators that decrease 2,3 BPG, thereby increasing hemoglobin-oxygen affinity and decreasing P50, are being studied as treatments for SCD. (See "Investigational therapies for sickle cell disease", section on 'Pyruvate kinase activation (mitapivat, etavopivat)'.)

●Others – It is likely that disease modifying genes and environmental factors that affect other aspects of Hb polymerization and the vaso-occlusive and hemolytic process, including vascular endothelial cells or mediators of inflammation, are also important contributors to disease heterogeneity. Many have been described but few have been independently verified [7]. (See "Basic genetics concepts: DNA regulation and gene expression", section on 'Genetic variation'.)

HB S POLYMERIZATION AND FIBER FORMATION — Hb S polymerization is the primary, indispensable, initiating pathophysiologic event in SCD from which all of the other pathophysiologic changes arise [8]. Therapies that target primarily Hb S polymerization reduce both vaso-occlusion and hemolytic anemia.

Two approved drugs and gene therapy approaches have a primary effect on Hb S polymerization:

●Hydroxyurea induces increased levels of Hb F that interrupts the polymerization process. (See "Hydroxyurea use in sickle cell disease", section on 'Mechanism of action'.)

●Voxelotor prevents polymerization by keeping the Hb S molecule in the oxy configuration, which cannot polymerize. (See "Disease-modifying therapies to prevent pain and other complications of sickle cell disease".)

●Some gene therapy approaches can greatly increase Hb F or add a Hb F-like Hb A molecule, and these appear to abate acute vaso-occlusive events and correct hemolytic anemia [9-11]. (See "Investigational therapies for sickle cell disease", section on 'Therapies with curative intent' and "Hematopoietic stem cell transplantation in sickle cell disease".)

Deoxygenation-sickling cycle — Normal Hbs are soluble in the cytoplasm of the red blood cell (RBC). Hb S forms insoluble polymers when deoxygenated.

The primary modulator of oxygen affinity in the RBC is 2,3 bisphosphoglycerate (2,3-BPG, also called 2,3-DPG). Increased 2,3-BPG favors Hb S polymerization in three ways [12,13]:

●It lowers oxygen affinity, increasing deoxy Hb S.

●It reduces intracellular pH, increasing deoxy Hb S.

●It alters the conformation of deoxy Hb S.

Reducing 2,3-BPG by activators of RBC pyruvate kinase is being studied as a method of decreasing P50 and reducing Hb S polymerization. (See "Investigational therapies for sickle cell disease", section on 'Reducing Hb S polymerization'.)

The structure of Hb S when in solution (not polymerized) is nearly identical with that of Hb A, and the two Hbs have identical oxygen binding curves in dilute solution [14]. However, at concentrations above the solubility of deoxy Hb S that cause polymerization, the Hb-oxygen binding curve is progressively right shifted (figure 3) [15-17]. Since Hb S polymerizes only when it is in the tense (T) quaternary conformation, the polymer binds oxygen non-cooperatively and with low affinity [18].

Hb polymerization — The polymerization of Hb S is a simple phase change from solution to gel. It is an endothermic process facilitated by hydrophobic interactions [19-21]. The mechanism of polymer formation involves the self-association of Hb molecules; no accessory proteins are involved. It is entropically driven, resulting from the release of ordered water molecules from the surface of free Hb.

The assembly process obeys classic chemical rules of kinetics and thermodynamics, with a polymerization rate that depends on the concentration of Hb S, intracellular oxygen tension, pH, and temperature [22-24]. The glutamic acid at amino acid 7 seems to be especially important in preventing polymerization, via its electrostatic (repulsive) interactions with asparagine at amino acid 74 [25].

The contact between the valine 7 on the beta2 subunit and the acceptor site on the partner strand is only possible when Hb S is in the T conformation. In the R (relaxed, oxygenated) state, Hb S cannot fit into the polymeric structure. (See "Structure and function of normal hemoglobins", section on 'Oxygenation and deoxygenation'.)

●Promoting polymerization – Polymerization is enhanced by the following [20,26]:

•Deoxygenation – The contact between the valine at amino acid 7 and the acceptor site in the partner strand is only possible when Hb S is in the T state; R state Hb S cannot fit into the polymeric structure. Hb S can undergo innumerable cycles of deoxygenation-induced polymerization and reoxygenation-induced depolymerization.

•Higher Hb S concentration

-In adults who are homozygous for Hb S, Hb S can make up >90 percent of all Hb, with approximately 3 percent Hb A2 and variable amounts of Hb F, or it can be as low as 80 percent if Hb F is greatly increased (table 1).

-In sickle-beta⁺ thalassemia, mutations that produce fewer normal beta globin chains are associated with more severe disease because Hb S concentrations are higher, favoring polymerization.

-The marked cellular dehydration seen in Hb SC RBCs increases the concentration of Hb S, favoring polymerization [27,28].

-The virtual absence of vaso-occlusive complications and hemolysis in compound heterozygotes with gene deletion Hb S-HPFH (hereditary persistence of fetal hemoglobin) is due to pancellular levels of Hb F of approximately 30 percent, inhibiting Hb S polymerization [29-31]. (See "Fetal hemoglobin (Hb F) in health and disease", section on 'Non-deletion HPFH'.)

-Results of gene therapy trials that have resulted in >40 percent Hb F or a Hb F-like Hb A have prevented new vaso-occlusive events and corrected hemolytic anemia. (See "Investigational therapies for sickle cell disease", section on 'Therapies with curative intent' and "Hematopoietic stem cell transplantation in sickle cell disease".)

-Cellular dehydration causes molecular crowding. Resulting increases in Hb S concentration are accentuated by membrane damage, creating a cycle of increased sickling. Because of crowding, Hb S can polymerize at higher than expected oxygen saturations. (See 'Cellular dehydration' below.)

•Hb D Los Angeles (HBB p.glu121gln) polymerizes much more readily than a solution containing only Hb S; the glu112gln substitution stabilizes the polymer [32]. Compound heterozygotes for Hb S and Hb D Los Angeles have a severe clinical phenotype. (See "Hemoglobin variants including Hb C, Hb D, and Hb E", section on 'Hb D'.)

•Acidosis – A decrease in blood pH below 7.4 in tissue capillaries yields twice the normal decrease in oxygen affinity and a large release of oxygen from red cells, significantly increasing their risk of sickling. Clinically, this means that even mild transient acidosis (with a corresponding drop of intracellular pH) could induce sickling, leading to further tissue hypoxia. The latter was unethically demonstrated when patients with sickle cell anemia were infused with low pH isotonic solutions and acute painful episodes ensued.

●Reducing polymerization – Polymerization can be inhibited either directly or indirectly [4,30,33]:

•Direct inhibition – Direct inhibition is achieved by the following:

-Increasing Hb-oxygen affinity (decreasing deoxy-Hb S) with medications like voxelotor or by decreasing 2,3 BPG with therapies under development that activate pyruvate kinase. (See "Investigational therapies for sickle cell disease", section on 'Reducing Hb S polymerization'.)

-Increased Hb F occurs naturally with HPFH, or via therapeutic interventions. Hb F contains gamma chains, which are not incorporated into the polymer.

-Increasing Hb A2. Hb A2 contains delta chains, which are not incorporated into the polymer.

•Indirect inhibition – Indirect inhibition is achieved by the following, both of which reduce the mean corpuscular hemoglobin concentration (MCHC):

-Concomitant alpha thalassemia.

-Hydrating the sickle RBC using agents that inhibit cation and water loss.

Polymer structure — Following a change that induces Hb S polymerization, there is a measurable lag time before a signal reflecting the presence of detectable polymer. The progress of polymer formation is exponential. Once the polymer nucleus is formed (homogeneous nucleation), subsequent addition of molecules is highly favored (heterogeneous nucleation) and fiber growth becomes very rapid (approximately 250 hemoglobin tetramers per second) [34]. Hb S must be in the T quaternary conformation and must be unliganded for nucleation to occur [35].

The Hb S polymer is made of seven twisted double Hb S tetramer strands, one in the center of the fiber surrounded by the other six. These fibers elongate and align with other polymerized Hb fibers forming bundles of fibers; these are nearly visible in a transmission electron micrograph (EM) (picture 1). Longitudinal views reveal a subtle but regular helix structure [36]. Parallel bundles of long fibers are oriented along the axis of sickling [37-40]. These longitudinal polymer bundles give sickle cells their classic "sickle" shape and other highly variable shapes, as illustrated in the scanning EM (picture 2). Several intermolecular and intramolecular interactions appear to be important in the polymer structure. X-ray diffraction studies subsequently refined the structure at a resolution of 0.2 nm (2.0 Å) and provided information on molecular orientation and contacts.

In some RBCs, polymer is nucleated in several orientations. In cells that assume a holly leaf shape, bundles of Hb S fibers point in the direction of each projection. (See 'Formation of sickled cells' below.)

Polymerized Hb S damages the RBC membrane, interfering with membrane function, cell deformability, and cell lifespan. (See 'Effects on the RBC' below.)

EFFECTS ON THE RBC — Sickle fibers initiate a cascade of events that include damage to the red blood cell (RBC) membrane, cellular dehydration, deformation of the cell into a sickle shape, and hemolysis.

Membrane damage — The RBC membrane can undergo reversible changes related to the Hb S polymer. Following deoxygenation, RBCs containing Hb S assume a variety of distorted shapes readily appreciated by light microscopy (picture 3) and even more clearly by scanning electron microscopy (picture 2) [41].

However, the membrane can only withstand a finite number of these cycles before it is irreversibly injured and the abnormal cell shape becomes irreversible. The accumulation of membrane damage ultimately renders cells irreversibly sickled regardless of the physical state of their Hb molecules, which may be in solution [42].

Sickle cells appearing on blood films are irreversibly sickled cells (ISCs) [43]. Their numbers remain generally constant over time but decrease early in acute vaso-occlusive episodes, while dense cell numbers increase. Later in the acute vaso-occlusive event, dense cells rebound to levels higher than in the steady state, while ISCs mostly disappear.

Cell membrane damage can cause release of vesicles and membrane particles, which have been linked to increased endothelial adhesivity and hypercoagulability. (See 'Adhesion of sickled cells to the vascular endothelium' below and 'Hypercoagulable state' below.)

●RBC microparticles can be derived from reticulocytes and mature RBCs, and they can take several different forms. Large right-side-out particles display phosphatidylserine, which is prothrombotic [44].

●Microparticles derived from sickle RBCs increased neutrophil adhesion to cultured endothelial cells, along with increased levels of messenger RNA and protein for intercellular adhesion molecule-1 (ICAM-1), an effect that was reduced when microparticles were isolated from hydroxyurea-treated patients and increased when microparticles were isolated during acute painful episodes.

●Microparticles might affect pathophysiology by triggering a proinflammatory phenotype [45].

Cellular dehydration — Polymerized Hb fibers interact with cell membranes and activate volume-regulating transport systems, leading to RBC dehydration [46,47]. Sickle RBCs can be rigid, poorly deformable, and fragile due to the presence of Hb S polymer alone.

This dehydration of sickle RBCs involves increased activity of the membrane K-Cl cotransporter, the calcium-dependent (Gardos) potassium channel, and mechanosensitive channels such as Piezo-1 [48]. (See "Red blood cell membrane: Structure and dynamics", section on 'Piezo-1' and "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)", section on 'KCNN4 (Gardos channel gene)'.)

Increasing density adversely affects RBCs by increasing the concentration of Hb S, which in turn fosters polymerization and downstream consequences [49]. (See 'Hb S polymerization and fiber formation' above.)

Formation of sickled cells — Hb S polymer distorts and damages the RBC. Mature RBCs and reticulocytes can accumulate polymerized hemoglobin and form sickled cells, whereas immature, nucleated RBC precursor cells do not sickle [50-52].

Sickled cells are dense, dehydrated, fragile cells with limited deformability that likely impairs their ability to traverse the microcirculation [53]. However, simple mechanical obstruction of vessels by misshapen cells is not considered the principal mechanism of vaso-occlusion. (See 'Vaso-occlusion' below.)

The majority (perhaps 95 percent) of cells do not contain measurable polymer during their flow through arterioles and capillaries [54]. In contrast, if these cells were equilibrated at the oxygen tensions in the microcirculation, virtually all of them would contain polymer and as a result would have markedly decreased deformability. Thus, kinetics is the critical determinant of cell shape and morphology [23]. (See 'Vascular obstruction' below.)

The classic sickle shape that occurs with slow deoxygenation is due to homogeneous nucleation in which one domain propagates by fiber growth and alignment to distort the cell into an elongated shape. With somewhat more rapid deoxygenation, a few independent domains of heterogenous polymer nucleation will induce a more irregular shape [55].

Formation of other shapes depends on the kinetics of Hb S polymerization, which in turn depends on the delay time in the microcirculation where deoxygenation occurs [23,54].

●"Holly leaf" cells form when different fibers are aligned in different orientations. Because the shape of the sickle cell depends on the number of independent polymer domains, it is possible to convert a holly leaf cell into an elongated sickle shape by partial reoxygenation [56]. (See 'Polymer structure' above.)

●When deoxygenation is rapid, multiple spherulitic domains result in a granular or cobblestone texture ("sack of potatoes" aspect) with no gross distortion of cell shape.

The distortion of cell shape by projections of aligned Hb S fibers plays a critical role in the pathogenesis of the membrane lesion. (See 'Membrane damage' above.)

Sickle RBCs can hemolyze and precipitate vascular occlusion. (See 'Hemolysis' below and 'Vaso-occlusion' below.)

HEMOLYSIS — Vaso-occlusion and hemolytic anemia are actually interrelated, as both are consequences of Hb S polymerization and red blood cell (RBC) damage. For example, reticulocytes, greatest in number in patients with the most severe hemolysis, are thought to be the cells that might initiate vaso-occlusion.

Sites of RBC destruction — Sickle cells undergo intravascular and extravascular hemolysis, both of which contribute to anemia and increased RBC turnover, with a high percentage of reticulocytes. Approximately two-thirds of hemolysis is extravascular and one-third is intravascular. The intravascular hemolysis component varies among patients as a result of differences in the RBC distribution of Hb F and Hb S concentrations and cellular hydration.

The best biomarker for hemolysis is RBC lifespan. The percentage of reticulocytes and the Hb F level correlate strongly with the RBC lifespan; in contrast, the Hb and hematocrit are not correlated RBC lifespan [57]. Irreversibly sickled cells (ISCs) have a short survival, and their numbers reflect the rate of hemolysis [58].

Hemolytic anemia occurs continuously, even in the absence of clinically apparent acute vaso-occlusive events. Patients with the most severe anemia have fewer acute vaso-occlusive events, but they have increased mortality and more chronic organ damage [59]. Higher rates of hemolysis with nitric oxide (NO) scavenging might cause progressive vascular damage. (See 'Clinical consequences and therapeutic implications of SCD pathophysiology' below.)

●Intravascular — Intravascular hemolysis releases free heme and other cellular contents including arginase and lactate dehydrogenase (LDH) into the circulation. Arginase degrades arginine, the substrate of the NO synthases; LDH is one measure of intravascular hemolysis in SCD. Downstream effects of intravascular hemolysis include:

•Generation of reactive oxygen species

•Scavenging of NO [60]

•Generation of carbon monoxide (CO) from heme catabolism [20,26]

•Endothelial damage [61,62]

•Interference with the ability of ADAMTS13 to cleave large von Willebrand factor multimers [63]

•Activation of neutrophils and formation of neutrophil extracellular traps (NETs) [64]

●Extravascular – Extravascular hemolysis occurs in macrophages of the reticuloendothelial system in the liver, spleen (when still intact), and bone marrow.

Organ system effects of hemolysis — The following subphenotypes of SCD are associated with high levels of hemolysis (gauged indirectly by bilirubin levels, reticulocyte count, LDH, and RBC microparticles):

●Leg ulcers

●Priapism

●Kidney disease/albuminuria

●Hypertension

●Stroke and increased transcranial Doppler (TCD) flow velocity

●Hyperbilirubinemia/cholelithiasis

●Pulmonary hypertension (PH)

●Increased tricuspid regurgitant jet velocity on echocardiography, associated with PH

●Mortality

The mechanisms of these effects are incompletely understood and likely multifactorial, and they may differ depending on the specific organ system.

●Efferocytosis – Efferocytosis refers to phagocytosis of apoptotic cells by macrophages; it is important in resolving tissue injury. (See "An overview of the innate immune system", section on 'Efferocytosis'.)

A study using mouse models and in vitro assays demonstrated that macrophages from sickle cell mice had impaired efferocytosis, resulting in accumulation of damaged cells in the liver and increased hepatic transaminases [65]. The effect of free heme on macrophage reprogramming may contribute to sickle hepatopathy [66]. (See "Hepatic manifestations of sickle cell disease".)

●Nitric oxide – In addition to its vasodilatory activity, NO also modulates hemoglobin function, blocks the activation of platelets, and attenuates the adhesivity of endothelial cells for sickle cells [67-69].The apparently limited bioavailability of NO in SCD may be related to depleted stores of the NO precursor L-arginine, sequestration of plasma NO by free hemoglobin, oxidative inactivation of NO, increased levels of arginase, or failure to deliver NO to the RBC membrane [70-77]. Such loss of NO regulation, associated especially with high degrees of intravascular hemolysis, has been associated with endothelial dysfunction and end-organ vasculopathy including leg ulcers, nephropathy, priapism, PH, and death [73,78,79].

Sickle cells loaded in vitro with NO and reoxygenated exhibited decreased epinephrine-activated cell adhesion to endothelium, decreased ability to mediate leukocyte adhesion in vitro, and decreased vessel obstruction in vivo [80].

Effects of concomitant alpha thalassemia — Many populations with SCD have high rates of concomitant alpha globin gene deletions that cause alpha thalassemia (30 to >60 percent of individuals). The most common gene deletion removes a 3.7 kb fragment that leaves one functional alpha globin gene/chromosome intact. Patients can be either heterozygous (a deletion on one chromosome) or homozygous (a deletion on both chromosomes). (See "Diagnosis of thalassemia (adults and children)", section on 'Alpha thalassemias'.)

Alpha thalassemia modulates SCD by reducing the intracellular concentration of Hb S. This decreases Hb S polymer-induced cellular damage, which ameliorates hemolysis. Concomitant alpha thalassemia is usually associated with fewer complications related to hemolysis such as those listed above. (See 'Hemolysis' above.)

Hematologic and laboratory characteristics in SCD-alpha thalassemia include average Hb levels of 9 to 10 g/dL, an increase in Hb A2 to approximately 4 percent (which is due to post-translational mechanisms), microcytosis, and less evidence of hemolysis compared with individuals without concomitant alpha thalassemia (lower reticulocyte count, bilirubin, and LDH, and fewer dense cells and ISCs); RBC lifespan is increased. Individuals with homozygous alpha thalassemia have more pronounced hematologic changes and reductions in hemolysis than individuals who are heterozygous for alpha thalassemia.

However, individuals with concomitant SCD and alpha thalassemia have comparable or perhaps increased rates of acute vaso-occlusive events (painful episodes, acute chest syndrome). This has been ascribed to increased blood viscosity that results from their higher Hb levels [81].

Microcytosis increases intracellular levels of Hb A2. SCD-alpha thalassemia is a phenocopy of Hb S-beta⁰ thalassemia and is very difficult to distinguish without genetic testing [82]. The clinician should take care to establish the correct genetic diagnosis before providing genetic counseling for individuals designated as having Hb S-beta⁰ thalassemia based solely on a protein-based method of analysis such as high-performance liquid chromatography (HPLC) or electrophoresis [82]. (See "Overview of compound sickle cell syndromes".)

VASO-OCCLUSION

Vascular obstruction — A common assumption is that the pathophysiology of SCD involves only microvascular obstruction. This is based on the expectation that the smallest vessels should be most susceptible to vaso-occlusion. However, studies in animal models and in humans have identified several sites of impaired blood flow and vaso-occlusion:

●Large arteries of SCD patients with stroke [83]

●Small vessels [84]

●Arteriolar-capillary bifurcations and venular junctions [85]

●Capillaries [86-88]

●Post-capillary venules [86,89]

This variation may be due to the kinetics of polymerization, which includes a delay time between the generation of deoxy-Hb S and polymer formation, resulting in a delay in cell sickling until they have passed into a vessel too large to be occluded by a few sickled cells [22].

It seems reasonable that the initiation of vaso-occlusion varies over time and place according to differences in blood shear rates and endothelial activation states, vascular bed characteristics, vascular tone, differential responses to endothelial cell agonists, and varying expression of cellular and soluble adhesion molecules [90].

However, vaso-occlusive complications in SCD are unlikely to be due to simple vascular obstruction by sickled cells, as was previously thought. Rather, sickle cells may adhere to the vascular endothelium and induce changes that lead to vaso-occlusion, with sickled cells accumulating proximal to the initial site of adherence ("log-jamming") after other changes cause occlusion. These initial vascular changes include endothelial damage, inflammation, and vasoconstriction.

The initiation of vaso-occlusion is a complex, highly stochastic process. The mechanism involves several events including changes in RBC volume, shape, and membrane properties, leading to increased RBC adherence to the endothelium, along with vasoconstriction, inflammatory changes, and likely other changes that remain incompletely understood [86,91,92]. (See 'Adhesion of sickled cells to the vascular endothelium' below.)

Contributing mechanisms to vaso-occlusion are summarized in the table (table 2). Details and supporting studies are discussed below.

Adhesion of sickled cells to the vascular endothelium — Profound differences are present among endothelial cells isolated from different organs, perhaps accounting for some of the organ specificity of vaso-occlusive events [93,94]. In a 1980 report, abnormal adhesivity of sickle RBCs to endothelium was documented in vitro and in vivo, and it appeared to correlate with disease severity [95]. Ischemia-reperfusion-induced injury can extend the vaso-occlusion-initiated tissue damage [96,97].

Some of the cellular findings are depicted in the figures illustrating adhesion of sickled cells to the vasculature (figure 4) and subsequent trapping of denser cells (figure 5). In studies of human sickle cells perfused into rat vessels, the more adhesive, low density sickled cells such as reticulocytes adhered to the vascular endothelium to initiate occlusion, and poorly deformable, dense cells become trapped in the regions of cell adhesion [86].

Examples of contributing mechanisms include:

●Circulating factors – Cytokines, chemokines, histamine, thrombin, and heme have been implicated in endothelial adherence and endothelial cell activation [98-105]. Blood cells including activated monocytes and platelets may also contribute [106,107]. Adhesion is also increased by epinephrine [108-110].

Soluble adhesion markers (soluble ICAM-1, E-selectin, and VCAM-1) were elevated in patients with SCD, signifying endothelial dysfunction or inflammation with endothelial activation; the degree of elevation correlated with the severity of pulmonary hypertension and risk of death, although these are poor prognostic markers in individual patients [111].

Microparticles containing RBC membrane, released following membrane damage, may also contribute. (See 'Membrane damage' above.)

●Adhesion molecules – Adhesion molecules on endothelial cells and the adhesion of sickle cells (reticulocytes and mature RBCs) to these molecules has been the subject of considerable investigation [112-115].

•Endothelial cells express P-selectin upon hypoxic or ischemia/reperfusion injury [116-118]. Preclinical models support a role for P-selectin in binding of sickle cells to endothelial cells [119-122].

•Mouse models also suggest that E-selectin could increase vaso-occlusion [123].

•Infection or inflammation might initiate vaso-occlusion via expression of adhesion molecules [124-128].

•Complement components may also play a role [129,130]. (See "Complement pathways".)

•Endothelial cells in different organs and different vascular beds appear to express different combinations of adhesion molecules [131]. (See "The endothelium: A primer", section on 'Endothelial heterogeneity'.)

•Crizanlizumab, a P-selectin-blocking antibody, decreased acute vaso-occlusive events and is approved for treatment. (See "Investigational therapies for sickle cell disease", section on 'Decreasing cell adhesion'.)

There appears to be an inverse correlation between the severity of hemolysis and vaso-occlusive events. A reduction in the frequency of vaso-occlusive events in individuals with more severe hemolysis might be a result of lower blood viscosity associated with more severe anemia. (See 'Hemolysis' above.)

Vasoconstriction — Vasoregulatory factors probably influence vaso-occlusion by affecting flow velocity and the microcirculatory transit time. These effects could initiate or contribute to the evolution of vaso-occlusion already in progress [132].

Contributing factors may include:

●Hypoxia – Hypoxia induces vasoconstriction via factors such as endothelin (ET-1) and placental growth factor [133-139]. In most acute vaso-occlusive events, there is no evidence of preceding hypoxia.

●NO – Effects of NO on SCD physiology are complex. In addition to its role in vasodilation, NO blocks platelet activation and attenuates the adhesivity of the endothelium for sickle cells; vasoconstriction might be affected by its reduced bioavailability [59,140]. Reduced NO occurs when hemolysis releases from heme, which is an NO scavenger. (See 'Hemolysis' above.)

The vasoregulatory actions of catecholamines might be a potential component of the association between vaso-occlusive pain events and "stress." Epinephrine-activated sickle erythrocytes also induce lymphocyte and monocyte adherence to the endothelium [141]. (See 'Stress or stress hormones' below.)

Reduced blood flow in response to temperature, especially cold, was ascertained by plethysmography and found to be greater in people with SCD than in controls [142]. Individuals with higher anxiety also experienced more rapid vasoconstriction. Augmented vasoconstrictive responses and progressive decreases in perfusion with repeated thermal stimulation in SCD were indicative of autonomic hypersensitivity in the microvasculature.

Nocturnal vasoconstriction, measured during polysomnography in 212 children and adolescents with SCD, was predictive of subsequent acute painful events [143]. The role of the autonomic nervous system in the control of the microvasculature has been reviewed [144].

Inflammation — Inflammatory changes mediated by white blood cells (WBCs) and mast cells likely contribute to vaso-occlusion, but the therapeutic use of anti-inflammatory agents has not been demonstrated to reduce complications. In a study of 70 children with SCD, high-sensitivity C-reactive protein (hsCRP) levels, a marker of low-grade systemic inflammation, emerged as the most significant laboratory correlate of hospitalization for pain or vaso-occlusive events [145]. (See "Acute phase reactants".)

●Neutrophils – There is a correlation between high WBC count and rates of pain, hemorrhagic stroke, and mortality in SCD, although this may represent an association with other aspects of disease biology and may not be causally related [146,147]. Some have suggested that hydroxyurea might improve vaso-occlusive events by decreasing neutrophil counts; however, dosing is titrated based on neutrophil counts, making this challenging to evaluate. (See "Hydroxyurea use in sickle cell disease", section on 'Other mechanisms'.)

Administration of granulocyte colony-stimulating factor (G-CSF) to patients with SCD has resulted in extreme leukocytosis followed by severe vaso-occlusive events, some fatal [148]. (See "Overview of the management and prognosis of sickle cell disease", section on 'Avoidance of G-CSF'.)

The mechanisms by which neutrophils might foster vaso-occlusion are incompletely understood. Neutrophils are less deformable than RBCs and do not pass through the microvasculature as easily [149]. Neutrophils from individuals with SCD show enhanced expression of oxidant and antiinflammatory genes relative to controls [150]. Neutrophils bound to the endothelium might provide a nidus of adhesion to allow RBCs to adhere to the vasculature in areas of low flow or to take longer to transit through a low oxygen region and sickle [90,110,141,151-154].

●Monocytes – SCD patients have greater numbers of patrolling monocytes with high heme-oxygenase (HO)-1 expression compared with controls [155]. Individuals with a recent vaso-occlusive episode had lower numbers of these patrolling monocytes.

●Mast cells – Mast cell-derived cytokines may contribute to neurogenic inflammation and nociceptive pain (pain associated with inflammation). (See "Mast cells: Development, identification, and physiologic roles".)

●Cytokines and other inflammatory mediators – In sickle mice, C5a, a proinflammatory mediator, promotes inflammation and vaso-occlusion [156]. (See "Regulators and receptors of the complement system", section on 'C5a receptor'.)

Antiinflammatory therapies, including intravenous immune globulin (IVIG), statins, and mast cell inhibitors, had some benefits in sickle mice and in limited observations in patients, but these therapies are not a component of routine care. [157-159]

When used to treat acute vaso-occlusive events, glucocorticoids were associated with an increased rate of hospital readmission, and among >5000 patients of all ages, glucocorticoid exposure was associated with increased likelihood of hospitalization for a vaso-occlusive episode (OR 3.8, CI 2.4-5.6) [160,161].

In a randomized trial in 49 individuals with SCD and elevated hsCRP who were receiving hydroxyurea, patients assigned to receive canakinumab, an inhibitor of the proinflammatory cytokine interleukin (IL)-1beta, or placebo over 24 weeks, canakinumab reduced hospitalization, vaso-occlusive episodes, and fatigue, although it did not significantly reduce daily pain [162]. Reductions were observed in some markers of inflammation (hsCRP and WBC count) but not others (VCAM-1, P-selectin, or IL-8).

Other contributing factors

Increased blood viscosity — When the hematocrit of sickle cell blood is increased in vitro, even with the addition of non-sickle RBCs, blood viscosity is increased and blood flow properties are impaired [163]. This phenomenon provides the basis for the recommendation to avoid post-transfusion hemoglobin >10 g/dL when the hemoglobin S level is greater than 50 percent [164,165]. However, in practice when the hemoglobin S levels are <20 percent, the total hemoglobin can be increased to approximately 12 g/dL with less concern for hyperviscosity syndrome [163]. (See "Red blood cell transfusion in sickle cell disease: Indications and transfusion techniques", section on 'Risk of hyperviscosity syndrome from simple transfusion'.)

Stress or stress hormones — Mental stress (or even anticipation of a stressful event) causes vasoconstriction; this is true in individuals with and without SCD [166-168]. Stress hormones may increase pain by increasing vasoconstriction and autonomic nervous system reactivity. (See 'Vasoconstriction' above.)

Epinephrine may enhance adhesivity of sickle RBCs, lymphocytes, and monocytes to the vascular wall [108,110,141].

Psychosocial support, treatment of insomnia, and antidepressant therapy for individuals with depression are an important supplement to analgesia in management and prevention. (See "Acute vaso-occlusive pain management in sickle cell disease", section on 'Adjuvant therapies'.)

HYPERCOAGULABLE STATE — SCD is a hypercoagulable state with multifactorial mechanisms. Contributing factors may include [169-173]:

●Induction of inflammatory cytokines

●Microparticles derived from RBC membranes (see 'Membrane damage' above)

●Endothelial dysfunction

●Cyto-adhesive proteins mediating RBC adherence to the endothelium

●Ischemia-reperfusion injury

●Expression of tissue factor on circulating monocytes or microparticles

●Activated factor XII

●Platelet activation

●Activation of coagulation pathways and thrombin generation

●Free heme from intravascular hemolysis, leading to:

•Generation of oxygen-free radicals

•Vasoconstriction due to NO depletion

Endothelial NO diffuses to smooth muscle, where it binds and activates the heme of soluble guanylate cyclase, which converts guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP) and produces vasodilation by smooth muscle relaxation. NO depletion is a result of RBC release of heme into plasma. Heme reacts with NO via the dioxygenation reaction forming inert nitrate. NO is further depleted by its reaction with reactive oxygen species and by the destruction of arginine by arginase. Endothelial changes are discussed separately in more detail. (See "The endothelium: A primer", section on 'Procoagulant properties'.)

Venous thromboembolism (VTE) is a common complication of SCD. Up to 12 percent of patients have had a thromboembolic event by age 40 years, although rates twice as high have been reported [174]. (See "Overview of the clinical manifestations of sickle cell disease", section on 'Venous thromboembolism'.)

VTE prophylaxis is appropriate in most adults with SCD hospitalized with an acute medical illness. (See "Overview of the management and prognosis of sickle cell disease", section on 'Thromboembolism prophylaxis'.)

VTE treatment is similar to individuals without SCD.

CLINICAL CONSEQUENCES AND THERAPEUTIC IMPLICATIONS OF SCD PATHOPHYSIOLOGY — Vaso-occlusion and hemolysis both contribute to the phenotypes of SCD. Clinical manifestations according to organ system are discussed in separate topic reviews listed above. (See 'Introduction' above.)

If possible, therapies for SCD should reduce vaso-occlusion and hemolysis, as exemplified by the effects of inducing high levels of Hb F.

As they are slow to develop, the beneficial effects of reduced hemolysis on pulmonary and systemic vascular disease might take many years to become apparent.

If interventions only reduce hemolysis, then vaso-occlusive complications may not improve. This is suggested by the natural history of concomitant SCD with alpha thalassemia and the findings from a clinical trial of senicapoc, in which reduction of hemolysis did not decrease vaso-occlusive events [175].

The mechanisms of various interventions, approved and investigational, are discussed separately.

●(See "Hydroxyurea use in sickle cell disease".)

●(See "Disease-modifying therapies to prevent pain and other complications of sickle cell disease".)

●(See "Hematopoietic stem cell transplantation in sickle cell disease".)

●(See "Investigational therapies for sickle cell disease".)

PATIENT PERSPECTIVE TOPIC — Patient perspectives are provided for selected disorders to help clinicians better understand the patient experience and patient concerns. These narratives may offer insights into patient values and preferences not included in other UpToDate topics. (See "Patient perspective: Sickle cell disease".)

SUMMARY

●Hb S – The genetic variant that produces sickle hemoglobin (Hb S) is a point mutation in the HBB gene, which encodes beta globin chains (figure 1); it results in substitution of valine for glutamic acid at amino acid 7 (glu7val; previously designated as glu6val). Sickle cell disease (SCD) occurs when Hb S is present in the homozygous state (sometimes designated Hb SS) or in compound heterozygosity with certain other HBB variants (Hb C, D, or E), or with beta thalassemia (figure 2). Other gene variation can contribute to SCD severity. As an example, variants that increase Hb F production reduce the likelihood and severity of SCD complications. (See 'Genetics' above.)

●Hb S polymerization – Hb S can polymerize when in the deoxygenated state. This is the primary, indispensable, initiating pathophysiologic event in SCD. Polymerization is increased by higher Hb S concentrations, increased 2,3 BPG, and acidosis. (See 'Hb S polymerization and fiber formation' above.)

●RBC changes – Polymerized Hb S can cause red blood cell (RBC) membrane damage, which leads to cellular dehydration. This in turn further concentrates Hb S and increases cell sickling (picture 1). Abnormal RBC morphologies include the classic sickle cell shape (picture 2) and others (picture 3). (See 'Effects on the RBC' above.)

●Hemolysis – SCD is associated with continuous intravascular and extravascular hemolysis. Intravascular hemolysis releases free heme, which scavenges nitric oxide (NO), impairs phagocytosis of dead cells, and further increases vasoconstriction and inflammation. Hemolysis correlates with several SCD complications (leg ulcers, kidney disease, hypertension, stroke, pulmonary hypertension, cholelithiasis, and death). (See 'Hemolysis' above.)

●Vaso-occlusion – Many interrelated processes contribute to the pathophysiology of vaso-occlusion, as summarized in the table (table 2). Circulating factors (cytokines, chemokines, histamine, thrombin, heme) and adhesion molecules (selectins, complement components) contribute significantly to the initial adhesion of sickle RBCs to the vascular endothelium. Hypoxia and reduced bioavailability of NO contribute to vasoconstriction. Inflammation, increased blood viscosity, and stress hormones may exacerbate vaso-occlusion. All types of vessels can be involved. (See 'Vaso-occlusion' above.)

●Therapeutic implications – Approved therapies for SCD and new agents under investigation target different features of pathophysiology. Therapies that target Hb polymerization directly (hydroxyurea or gene therapy) are the most efficacious. (See 'Clinical consequences and therapeutic implications of SCD pathophysiology' above and "Hydroxyurea use in sickle cell disease" and "Disease-modifying therapies to prevent pain and other complications of sickle cell disease" and "Investigational therapies for sickle cell disease".)

ACKNOWLEDGMENT — UpToDate gratefully acknowledges Stanley L Schrier, MD (deceased), who contributed as Section Editor on earlier versions of this topic and was a founding Editor-in-Chief for UpToDate in Hematology.