Sickle cell crises saq

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I now have comprehensive, well-sourced information to write a thorough SAQ answer on sickle cell crises. Here is the complete answer:

Sickle Cell Crises - Short Answer Question (SAQ)

1. Background: Pathophysiology of Sickle Cell Disease

Genetic basis: Sickle cell disease (SCD) results from a point mutation - substitution of glutamic acid by valine at the 6th position of the beta-globin chain, producing HbS instead of HbA. This autosomal recessive condition is most severe in homozygotes (HbSS).

Mechanism of sickling: Under deoxygenated conditions (low O2 saturation), HbS polymerizes and aggregates inside erythrocytes, distorting them into a rigid sickle shape. In homozygotes, sickling can occur at oxygen saturations as high as 85% (vs. ~40% in trait carriers).

Downstream injury (Barash Clinical Anesthesia, Table 24-22):

RBC and platelet adhesion to endothelium
Activation of coagulation and thrombosis
Ischemia-reperfusion injury
Intravascular hemolysis releasing free Hb, which scavenges NO (nitric oxide) - causing endothelial dysfunction and vasoconstriction
Leukocyte activation and free radical injury
TLR4-mediated inflammatory amplification by free heme

Peripheral blood film showing sickle cells:

Peripheral blood film showing elongated, curved sickle cells among normal round red blood cells

(Quick Compendium of Clinical Pathology, 5e)

2. Types of Sickle Cell Crises

A. Vaso-occlusive Crisis (VOC) - Pain Crisis

Most common type. The first and most frequent manifestation of SCD.

Precipitants:

Cold exposure
Dehydration
Infection
Alcohol
Hypoxia (altitude, unpressurised flight)
Acidosis

Mechanism: Sickled cells obstruct the microcirculation, causing musculoskeletal ischemia. Vaso-occlusion involves not just HbS polymerization but also activated vascular endothelium, altered NO metabolism, hypercoagulability, and ischemia-reperfusion injury.

Features: Severe pain in bones, joints, abdomen, back. Dactylitis (hand-foot syndrome) in infants.

Management:

Mild: oral analgesics, rest, hydration
Severe: IV opioids (morphine or hydromorphone), IV fluids, supplemental O2, RBC transfusion in refractory cases, consider regional analgesia

Long-term complications:

Autosplenectomy (recurrent splenic infarction)
Avascular necrosis (hip most common, then shoulder)
Papillary necrosis + renal complications
Stroke/TIA (up to 30% of HbSS patients)
Liver infarction
Pulmonary hypertension (in ~10-30% of adults)

B. Acute Chest Syndrome (ACS)

Most common cause of death in SCD. Second most common reason for hospitalization.

Definition: New pulmonary opacity involving at least one complete lung segment, with at least one of: chest pain, fever, tachypnea, wheezing, cough - in a patient with SCD.

Causes: Fat embolism (from infarcted bone marrow), pulmonary infection, thromboembolism, hypoventilation from rib/sternum infarct.

Features:

Chest pain, dyspnea, cough, wheezing
Fever, tachypnea
Hypoxemia
New pulmonary infiltrates on chest X-ray
Leukocytosis

Mortality: 1-20%.

Management:

Supplemental O2
IV hydration
Analgesia
Incentive spirometry / respiratory support
Empirical antibiotics (pending sputum cultures)
RBC transfusion (simple or exchange) for severe/refractory cases
Note: Postoperative ACS incidence = 10-20% after abdominal surgery. Preoperative transfusion and postoperative spirometry reduce risk.

C. Aplastic Crisis

Cause: Transient arrest of erythropoiesis, almost always due to Parvovirus B19 infection (can also be folic acid deficiency).

Features:

Acute fall in Hb below baseline
Low or absent reticulocyte count (key differentiator)
Viral prodrome often present
Can precipitate heart failure in severe anemia

Management:

Simple RBC transfusion if severe anemia
Isolation (parvovirus is infectious)
IV fluids
Do not transfuse for asymptomatic anemia unless reticulocyte count is low

Indications for emergency transfusion (Quick Compendium of Clinical Pathology, 5e): stroke, retinal artery occlusion, splenic sequestration crisis, ACS, aplastic crisis.

D. Splenic Sequestration Crisis

Epidemiology: Occurs primarily in infants and young children (before autosplenectomy is complete). Rare in adults.

Mechanism: Sudden pooling of blood in the spleen due to partial or complete occlusion of venous drainage, causing rapid expansion of the spleen.

Features:

Acutely enlarged spleen
Hb 2+ g/dL below baseline
Life-threatening hypotension and hypovolemic shock
Often triggered by viral illness

Management:

IV fluid resuscitation
Type and screen
Admit for serial abdominal exams
RBC transfusion in 5-10 mL/kg aliquots - caution: autotransfusion from spleen on recovery can cause rebound rise in Hb and hyperviscosity
Recurrent severe cases: splenectomy

E. Hyperhemolytic Crisis

Features: Sudden, severe exacerbation of anemia with increased hemolysis.

May be caused by concomitant G6PD deficiency
Elevated bilirubin, elevated LDH, hemoglobinuria

3. Summary Table of Crises

Crisis	Key Feature	Reticulocytes	Cause	Emergency Tx
Vaso-occlusive	Severe bone/tissue pain	Normal/raised	Microvascular occlusion	Analgesia, fluids, O2
Acute chest syndrome	Pulmonary infiltrate + hypoxia	Normal	Fat embolism, infection	O2, antibiotics, exchange transfusion
Aplastic	Sudden severe anemia	Low/absent	Parvovirus B19	Simple transfusion, isolation
Sequestration	Splenomegaly + shock	Raised	Venous splenic occlusion	Fluids, careful transfusion
Hyperhemolytic	Worsening anemia + jaundice	Raised	G6PD deficiency	Supportive

4. Long-term / Chronic Complications of SCD

Stroke (up to 30% of HbSS)
Pulmonary hypertension (10-30% of adults)
Proliferative retinopathy
Renal disease - isosthenuria, gross hematuria, papillary necrosis, nephrotic syndrome, renal medullary carcinoma
Avascular necrosis of femoral/humeral head
Priapism (sustained painful erection >4 hr - treat with analgesia, hydration, O2, urology referral for aspiration)
Autosplenectomy - functional asplenia increasing risk of encapsulated organism sepsis

5. Management Principles

Acute

IV fluids (avoid dehydration)
Supplemental oxygen
Strong analgesia (IV morphine/hydromorphone for severe VOC)
Antibiotics if infection suspected

Preventive / Chronic

Intervention	Purpose
Hydroxyurea (hydroxycarbamide)	Induces HbF (target >20%), reduces frequency of painful crises and ACS, reduces transfusion requirements. Monitor for myelosuppression. Teratogenic - contraception required.
Penicillin V prophylaxis	Reduces pneumococcal sepsis risk (from age 3 months, when HbF wanes)
Vaccinations	Pneumococcal (PCV13 <2 yrs; PPV23 ≥2 yrs), meningococcal, Hib, influenza (annual)
Folic acid	Replaces folate consumed by chronic hemolysis
Chronic transfusion	Children with abnormal transcranial Doppler (TCD) velocities for stroke prevention; target HbS <30% in children, <50% in adults
Stem cell transplant	Only curative therapy; requires matched sibling donor

6. Key SAQ Points

Mutation: Glu → Val at 6th position of beta-globin chain (HbS)
Most common crisis: Vaso-occlusive (pain crisis)
Most common cause of death: Acute chest syndrome
Aplastic crisis cause: Parvovirus B19 - distinguished by absent reticulocytes
Sequestration crisis: Infants/young children; life-threatening; avoid over-transfusion
Hydroxyurea mechanism: Increases HbF, which prevents intracellular sickling
Autosplenectomy makes patients vulnerable to encapsulated bacteria (S. pneumoniae, H. influenzae, N. meningitidis)
Altitude/hypoxia can precipitate VOC even in pressurised aircraft

Sources: Emery's Elements of Medical Genetics and Genomics; Quick Compendium of Clinical Pathology, 5e; Barash Clinical Anesthesia, 9e; Morgan & Mikhail's Clinical Anesthesiology, 7e; Murray & Nadel's Respiratory Medicine; Harriet Lane Handbook, 23e; Swanson's Family Medicine Review

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