Study Guide

Sickle Cell Disease

Key Points:

Description:

Sickle Cell Disease (SCD) is an autosomal recessive disorder that affects erythrocyte function, which can lead to hemolytic anemia resulting in decreased hemoglobin and erythrocyte levels. The disease is characterized by vaso-occlusive episodes, visceral sequestration, and aplastic or hemolytic crisis. Due to the implications of sickle cell disease on the bone, orthopedic and musculoskeletal complications are severely disabling in pediatric patients. 

The major orthopaedic manifestations of sickle cell disease are osteonecrosis, osteomyelitis, septic arthritis, and bone infarction. Accordingly, effective management of the affected bone and joint require accurate diagnosis, understanding of the pathophysiology of the disease, and knowledge of the available medical and surgical approaches to treat orthopedic complications. 

Epidemiology:

Sickle cell disease is the most commonly inherited blood disorder in the United States, affecting roughly 90,000 to 100,000 individuals, with roughly 2 million Americans carrying the gene for sickle cell disease. 1 in 12 Black or African American children are born with sickle cell trait, and 1 in 500 Black or African American babies are diagnosed with sickle cell disease. 

Clinical Findings:

Imaging Studies:

Treatment:

Orthopedic Manifestations:

Osteonecrosis
Roughly 10% of patients with SCD develop osteonecrosis: the death of bone tissue due to the lack of blood supply. Sickle cell patients with osteonecrosis typically see tissue death throughout the epiphysis rather than just a focal weight bearing area. Osteonecrosis is commonly found in the femoral head, humeral head, and distal femur, often presenting as severe pain for the pediatric patient.
 
A plain radiograph can be used to visualize the lucency in the epiphysis which is followed by flattening, sclerosis, and subsequent collapse. 

An elevated hemoglobin to hematocrit ratio may be a strong predictor of osteonecrosis and can be used as a tool to determine the need for MRI screening to help detect early osteonecrosis. Early detection of osteonecrosis is vital in preventing collapse of the femoral or humeral head. 

Treatment options for patients who show symptoms prior to collapse include autologous bone marrow grafting, core decompression, osteotomy, and temporary unloading of the affected area. Osteotomies have shown to be a plausible salvage procedure for skeletally immature patients with osteonecrosis secondary to SCD. Conversely, a total hip arthroplasty has shown to have high complications in skeletally immature patients with sickle cell disease and may not be the ideal treatment for osteonecrosis. 

Osteomyelitis
Even though osteomyelitis is rare in children with SCD, it has been shown that osteomyelitis is more common in pediatric patients with SCD than the general population. A careful patient history is required to differentiate osteomyelitis from a bone infarction. A proper diagnosis can be made with a positive gram stain blood culture from the infected area. The main causative agent of osteomyelitis in North America is staphylococcus aureus or salmonella. However, the main causative agent in West Africa and Saudi Arabia is only salmonella. 
Patients with an infection are relatively more ill compared to patients with a bone infarct. Initial management may include intravenous fluids, oxygenation, and NSAIDs. A radionuclide bone scan may also determine whether a child has a bone infarct or osteomyelitis as patients with osteomyelitis will still have a normal level of marrow uptake, but present with an abnormal bone scan. Patients with osteomyelitis are also at risk of developing a multifocal infection, making early detection and early antibiotic treatment vital. 

Septic Arthritis
Septic arthritis occurs in 5% of pediatric patients with SCD. The most common cause is staphylococcus aureus and gram-negative species mainly affecting the hips. Due to sluggish circulation that leads to decreased opsonization of bacteria, sickle cell patients are more susceptible to infection. 
Septic arthritis presents through bone pain and simultaneous fever and can be confirmed with an elevated C-Reaction Protein level and Erythrocyte Sedimentation Rate. Treatment options for pediatric patients with septic arthritis include irrigation and debridement. The infected joint should be aspirated and cultured in order to identify the causative organism and generate the proper antibiotic therapy. 

Sickle Cell Crisis
Sickle Cell Crisis occurs when sickled blood cells block small blood vessels that carry blood to the bones, causing periods of pain that may last hours or days. Pain commonly occurs in the back, knees, legs, arms, chest or stomach. The pain is caused by substance P and usually begins when the child is between the ages of 2 and 3. This increases the risk of developing a bone infarct. Treatment for the sickle cell crisis includes hydroxyurea to provide pain relief during a bone crisis.  

Complications:

Treatment Related Complications
Treatment related complications, while rare, are associated with orthopaedic conditions such as osteoarthritis and osteomyelitis. Hydroxyurea and blood transfusions are often used to reduce the recurrence of pain crises by stimulating production of fetal hemoglobin (HbF) or transfusing non-pathological donor erythrocytes. However, hydroxyurea has been shown to suppress marrow activity and causes neutropenia which places patients at higher risk for osteomyelitis. Recurring blood transfusions also places patients at risk for hemochromatosis which has been associated with higher rates of osteoarthritis.

Infection vs. Infarction
The differentiation of a bone infarction and a bone infection can be clinically challenging due to their similarities in presentation. The table below provides a comparison between clinical presentation and imaging findings between the two complications in pediatric patients with sickle cell disease.

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Top Contributors:

Amog Mysore, BS; Alexander Chang, BS; Christina Hardesty, MD