Pathophysiology and Clinical Features
Under oxygen deprivation and dehydration, defective hemoglobin (HbS) polymerizes inside the cell. The erythrocyte is irreversibly damaged, becomes rigid, and assumes a characteristic sickle shape. These altered cells become trapped in small blood vessels.
The result is vaso-occlusion: blood stasis, acute tissue hypoxia, and infarctions. Concurrently, chronic hemolytic anemia develops as defective erythrocytes are rapidly destroyed by macrophages.
The primary marker of the disease is vaso-occlusive crises, causing excruciating pain and leading to tissue death. The disease predominantly affects individuals of African descent (less commonly those of Middle Eastern origin). In patients with severe homozygous disease, life expectancy is reduced by 25–30 years (many do not survive past 50 years).
Radiological signs of bone marrow hyperplasia
Due to ongoing hemolysis, the body requires more red blood cells, leading to massive red bone marrow hyperplasia. On MRI, this manifests as reconversion: normal fatty bone marrow is replaced by tissue with intermediate signal intensity on T1-weighted images (T1-WI). This process can even involve the epiphyses.
On standard radiographs, we observe:
- Thinning of the cortical layer and significant widening of the medullary canal.
- Loss of clear differentiation between the cortex and trabecular bone.
- Coarse trabecular pattern of flat and long tubular bones.
Bones appear osteopenic, increasing the risk of fractures. A typical finding is vertebral microfractures, causing the endplates to acquire a biconcave shape (biconcave morphology).
Bone infarctions and spinal pathology
Bone infarctions in sickle cell anemia occur at least 50 times more frequently than infectious complications. Infants develop dactylitis — infarctions of the metacarpals and phalanges with lytic lesions, accompanying periostitis, and severe edema. This can lead to asymmetrical shortening of the bones.
In the spine, the 'H-shaped' vertebra ('H-shaped' vertebra) sign is pathognomonic. It occurs due to venous thromboembolism in the center of the endplate, leading to its focal collapse (seen on X-ray as a 'stepped depression'). The adjacent undamaged vertebra may compensatorily elongate upward, forming a 'tower' vertebra ('tower' vertebra).
Chronic ischemia of long bones presents a 'bone-within-a-bone' appearance due to cortical splitting, and also leads to endosteal and diffuse medullary sclerosis.
Differential diagnosis: Infarction or Osteomyelitis?
Distinguishing sterile bone infarction from osteomyelitis (which occurs in 18% of patients) clinically and radiologically is extremely difficult. On MRI, both conditions present with poorly marginated bone marrow edema, periostitis, and soft tissue edema.
Key MRI differential criteria:
- Acute infarction: the lesion typically has a characteristic serpentine (serpentine) contour.
- Osteomyelitis: fluid collection in the medullary canal communicates with the surrounding soft tissues through a cortical bone defect. Post-contrast fat-suppressed T1-weighted images show 'geographic' enhancement zones.
Additionally, in severe osteomyelitis, radiography may demonstrate increased bone destruction, multilayered periostitis, and sequestrum formation.
Briefly: answers to topical questions
What splenic changes are detected by diagnostic imaging in the early and late stages of the disease?
In the early stages of the disease, splenomegaly is detected, whereas in the late stages, signs of infarction and atrophy are determined.
- Splenomegaly — typical for early childhood.
- Splenic infarction — occurs due to massive sequestration and vascular occlusion.
- Organ atrophy — develops later due to recurrent infarctions.
- Autosplenectomy (autosplenectomy) — complete loss of splenic function as a result of recurrent infarctions.
What cerebral complications are characteristic of sickle cell anemia and how are they visualized on MRI?
The main cerebral complication is ischemic or hemorrhagic stroke, as well as the development of cerebral vasculopathy. The following changes are visualized on MRI:
- Cerebral infarction — typically localized in watershed infarction zones (watershed infarction / borderzones) between major arterial territories.
- Moyamoya disease (moyamoya disease) — manifests as progressive stenosis of the terminal portion of the internal carotid artery.
- 'Puff of smoke' sign (puff of smoke) — multiple small areas of blood flow signal void (flow voids) in the basal ganglia.
- Calvarial thickening — diffuse thickening of the diploic space (diploic space) due to enhanced hematopoiesis.
What are the radiological signs of avascular necrosis of the bones in sickle cell anemia?
Avascular necrosis most commonly affects the epiphyses of the femoral and humeral heads, leading to their collapse. Radiological signs depend on the stage and imaging modality:
- Epiphyseal edema — an early sign detected on MRI.
- Areas of lysis and sclerosis — mixed bone structure changes on radiographs.
- Subchondral fracture (subchondral fracture) — a typical radiological manifestation of necrosis.
- Epiphyseal sclerosis (epiphyseal sclerosis) — accompanies joint head collapse.
- Secondary osteoarthritis (secondary osteoarthritis) — an inevitable outcome of chronic joint ischemia.
To memorize vertebral changes: letter 'H' — Necrosis (collapse of the center of the endplate forms the H-shaped vertebra), letter 'T' — Tall (compensatory growth of the adjacent vertebra forms the Tower vertebra).
Class and exam questions
Which parts of the bones are most frequently affected by infarctions in adult patients?
In adolescents and adults, bone marrow infarctions are most commonly localized in the metaphyses and epiphyses of long tubular bones.
What is the etiology of osteomyelitis in this condition?
The most frequent causative agent is various bacteria of the genus Salmonella. About 10% of cases are caused by Staphylococcus aureus.
What is the "bone-within-bone" sign?
This is a specific radiological sign of chronic diaphyseal ischemia in tubular bones, resulting from splitting of the thickened cortical layer.
What else to study on this topic
- Avascular necrosis of the epiphyses (femoral and humeral heads)
- Extramedullary hematopoiesis: localization and radiological signs
- Compound heterozygous forms (HbS-C and HbS-thalassemia)
- Secondary osteoarthritis and myelofibrosis as outcomes of ischemia
- Rib and sternal infarctions (mimicking cardiopulmonary pathology)
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