15.2 Sickle Cell Disease

Pathophysiology

Sickle cell disease (SCD) is an autosomal recessive hemoglobinopathy caused by a point mutation in the beta-globin gene that substitutes valine for glutamic acid at the sixth amino acid position, producing hemoglobin S (HbS). Under conditions of hypoxia, acidosis, dehydration, or fever, deoxygenated HbS polymerizes into rigid rod-like fibers, distorting the red blood cell into the classic sickle shape. Sickled cells are (1) fragile, causing chronic hemolytic anemia, and (2) rigid and adhesive, causing vaso-occlusion in the microvasculature. These two mechanisms -- hemolysis and vaso-occlusion -- explain essentially every clinical manifestation of the disease and are the lens the exam expects you to apply to each vignette.

The most common genotype is homozygous HbSS (sickle cell anemia), but compound heterozygous states -- HbSC disease and HbS-beta-thalassemia -- also cause clinically significant disease, generally milder than HbSS.

Newborn Screening

Universal newborn screening uses hemoglobin electrophoresis or isoelectric focusing on a heel-prick dried blood spot, reported as a hemoglobin pattern:

Exam trap: The presence of any HbA on the newborn screen rules out homozygous sickle cell disease -- HbA can only come from a normal beta-globin allele. "FS" with no HbA at all is what confirms disease, not trait. All positive screens require a confirmatory test and referral to a pediatric hematologist, and empiric penicillin prophylaxis is started immediately while confirmation is pending given the high infection risk if the diagnosis is missed.

Vaso-Occlusive Crisis (VOC)

VOC ("pain crisis") is the most common reason for hospital presentation, typically involving the long bones, back, chest, or abdomen. In infants and toddlers, VOC in the small bones of the hands and feet is called dactylitis (hand-foot syndrome) and is often the first clinical manifestation of SCD, typically appearing between 6 months and 2 years (once HbF has declined enough to unmask HbS polymerization).

Management of VOC:

• IV or oral hydration at roughly maintenance rate -- avoid over-hydration, which can precipitate acute chest syndrome by increasing pulmonary edema risk
• Scheduled (around-the-clock), not PRN, opioid analgesia -- undertreated pain is a common exam trap and a real clinical error; individualized dosing based on the patient's prior effective regimen
• Oxygen only if hypoxic (SaO2 <95% or below baseline) -- supplemental oxygen in a normoxic child does not reverse sickling and can suppress erythropoiesis
• Incentive spirometry to reduce atelectasis and the risk of progression to acute chest syndrome
• Identify and treat precipitants: infection, dehydration, cold exposure, acidosis

Acute Chest Syndrome (ACS)

ACS is the leading cause of death in SCD and the complication the exam tests most aggressively for recognition. Definition: a new pulmonary infiltrate on chest X-ray involving at least one complete lung segment, plus at least one of: fever, cough, chest pain, tachypnea, wheezing, or hypoxia. ACS can be triggered by infection, fat embolism (from bone marrow infarction), or pulmonary vaso-occlusion/infarction, and can also be precipitated by aggressive fluid resuscitation or under-treated VOC with splinting/atelectasis.

Exam trap: A child admitted for VOC who develops fever and a new infiltrate a day or two later has evolved into ACS -- this is a distinct diagnosis requiring escalated management, not just "worse pain crisis."

Management of ACS:

• Empiric antibiotics covering both typical and atypical organisms (e.g., a third-generation cephalosporin plus a macrolide)
• Supplemental oxygen to maintain adequate saturation
• Bronchodilators if wheezing/reactive component present
• Incentive spirometry
• Judicious fluid management (avoid over-hydration)
• Simple or exchange transfusion for hypoxia, multilobar involvement, or clinical deterioration -- exchange transfusion is preferred in severe or rapidly progressive cases because it lowers the percentage of HbS without raising viscosity as much as simple transfusion

Splenic Sequestration

Acute splenic sequestration is a life-threatening complication in which a large volume of blood pools acutely in the spleen, causing sudden splenic enlargement, a precipitous drop in hemoglobin (often >=2 g/dL below baseline), thrombocytopenia, and hypovolemic shock. It typically occurs in infants and young children (before autosplenectomy is complete, roughly ages 6 months-2 years, sometimes up to age 5-6) and is a recognized cause of sudden death if unrecognized. Management is emergent volume resuscitation and simple blood transfusion; recurrence risk is high, so recurrent sequestration is an indication for elective splenectomy.

Aplastic Crisis

Aplastic crisis (distinct from aplastic anemia) is a transient, severe drop in hemoglobin caused by parvovirus B19 infection, which infects erythroid precursors and temporarily halts red cell production. In children with SCD, who already have shortened red cell survival from chronic hemolysis, even a brief cessation of erythropoiesis produces a precipitous fall in Hb. The hallmark laboratory finding is reticulocytopenia -- unlike a typical hemolytic exacerbation, where reticulocytes are elevated. The child may appear acutely more anemic and fatigued without the severe pain of a classic VOC. Management is supportive: transfusion for symptomatic anemia, monitoring until marrow recovery (typically 1-2 weeks), and counseling that household contacts who are pregnant should be informed because parvovirus B19 can cause fetal hydrops.

Infection Risk and Penicillin Prophylaxis

Repeated splenic infarction leads to functional asplenia by roughly 2-4 years of age, placing children with SCD at high risk for overwhelming infection with encapsulated organisms -- most importantly Streptococcus pneumoniae, and also Haemophilus influenzae type b and Neisseria meningitidis. Any febrile child with SCD requires urgent evaluation, blood culture, and empiric parenteral antibiotics (covering pneumococcus) given the risk of rapidly fatal sepsis.

Prevention:

• Oral penicillin V prophylaxis starting by 2 months of age (as soon as the diagnosis is confirmed/suspected), continued at minimum through age 5, when the highest-risk period for pneumococcal sepsis has passed (many centers continue longer in children with prior invasive pneumococcal disease or after splenectomy)
• Full, on-schedule pneumococcal conjugate and polysaccharide vaccination, plus routine immunizations including annual influenza vaccine

Stroke Risk and TCD Screening

Children with HbSS are at markedly elevated risk of overt and silent cerebral infarction from vaso-occlusion in the large intracranial vessels. Transcranial Doppler (TCD) ultrasound measures blood flow velocity in the middle cerebral artery and is used as an annual screening test from age 2 to 16 in children with HbSS/HbS-beta0-thalassemia. A time-averaged mean velocity >=200 cm/s is considered abnormal and identifies children at substantially elevated stroke risk; these children are started on chronic transfusion therapy, which has been shown to dramatically reduce stroke incidence in this population. Velocities in the 170-199 cm/s range are "conditional" and warrant more frequent repeat screening.