Most people are aware of carpal tunnel syndrome as a common nerve entrapment problem in the wrist and hand. There is a similar type of nerve entrapment in the ankle, which is not as common. Entrapment of the tibial nerve as it passes through a tunnel on the medial side of the ankle is called tarsal tunnel syndrome.

Nerve entrapment syndromes don't occur with as much frequency in the lower extremity as they do in the upper extremity. As a result, tarsal tunnel syndrome (TTS) is considered by some to be a rare condition, leading to it being frequently overlooked as a source of foot pain.¹ The location of pain on the plantar surface of the foot produced by TTS also might cause it to be mistaken for plantar fasciitis. TTS also can be mistaken for proximal nerve compression pathologies, such as herniated discs in the lumbar region.

Anatomical Considerations

As the tibial nerve exits the deep posterior compartment, it passes around the medial side of the ankle on its way to termination in the toes. Near the medial malleolus, it divides into three branches. Just after it divides into these three branches, they all pass under a fascial band on the medial side of the ankle called the flexor retinaculum (Figure 1). The retinaculum is connected superiorly to the medial malleolus and inferiorly to the medial side of the calcaneus. The space under the retinaculum is the tarsal tunnel. There are several other structures that pass through the tunnel, including the tendons of tibialis posterior, flexor digitorum longus and flexor hallucis longus, and the posterior tibial artery and vein.

Tarsal tunnel syndrome results when the tibial nerve or its branches are exposed to compressive or tensile stress within the tarsal tunnel. Nerve compression occurs from pressure outside the tunnel such as a direct blow to the medial side of the ankle or from force within the tunnel from synovial ganglions or bony prominences.^2,3

A swelling of synovial tendon sheaths (tenosynovitis) also could compress the tibial nerve.

Tensile forces on the tarsal tunnel nerves also cause symptoms. Neural tension results from either a sudden or chronic stretch of the nerve. Sudden nerve stretch happens in acute injuries while chronic stretching results from postural distortions such as a calcaneal valgus foot alignment.

Peripheral neuropathies like TTS can be linked to systemic disorders such as diabetes, muscular sclerosis, rheumatoid arthritis and hyperthyroidism.⁴ Note that some medications might cause sensitivity in the distal lower extremity nerves that could be mistaken for compression pathologies in the tarsal tunnel.

Identifying the Condition

A client with TTS reports sharp, shooting pain sensations around the medial ankle and along the plantar surface of the foot. In addition to pain, there might be paresthesia, numbness or motor weakness in the muscles of the foot. Symptoms ordinarily are worse after long periods of standing or walking, but also might be aggravated during the night if the nerve is in a compromised position for prolonged periods. Ask about recent trauma involving sudden compressive or tensile loads on the nerve, as recent injuries might be responsible for the symptoms. It's important to ask about systemic disorders that might cause TTS, or be related to it.

There are no clear visible signs of tarsal tunnel syndrome, but certain postural disorders such as calcaneal varus or valgus can aggravate the condition. Although uncommon, if TTS is severe or has been present for a long time some atrophy of the muscles innervated by the divisions of the tibial nerve might be apparent. Placing pressure directly on the tarsal tunnel is one of the most valuable ways of identifying this condition and is sometimes called the tarsal compression test. If the pressure reproduces the client's primary pain or other neurological sensations, it's a good indication of tarsal tunnel syndrome.

A special orthopedic test called the dorsiflexion-eversion test also is used to identify the condition. In this test, the client is in a supine position. The ankle is passively moved into maximum dorsiflexion and eversion while the toes are held in hyperextension (Figure 2). The position is held for five to 10 seconds. If symptoms develop, it's a positive sign of TTS.

Identifying nerve compression pathologies like TTS is important so proper treatment can be administered. If the client reports foot pain, there might be a tendency to use additional pressure around the ankle or foot in an effort to "work it out." This would be a mistake with a nerve compression pathology like TTS. Accurate identification will guide the most appropriate treatment.

References

1. DeLisa JA, Saeed MA. The tarsal tunnel syndrome. Muscle Nerve. Nov-Dec 1983;6(9):664-670.
2. Nagaoka M, Satou K. Tarsal tunnel syndrome caused by ganglia. J Bone Joint Surg Br. 1999;81(4):607-610.
3. Boc SF, Hatef J. Space-occupying lesions as a cause of tarsal tunnel syndrome. J Am Podiatr Med Assoc. 1995;85(11):713-715
4. Walker R. Diabetes and peripheral neuropathy: keeping people on their own two feet. Br J Community Nurs. Jan 2005;10(1):33-36.