Transient synovitis
Transient synovitis of hip is a self-limiting condition in which there is an inflammation of the inner lining of the capsule of the hip joint. The term irritable hip refers to the syndrome of acute hip pain, joint stiffness, limp or non-weightbearing, indicative of an underlying condition such as transient synovitis or orthopedic infections. In everyday clinical practice however, irritable hip is commonly used as a synonym for transient synovitis. It should not be confused with sciatica, a condition describing hip and lower back pain much more common to adults than transient synovitis but with similar signs and symptoms.
Transient synovitis usually affects children between three and ten years old. It is the most common cause of sudden hip pain and limp in young children. Boys are affected two to four times as often as girls. The exact cause is unknown. A recent viral infection or a trauma have been postulated as precipitating events, although these are reported only in 30% and 5% of cases, respectively.
Transient synovitis is a diagnosis of exclusion. The diagnosis can be made in the typical setting of pain or limp in a young child who is not generally unwell and has no recent trauma. There is a limited range of motion of the hip joint. Blood tests may show mild inflammation. An ultrasound scan of the hip joint can show a fluid collection. Treatment is with nonsteroidal anti-inflammatory drugs and limited weight-bearing. The condition usually clears by itself within seven to ten days, but a small group of patients will continue to have symptoms for several weeks. The recurrence rate is 4–17%, most of which is in the first six months.
Symptoms and signs
Transient synovitis causes pain in the hip, thigh, groin or knee on the affected side. There may be a limp with or without pain. In small infants, the presenting complaint can be unexplained crying. 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. 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.
Complications
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, but further studies did not confirm any link between the two conditions.
Diagnosis
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, 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, 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. An anteroposterior and frog lateral view of the pelvis and both hips is advisable.
An ultrasound scan of the hip can easily demonstrate fluid inside the joint capsule, 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 that can differentiate between septic arthritis and transient synovitis. 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.
Differential diagnosis
Pain in or around the hip and/or limp in children can be due to a large number of conditions. Septic arthritis 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 all point to septic arthritis, but a high index of suspicion remains necessary even if these are not present. Osteomyelitis can also cause pain and limp.Bone fractures, such as a toddler's fracture, 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 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 or by testicular disease. Rarely, there is an underlying rheumatic condition or bone tumour.