Basic defect in HGB• SCD is a type of hemoglobinopathy• β-globin chain defect – Valine replaces glutamine at the 6th position from aminoterminal• As a result HGB molecules develop sticky points• Sticky points are exposed when the HGB is deoxygenated• Hence, HGB molecules polymerize to form fibers / long crystals
Red Blood Cells from Sickle Cell Anemia• Deoxygenation of SS erythrocytes leads to intracellular hemoglobin polymerization, loss of deformability and changes in cell morphology. OXY-STATE DEOXY-STATE
Sickle cell disease Point Mutation DNA mRNA AMINOACID C T C G A G G A G C A C G T G G U G
Deoxyhemoglobin S Polymer StructureA) Deoxyhemoglobin S B) Paired strands of C) Hydrophobic pocket 14-stranded polymer deoxyhemoglobin S for 6b Val (electron micrograph) (crystal structure) D) Charge and size prevent 6b Glu from binding.Dykes, Nature 1978; JMB 1979Crepeau, PNAS 1981 Wishner, JMB 1975
Basic defect in HGBPolymerization gives abnormal physiochemical properties to HGB - sickle hemoglobin (HbS) - that are responsible for the disease
PathogenesisAggregated HbS molecules assemble into long needle-like fibers within red cells, producing a distorted sickle or holly-leaf shape
Photomicrograph of red blood cells, showingabnormal shape characteristic of sickle cell anemia
PathogenesisThe major pathologic manifestations: 1. Chronic hemolysis 2. Microvascular occlusions, and 3. Tissue damageSeveral variables affect the rate and degree of sickling
PathogenesisSeveral variables affect the rate and degree of sickling: 1. Interaction of HbS with the other types of hemoglobin in the cell 2. Mean cell hemoglobin concentration (MCHC) 3. Intracellular pH 4. Transit time of red cells through microvascular beds
Mechanisms of RBC damage• Severe derangement in membrane structure due to distorsion of membrane by needles of HBS crytals – influx of Ca++ – efflux of K+ and H2O• Irreversibly sickled cells
Pathophysiology HgbS fibers are rigid Hgb S fibers deform RBC membranes Membrane disruption exposes transmembrane proteins and lipids that are pro-inflammatory Progressive sickling makes cells dense and inflexibleFrenette et al., Journal of Clinical Investigation 117(4): 850-858, 2007
SCD usually manifests early in childhood– For the first 6 months of life, infants are protected largely by elevated levels of Hb F; soon thereafter, the condition becomes evident.– The following 3 prognostic factors have been identified as predictors of an adverse outcome: (1) dactylitis in infants younger than 1 year, (2) Hb level of less than 7 g/dL, and (3) leukocytosis in the absence of infection.
Painful Crisis• The most common clinical picture during adult life is vasoocclusive crisis. – The crisis begins suddenly, sometimes as a consequence of infection or temperature change, such as an air-conditioned environment during a hot summer day. However, often, no precipitating cause can be identified. – Severe deep pain is present in the extremities, involving long bones. The abdomen is affected with severe pain resembling acute abdomen. The face also may be involved. Pain may be accompanied by fever, malaise, and leukocytosis. The person in crisis is in extreme discomfort. – The crisis may last several hours to several days and terminate as abruptly as it began.
Sickle-Cell Disease: Bone Infarcts, Upper Tibiae
Anemia• Anemia is universally present. – It is chronic and hemolytic in nature and usually very well tolerated. – While patients with an Hb level of 6-7 g/dL who are able to participate in the activities of daily life in a normal fashion are not uncommon, their tolerance for exercise and exertion tends to be very limited. – Anemia may be complicated with megaloblastic changes secondary to folate deficiency. These result from increased RBC turnover and folate utilization. Periodic bouts of hyperhemolysis may occur.
Aplastic Crisis• This is caused by infection with the Parvovirus B-19 (B19V). The virus infects RBC progenitors in bone marrow, resulting in cessation of erythropoiesis.• Coupled with greatly shortened RBC lifespan, usually 10-20 days, a very rapid drop in Hb occurs.• The condition is self-limited, with bone marrow recovery occurring in 7-10 days, followed by brisk reticulocytosis.
Hand-Foot Syndrome• Problem occurring in infancy is hand-foot syndrome. – This is a dactylitis presenting as painful swelling of the dorsum of the hand and foot. – Cortical thinning and destruction of the metacarpal and metatarsal bones appear on radiographs 3-5 weeks after the swelling begins. – Leukocytosis or erythema does not accompany the swelling.
Spleen• The spleen enlarges in the latter part of the first year of life.• Splenic sequestration crisis occurs due to a sudden very painful enlargement of the spleen due to pooling of large numbers of sickled cells..• The spleen undergoes repeated infarction• Over time, the spleen becomes fibrotic and shrinks. This is, in fact, an autosplenectomy.• Spleen is nonfunctional. Failure of opsonization cause inability to deal with infective encapsulated microorganisms, particularly Streptococcus pneumoniae.
Clinical intraoperative photograph showing massivesplenic infarction in a child with sickle cell anemia. Note the omental adhesions adherent to site of infarction
Splenic infarcts and white pulp hyperplasia (follicular lymphoid hyperplasia)
This photo shows the small, fibrotic residual spleen of an 11 year old child who had a history of many sickle crises and splenic sequestrations
“Autoinfarcted” splenic remnant in sickle cell disease
Infections• Pneumococcal infections are common in childhood.• During adult life, infections with gram- negative organisms, especially Salmonella, predominate.• Of special concern is the frequent occurrence of Salmonella osteomyelitis in areas of bone weakened by infarction.
Acute Chest Syndrome• Acute chest syndrome• Acute chest syndrome (ACS) refers to the combination of respiratory symptoms, new lung infiltrates, and fever.• Infection may set off a wave of local ischemia that produces focal sickling, deoxygenation, and additional sickling.• Treatment include – oxygen therapy, empiric antibiotic, analgesics. – Careful hydration that avoids volume overload. – Simple transfusion administered early may halt progressive respiratory deterioration. – Progress symptoms and failure to improve requires erythrocytapheresis (exchange transfusion).
Acute chest syndromePhotograph of the coronally sectioned lungs shows dark peripheral areas in both lungs, findings consistent with infarct, and central, red areas of recent pulmonary hemorrhage
CNS• Central nervous system involvement is one of the most devastating aspects of SCD. – It is most prevalent in childhood and adolescence. – The most severe manifestation is stroke, resulting in varying degrees of neurological deficit (silent strokes) – The stroke is mostly thrombotic, but it may also be hemorrhagic. – All children with SCD should be screened with transcranial Doppler.
Heart• The heart is involved due to chronic anemia and microinfarcts. – Hemolysis and blood transfusion lead to hemosiderin deposition in the myocardium. – Both ventricles and the left atrium are all dilated. – Usually, a systolic murmur is present, with wide radiation over the precordium.
Liver• Chronic hemolysis with hyperbilirubinemia is associated with the formation of bile stones.• Cholelithiasis may be asymptomatic or result in acute cholecystitis, requiring surgical intervention.• In addition, a hepatopathy may be present.
Lungs• Blood in the pulmonary circulation is deoxygenated, resulting in a high degree of polymer formation.• The lungs develop areas of microinfarction. The resulting areas that lack oxygenation aggravate the sickling process.• Pulmonary hypertension may develop. This may be due in part to the depletion of nitric oxide. Various studies have found that more than 40% of adults with SCD have pulmonary hypertension that worsens with age.
Kidneys• The kidneys lose concentrating capacity.• Isosthenuria results in a large loss of water, further contributing to dehydration in these patients. Renal failure may ensue, usually preceded by proteinuria.• Nephrotic syndrome is uncommon but may occur.
Defective Urine Concentrating Ability in Sickle Cell Trait• High osmolality and low O2 sat of the renal medulla are conditions that favor polymerization.• Hemoglobin polymerization correlates inversely with urine concentrating ability.• a-Thalassemia reduces %HbS, and polymerization potential. (-a/-a) (-a/aa) (aa/aa) 1000 Urinary Osmolality (mOsm/kg H2O) 900 800 700 600 500 400 300 20 35 50 Percent Hemoglobin S Gupta, Kirchner, Nicholson, Adams, Schechter, Noguchi, Steinberg, JCI 1991
Extremities• Leg ulcers are a chronic painful problem. They result from minor injury to the area around the malleoli. Because of relatively poor circulation, compounded by sickling and microinfarcts, healing is delayed and infection is established.• Repeated infarction of joints, bones, and growth plates leads to aseptic necrosis, especially in weightbearing areas such as the femur. This complication is associated with chronic pain and disability and may require changes in employment and lifestyle.
Others• Priapism is a serious complication and tends to occur repeatedly. When it is prolonged, it may lead to impotence.• Pregnancy represents a special area of concern. The high rate of fetal loss is due to spontaneous abortion. Placenta previa and abruption are common due to hypoxia and placental infarction. At birth, the infant often is premature or has low birth weight.
Clinical SyndromesDisease Severity is Genotype –DependentGenotype Worsening Disease SeverityHgb SSHgb S / β0 thalassemiaHgb SCHgb S / α thalassemiaHgb S / DHgb S / AHgb S / E AsymptomaticHgb S / β+ thalassemia + / - Mild AnemiaHgb S / HPFH
Diagnosis Hemoglobin Electrophoresis Cellulose acetate, pH 8.4Embury SH et al. Diagnosis of sickle cell syndromes. In: UpToDate, Rose, BD(Ed),UpToDate, Waltham, MA, 2008.
History of Sickle Cell Disease• 1927 Hahn and Gillespie: ↓ SaO2 and ↓ pH = ↑ sickling• 1949 Linus Pauling: Applied Gel Electrophoresis to separate Hgb A, Hgb AS, Hgb SS, and Hgb F Disease Molecular Defect * 1st-ever demonstration• 1956-1959 Hunt and Ingram identified B6 GluVal• 1950s Xray diffraction (Perutz and Mitchison) and 1960s electron microscopy (Dobler and Bertles): deoxygenation hemoglobin Hgb S polymerization sickling
James Bryan Herrick and his interndiscovered sickle cell anemia.
Immunohemolytic anemia - Warm antibody type• Common type – 50% cases are idiopathic – 50% -secondary• Autoantibodies are IgG, sometime IgA• Antibodies are active at 370C• Many cases Ab are directed against Rh blood group antigen• Moderate splenomegaly
Immunohemolytic anemia• Drug induced hemolysis 1.Hapten type – • Pencillin 2.Autoantibody model – • alpha-methyldopa
Drug induced hemolysis -- Hapten type (adsorption)§Eg. Pencillin,
Drug induced immune hemolytic anemia -- :• Autoantibody induction• Common with alpha-methyldopa,• Drug intiates Antibody production against native erythrocyte antigens (Rh blood group antigens)• Only 1% develop Significant hemolysis
Cold agglutinin type• Ab are IgM & are most active in vitro at 0- 4oC• Agglutination of red cells & fixation of complement occur at distal body part – Temp.300C –intravascular hemolysis• Vascular obstruction-Pallor, cyanosis of body part exposed to cold temperature (Raynaud phenomenon)
Cold hemolysins (paroxysmal cold hemoglobinuria)• Acute intermittent massive hemolysis after exposure to cold• Hemolysis is complement dependent• Autoantibodies are IgG & directed against P blood group antigen (Donath – Landsteiner Ab)
Cold hemolysins (paroxysmal cold hemoglobinuria)• They bound to red cells & complement at low temp. when temp. is elevated complement cascade is activated• Follows Mycoplasmal pneumonia, measles, mumps, viral/ flu syndrome• Mechanism of autoAntibody production is unknown
Sickle Cell Disease• Hereditary Hemoglobinopathy• Structurally abnormal Hb• Hb – Tetramer of 4 globin chains / two pairs of similar globin chains• Adult : – 96 % Hb A (α2 β2) – 3 % Hb A2 (α2 δ2) – 1 % Hb F (α2 γ2)
Sickle cell disease Point Mutation DNA mRNA AMINOACID C T C G A G G A G C A C G T G G U G
Sickle Cell Disease• Point mutation – substitution of valine for glutamic acid at 6th position of globin chain – HbS• Homozygous for sickle mutation – all Hb is HbS• Heterozygous for sickle mutation – only 40% of Hb is HbS
Sickle Cell DiseaseHbS – Different physiochemical properties• On deoxygenation HbS molecule undergo aggregation & polymerization – HbS fibre formation – Sickle shape – Intially – sickling is reversible – Later – irreversibly sickled
Sickle Cell Disease• Defect in membrane phosphorylation• Detachment of cell membrane from underlying membrane skeleton• Cells – dehydrated and dense
Sickle Cell Disease• Sickling depends on 1. Amount of HbS and its interaction with other Hb chains – Heterozygotes – less tendency for sickling – No hemolysis/ anemia (sickle cell trait) – Homozygotes – Full blown SCD
Sickle Cell Disease• HbF – Inhibits polymerization of HbS• New born do not manifest disease till 6 months• HbC has greater tendency to aggregate with HbS
Sickle Cell Disease• Rate of HbS polymerization affected by Hb concentration per cell – Dehydration -↑ MCHC - ↑sickling – Thalasemia - ↓MCHC – milder disease• Fall in PH - ↑ sickling
Sickle Cell DiseaseConsequences of HbS• Chronic hemolytic anemia – Splenic sequestration – life span 20 days• Occlusion of small blood vessel – ↑ expression of adhesion molecule promote adhesiveness of red cells to endothelial cell – narrowing of blood vessel – Ischemia and Infarction
Morphology• BM hyperplasia – Expansion of BM cavity – Resorption of bone with secondary new bone formation – X- ray of skull – crew hair cut appearnce• Extra medullary hematopoiesis• ↑ release of Hb & formation of Bilirubin – Jaundice, pigment gall stone
Sickle Cell Disease• Capillary stasis & thrombosis – Early stages – splenomegaly – Erythrostasis leads to thrombosis and infarction - Autosplenectomy• Infarction due to vascular occlusion – Bones, brain, kidney , liver and retina – Leg ulcers