|Other names||Toxic synovitis, transitory coxitis, coxitis fugax, acute transient epiphysitis, coxitis serosa seu simplex, phantom hip disease, observation hip|
|The hip joint is formed between the femur and acetabulum of the pelvis.|
|Symptoms||Groin pain, limp|
|Usual onset||3 to 10 years|
|Duration||< 5 days|
|Diagnostic method||Ruling out other potential causes|
|Differential diagnosis||Bone fracture, slipped capital femoral epiphysis, discitis, leukemia, septic joint, Lyme disease|
|Treatment||NSAIDs, limited weight-bearing|
|Frequency||Up to 3%|
Transient synovitis of hip, also called toxic synovitis, is a self-limiting inflammation of the lining of the hip joint. Symptoms often include pain in the groin, a limp, or refusal to walk. Symptoms often come on over a few days and there is generally no fever. Pain generally only occurs with significant movements at the hip. The child generally looks otherwise well.
The exact cause is unknown. A recent viral infection (most commonly an upper respiratory tract infection) or recent injury has been proposed as triggers. Diagnosis involves ruling out other potential causes. Blood tests may show mild inflammation. An ultrasound scan may show a fluid collection in the hip joint which may require aspiration to rule out a infection.
Transient synovitis affects up to 3% of children at some point in time. Most commonly it occurs between three and ten years of age and is the most common cause of hip pain in this age group. Rarely other age groups are affected. Boys are affected twice as often as girls.
Signs and symptoms
Transient synovitis causes pain in the hip, thigh, groin or knee on the affected side. There may be a limp (or abnormal crawling in infants) with or without pain. In small infants, the presenting complaint can be unexplained crying (for example, when changing a diaper). The condition is nearly always limited to one side. The pain and limp can range from mild to severe.
Some children may have a slightly raised temperature; high fever and general malaise point to other, more serious conditions. On clinical examination, the child typically holds the hip slightly bent, turned outwards and away from the middle line (flexion, external rotation and abduction). Active and passive movements may be limited because of pain, especially abduction and internal rotation. The hip can be tender to palpation. The log roll test involves gently rotating the entire lower limb inwards and outwards with the patient on his back, to check when muscle guarding occurs. The unaffected hip and the knees, ankles, feet and spine are found to be normal.
In the past, there have been speculations about possible complications after transient synovitis. The current consensus however is that there is no proof of an increased risk of complications after transient synovitis.
One such previously suspected complication was coxa magna, which is an overgrowth of the femoral head and broadening of the femoral neck, accompanied by changes in the acetabulum, which may lead to subluxation of the femur. There was also some controversy about whether continuous high intra-articular pressure in transient synovitis could cause avascular necrosis of the femoral head (Legg-Calvé-Perthes disease), but further studies did not confirm any link between the two conditions.
There are no set standards for the diagnosis of suspected transient synovitis, so the amount of investigations will depend on the need to exclude other, more serious diseases.
Inflammatory parameters in the blood may be slightly raised (these include erythrocyte sedimentation rate, C-reactive protein and white blood cell count), but raised inflammatory markers are strong predictors of other more serious conditions such as septic arthritis.
X-ray imaging of the hip is most often unremarkable. Subtle radiographic signs include an accentuated pericapsular shadow, widening of the medial joint space, lateral displacement of the femoral epiphyses with surface flattening (Waldenström sign), prominent obturator shadow, diminution of soft tissue planes around the hip joint or slight demineralisation of the proximal femur. The main reason for radiographic examination is to exclude bony lesions such as occult fractures, slipped upper femoral epiphysis or bone tumours (such as osteoid osteoma). An anteroposterior and frog lateral (Lauenstein) view of the pelvis and both hips is advisable.
An ultrasound scan of the hip can easily demonstrate fluid inside the joint capsule (Fabella sign), although this is not always present in transient synovitis. However, it cannot reliably distinguish between septic arthritis and transient synovitis. If septic arthritis needs to be ruled out, needle aspiration of the fluid can be performed under ultrasound guidance. In transient synovitis, the joint fluid will be clear. In septic arthritis, there will be pus in the joint, which can be sent for bacterial culture and antibiotic sensitivity testing.
More advanced imaging techniques can be used if the clinical picture is unclear; the exact role of different imaging modalities remains uncertain. Some studies have demonstrated findings on magnetic resonance imaging (MRI scan) that can differentiate between septic arthritis and transient synovitis (for example, signal intensity of adjacent bone marrow). Skeletal scintigraphy can be entirely normal in transient synovitis, and scintigraphic findings do not distinguish transient synovitis from other joint conditions in children. CT scanning does not appear helpful.
The term "irritable hip" refers to hip pain of sudden onset, joint stiffness, and limping, and is indicative of an underlying condition such as transient synovitis or infections (like septic arthritis or osteomyelitis). Often the term irritable hip is used as a synonym for transient synovitis.
Pain in or around the hip or limp in children can be due to a large number of conditions. Septic arthritis (a bacterial infection of the joint) is the most important differential diagnosis, because it can quickly cause irreversible damage to the hip joint. Fever, raised inflammatory markers on blood tests and severe symptoms (inability to bear weight, pronounced muscle guarding) all point to septic arthritis, but a high index of suspicion remains necessary even if these are not present. Osteomyelitis (infection of the bone tissue) can also cause pain and limp.
Bone fractures, such as a toddler's fracture (spiral fracture of the shin bone), can also cause pain and limp, but are uncommon around the hip joint. Soft tissue injuries can be evident when bruises are present. Muscle or ligament injuries can be contracted during heavy physical activity —however, it is important not to miss a slipped upper femoral epiphysis. Avascular necrosis of the femoral head (Legg-Calvé-Perthes disease) typically occurs in children aged 4–8, and is also more common in boys. There may be an effusion on ultrasound, similar to transient synovitis.
Neurological conditions can also present with a limp. If developmental dysplasia of the hip is missed early in life, it can come to attention later in this way. Pain in the groin can also be caused by diseases of the organs in the abdomen (such as a psoas abscess) or by testicular disease. Rarely, there is an underlying rheumatic condition (juvenile idiopathic arthritis, Lyme arthritis, gonococcal arthritis, ...) or bone tumour.
Treatment consists of rest, non-weightbearing and pain medication when needed. Nonsteroidal anti-inflammatory drug such as ibuprofen can shorten the disease course (from 4.5 to 2 days) and provide pain control with minimal side effects. If fever occurs or the symptoms persist, other diagnoses need to be considered.
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