Sickle Cell Disease — Causes, Crisis, Complications & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Sickle cell disease (SCD) encompasses a group of inherited haemoglobin disorders caused by a single point mutation in the beta-globin gene (HBB) — substituting glutamic acid for valine at position 6 of the beta-globin chain, producing haemoglobin S (HbS). In the homozygous form (HbSS — sickle cell anaemia, the most severe form), or compound heterozygous states (HbSC, HbS/beta-thalassaemia), deoxygenated HbS polymerises into rigid tactoids that distort red cells into the characteristic sickle shape. Sickled cells are rigid, adhesive, and short-lived — causing haemolytic anaemia, vaso-occlusion (microvascular obstruction by sickle cells), and endothelial dysfunction affecting every organ system. SCD affects approximately 8 million people worldwide, with 300,000 new cases born annually — predominantly in sub-Saharan Africa (highest global burden), Middle East, India, and parts of the Mediterranean. SCD is the most common serious inherited blood disorder globally. Historically considered a disease of childhood, modern management has dramatically improved survival — over 95% of patients in high-income countries now survive to adulthood.

SCD follows autosomal recessive inheritance. A child inherits one HbS allele from each parent (both of whom are carriers — sickle cell trait, HbAS — typically asymptomatic). Genotypes: HbSS (sickle cell anaemia — most severe, 60-70% of SCD); HbSC (compound heterozygote — milder course but still significant morbidity); HbS/beta-thalassaemia (HbS/beta0 — severity similar to HbSS; HbS/beta+ — milder). Sickle cell trait (HbAS) — carrier state — generally not associated with disease under normal conditions; may cause symptoms in extreme hypoxia (high altitude, severe dehydration, extreme exertion — cases of sudden death in military/athletic settings reported). The HbS mutation is maintained at high frequency in malaria-endemic regions because sickle trait provides 70% protection against severe Plasmodium falciparum malaria — explaining its geographic distribution. Risk factors for severe vaso-occlusive crises: dehydration, cold exposure, hypoxia, infection, acidosis, and stress. Modifier genes: foetal haemoglobin (HbF) level — higher HbF inhibits HbS polymerisation and significantly reduces disease severity; alpha-thalassaemia co-inheritance (reduces intracellular HbS concentration, mitigating haemolysis).

Haemolytic anaemia: chronic anaemia (Hb typically 60-90 g/L in HbSS); jaundice, scleral icterus, gallstones (pigment cholelithiasis from haemolysis — cholecystectomy often required). Vaso-occlusive (painful) crisis (VOC): most common acute complication — severe, acute pain (bone, chest, abdomen, joints) from microvascular obstruction causing ischaemia; precipitated by infection, dehydration, cold, fatigue; may require hospitalisation and opioid analgesia. Acute chest syndrome (ACS): life-threatening — new pulmonary infiltrate on CXR with respiratory symptoms and/or fever; microvascular occlusion + fat embolism from necrotic bone marrow + infection; mortality 3-9%; requires exchange transfusion and antibiotics. Stroke: cerebral infarction occurs in 11% of HbSS children by age 20 — predominantly large vessel (ICA, MCA); detected by transcranial Doppler (TCD) screening in children; prevented by regular blood transfusion programme in high TCD velocity children. Splenic sequestration crisis: pooling of blood in the spleen (children under 5 before autosplenectomy) — rapidly dropping Hb, massive splenomegaly — potentially fatal; requires blood transfusion. Aplastic crisis: parvovirus B19 infection causing transient red cell aplasia — Hb drops sharply; usually self-limiting. Functional asplenia: autosplenectomy from recurrent splenic infarction by age 5-6 — high risk of overwhelming bacterial infections (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis). Priapism: prolonged painful erection from penile vascular occlusion — emergency urology referral. Avascular necrosis: femoral and humeral heads. Pulmonary hypertension (25% of adults with HbSS). Renal disease: sickle nephropathy, haematuria. Retinopathy. Chronic pain: increasingly recognised as distinct from acute VOC — central sensitisation.

Newborn bloodspot screening (heel prick): identifies HbSS, HbSC, and HbS/beta-thal at birth — standard in UK NHS since 2006; also in USA since the 1980s; HPLC (high-performance liquid chromatography) or isoelectric focusing used. Haemoglobin electrophoresis or HPLC: confirms diagnosis — identifies HbS (migrates differently to HbA); confirms absence of HbA in HbSS; quantifies HbF (protective). FBC and reticulocyte count: baseline anaemia (Hb 60-90 g/L), reticulocytosis (4-10%); normocytic or slightly macrocytic MCV (higher if hydroxyurea therapy). Peripheral blood film: sickle cells, target cells, polychromasia, Howell-Jolly bodies (functional asplenia). Transcranial Doppler (TCD) ultrasound: annual screening in children aged 2-16; TAMV (time-averaged mean velocity) above 200 cm/s in MCA/ICA identifies high stroke risk — chronic transfusion programme indicated. Annual organ function monitoring: renal function, urine ACR, LFTs, echocardiography (pulmonary hypertension), retinal examination, MRI brain (silent infarcts). Genotyping: confirms specific SCD genotype and identifies modifying genes.

Preventive: penicillin V prophylaxis — lifelong starting at age 3 months (reduces invasive pneumococcal disease by 84%); vaccinations — pneumococcal (PCV13 + PPV23), meningococcal ACWY and B, Hib, influenza annually; folic acid supplementation. Acute VOC management: NICE recommends the SCD acute pain guideline — assess with pain score; IV/oral analgesia (weak opioid: codeine; strong: morphine, oxycodone; patient-controlled analgesia); regular paracetamol and NSAIDs; IV fluids; oxygen only if SpO2 below 95%; warming (avoid cold); anti-emetics. Acute chest syndrome: urgent exchange transfusion + broad-spectrum antibiotics (including atypical cover) + oxygen; ITU if severe. Disease-modifying therapy: hydroxyurea (hydroxycarbamide): induces HbF production (HbF inhibits HbS polymerisation); reduces VOC frequency by 50%, ACS by 30%, hospitalisation, transfusion requirement, and mortality; recommended for all adults with HbSS and HbSC with significant disease (NICE guideline); dose titrated to achieve HbF above 20% and MCV above 100 fl; monitor FBC. Crizanlizumab (Adakveo): anti-P-selectin monoclonal antibody — reduces VOC frequency by 45% (SUSTAIN trial); NICE approved for SCD patients aged 16 and over with 2+ crises/year not adequately controlled. Voxelotor (Oxbryta): increases Hb-O2 affinity, reducing HbS polymerisation and haemolysis; approved FDA 2019; not currently NICE approved for routine NHS use. Blood transfusion: for stroke, ACS, aplastic crisis, splenic sequestration, pre-surgery — simple or exchange transfusion (reduces HbS below 30%); iron chelation with desferrioxamine or deferasirox for transfusional iron overload. Curative options: allogeneic stem cell transplantation (alloSCT): curative for HbSS with matched sibling donor — event-free survival above 90%; available to children with severe disease, stroke, or refractory VOC; graft-versus-host disease is main risk. Gene therapy: exagamglogene autotemcel (Casgevy — CTX001) — CRISPR/Cas9-based gene editing to reactivate foetal haemoglobin production in patient's own stem cells; FDA and EMA approved (2023); transformative results (73 of 74 patients with HbSS achieved HbF above 20% — no further severe crises); becoming available in high-income countries.

Stroke is a devastating complication — cerebral infarction occurs in 11% of children with HbSS by age 20, predominantly large-vessel (ICA, MCA occlusion); silent cerebral infarction (detected on MRI without clinical stroke) affects 25–30%, causing cognitive impairment, academic difficulties, and progressive neurological dysfunction. Regular transcranial Doppler (TCD) screening identifies high-risk children (TAMV above 200 cm/s) who benefit from chronic exchange transfusion to reduce stroke risk by 90%. Acute chest syndrome (ACS) affects 50% of HbSS patients at least once — a combination of microvascular occlusion, fat embolism from necrotic bone marrow, and pulmonary infection causes pulmonary infiltrates, hypoxia, and chest pain; mortality 3–9% per episode; repeated ACS episodes lead to chronic lung disease and pulmonary hypertension. Avascular necrosis (AVN) of the femoral head and humeral head from vascular occlusion affects 50% of adults with HbSS by age 35, causing chronic pain and functional impairment; total hip or shoulder arthroplasty may be required. Pulmonary hypertension (elevated tricuspid regurgitation velocity above 2.5 m/s) affects 25% of adults with HbSS and is associated with significantly increased mortality (5-fold). Chronic kidney disease (sickle nephropathy) from renal medullary infarction, hyperfiltration, and glomerulopathy affects the majority of adult HbSS patients; end-stage renal disease requiring dialysis affects approximately 5% of adults. Hepatobiliary complications: pigment gallstones (from haemolysis) affect up to 75% of adults; sickle hepatopathy causes acute and chronic liver disease. Priapism (painful sustained erection) is an emergency in males — occurs in up to 35% of adolescent males with SCD; delayed treatment causes permanent erectile dysfunction. Leg ulcers around the medial malleolus affect 10–20% of adults with HbSS and are notoriously difficult to heal.

Genetic counselling: both prospective parents who are carriers (sickle cell trait, HbAS) — which is common in populations of African, Mediterranean, Middle Eastern, and South Asian heritage — should be counselled that each pregnancy carries a 25% risk of SCD (HbSS). Pre-conception genetic testing and carrier screening is offered in the UK (NHS Sickle Cell and Thalassaemia Screening Programme). Prenatal diagnosis by chorionic villus sampling (CVS) at 10-12 weeks or amniocentesis at 15-18 weeks allows diagnosis; pre-implantation genetic testing (PGT) via IVF can select unaffected embryos. Trigger avoidance reduces crisis frequency: maintain adequate hydration (increased in heat, exercise, illness); avoid extreme cold; avoid high altitudes (risk of hypoxia — commercial aircraft are pressurised to 6,000-8,000 ft altitude — generally safe; notify airline carrier); treat infections promptly; seek medical review for fever above 38.5°C given risk of bacterial sepsis. Travel: carry a sickle cell card; ensure access to adequate analgesia; hydrate well; inform treating physicians of diagnosis.

Attend Emergency Department immediately for: fever above 38.5°C in anyone with SCD — functional asplenia means potentially fatal bacterial sepsis; priapism lasting more than 2 hours — urology emergency to prevent permanent erectile dysfunction; severe chest pain or difficulty breathing — acute chest syndrome; sudden neurological symptoms (weakness, difficulty speaking, sudden severe headache) — stroke; rapidly dropping haemoglobin with severe pallor — aplastic crisis or splenic sequestration; pain crisis not responding to oral analgesia after 1-2 hours at home. Contact haematology specialist for: VOC frequency increasing despite hydroxyurea; intercurrent pregnancy (SCD in pregnancy is high-risk); planned surgery (requires exchange transfusion pre-operatively); new or progressive complications; consideration for gene therapy or bone marrow transplant.