Sinus tarsi syndrome is an impingement syndrome like carpal tunnel syndrome. Some nerves in the body must travel through narrow tunnels. An impingement syndrome means that a nerve gets pinched because of excess swelling, inflammation or build-up of scar tissue inside the tunnel.
The sinus tarsi is located inside the foot just above the heel bone. It sits between the talus (the top bone of the foot that is part of the ankle joint) and the calcaneus (heel bone). The sinus tarsi is part of the subtalar joint (the joint just above the heel bone). The sinus tarsi is filled with ligaments tying the two bones together and has a small nerve running through it to the skin on the outside of the foot, just forward of the lateral malleolus (outside ankle bone).
As with carpal tunnel syndrome, many forms of inflammation can cause swelling and pressure on the nerve. The most common cause of sinus tarsi syndrome is chronic sprains of the ankle that involve the subtalar ligaments. Gout and rheumatoid arthritis can also inflame the tissues in the sinus tarsi and cause pain.
A doctor suspects the diagnosis based on persistent pain on the outside or top of the foot near the ankle. The pain often is worse when the foot bends up, such as when walking up stairs.
The doctor can confirm the diagnosis by injecting a local anesthetic into the sinus tarsi to see if it relieves the pain.
Treatment can be given on the same visit by adding a corticosteroid into the injection site. Sometimes, night splinting of the ankle can help. If all else fails and pain persists, surgical treatment may be necessary and often can be performed arthroscopically.