Popliteal synovial cysts (Baker's cysts) are a common occurrence in children and adults. Synovial cysts in other locations and/or with atypical extension are less common and may be confounded with tumors or other medical conditions. In this article we describe the underlying disease, clinical presentation and clinical course in six children with a sudden onset of paraarticular soft tissue masses or non-specific chronic pain. Ultrasound was the initial imaging method used in all cases; this was supplemented by MRI in three patients. Four children were diagnosed to be suffering from juvenile idiopathic arthritis (JIA), one child from Lyme Arthritis, whereas in one child no underlying disease was identified. Well-demarcated hypoechogenic lesions without signs of perfusion extending from the shoulder (two patients), elbow (one patient), hip (one patient), knee (1) or ankle (one patient) far into the adjacent musculature were detected on ultrasonography. A direct connection to the joint was demonstrated in all cases. All synovial cysts in the five arthritic patients resolved rapidly with medical treatment for arthritis, whereas the cyst persisted in the non-JIA patient.
Uncommon synovial cysts occur in particular as a complication of arthritis. Ultrasonography is the initial and follow-up imaging method of choice, which can be supplemented by MRI in unusual cases. Apart from treatment for arthritis, no specific therapeutic interventions were required in the present cases.