Transient synovitis (TS), often called “irritable hip,” is a common, temporary condition involving inflammation of the synovium, the delicate lining of the hip joint capsule. This non-infectious, self-limiting process is the most frequent cause of sudden hip pain and a limp in young children. The term “transient” highlights the condition’s benign nature and short duration, as it typically resolves without lasting effects. TS most commonly affects children between the ages of three and ten years old, with boys affected more often than girls. The condition usually affects only one hip, resulting in acute symptoms that rarely indicate a serious problem.
Recognizing the Signs of Hip Irritation
The most noticeable sign of transient synovitis is the acute onset of a limp, where the child quickly shifts weight off the painful leg. This limping can range from a slight shuffle to a complete refusal to bear any weight on the affected side. Children may complain of pain felt directly in the hip or groin area, but the discomfort is also frequently referred to the thigh or the knee. This pain referral occurs because the same nerves supply both the hip and the knee joint.
The onset of symptoms is usually sudden. The child may hold the hip in a specific position, slightly flexed and externally rotated, to maximize the joint space and relieve pressure. Unlike serious joint infections, children with transient synovitis typically appear well, often with a normal temperature or only a mild fever below 101°F (38.5°C). A child with TS may be uncomfortable but is usually not systemically ill or toxic-looking.
Very young children, such as toddlers, may only present with unexplained crying or an unwillingness to crawl or walk rather than verbalizing pain.
What Causes the Inflammation
The precise mechanism that triggers transient synovitis remains unknown. However, the inflammation is theorized to be an immune-mediated response following a recent infection elsewhere in the body. A history of a mild viral illness, such as an upper respiratory infection or gastroenteritis, is reported in a significant number of cases.
This prior illness often occurred one to three weeks before the hip symptoms began, suggesting a delayed reaction. The immune system, having fought off the virus, mistakenly launches a temporary, sterile inflammatory attack on the synovial tissue of the hip joint. This reaction causes the joint lining to swell and produce excess fluid, which irritates the joint and causes pain. The inflammation is sterile because it is not caused by bacteria directly infecting the joint space.
Differentiating Transient Synovitis from Other Conditions
Transient synovitis is considered a diagnosis of exclusion because a painful limp could indicate a range of conditions, from benign to severe. The primary concern is always to rule out septic arthritis, a bacterial infection of the joint space that requires immediate intervention to prevent joint destruction. Diagnosis involves a thorough physical examination, laboratory tests, and imaging studies.
During the physical exam, a physician assesses the child’s ability to bear weight and the hip’s range of motion. Movement is often limited and painful, particularly when rotating the leg inward. Blood tests measure inflammatory markers, specifically the erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and the white blood cell (WBC) count.
In transient synovitis, these markers are usually normal or only mildly elevated. Septic arthritis, however, is associated with significantly higher levels, such as an ESR above 40 mm/hr and a WBC count over 12,000 cells per cubic millimeter.
Imaging studies are utilized to differentiate the causes of hip pain. An X-ray is often ordered to check for bone abnormalities, fractures, or signs of Legg-Calvé-Perthes disease. X-rays are typically normal in cases of transient synovitis.
A joint ultrasound is valuable because it can visualize the hip joint space and confirm the presence of an effusion (fluid accumulation within the joint capsule). While an effusion is present in both TS and septic arthritis, the combination of an effusion with low inflammatory markers and an afebrile, non-toxic-appearing child strongly suggests transient synovitis.
Managing the Condition and Expected Recovery
The management of transient synovitis is supportive, focusing on rest and controlling discomfort until the self-limiting inflammation resolves. The child should avoid weight-bearing activities and strenuous play until the limp and pain have completely disappeared. Restricted activity for several days is often recommended to allow the inflamed synovium to calm down.
Pain and inflammation are managed with over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, which decrease discomfort and joint swelling. The prognosis for transient synovitis is excellent. Most children experience marked improvement within 24 to 48 hours of starting treatment.
Complete resolution of symptoms typically occurs within seven to ten days, with most children fully returning to normal activities within two weeks. If the limp or pain persists beyond two weeks, or if the child develops a high fever or worsening symptoms, prompt follow-up is necessary to re-evaluate the diagnosis.