The tarsal tunnel is a passageway on the inner side of the ankle (the side of the big toe) where a big bundle of blood vessels and a large nerve passes through.  It is surrounded by firm fibrous tissue that can pinch, or create abnormal pressure, on the tissue within it.  In particular, the nerve that runs within it, called the posterior tibial nerve, can become irritated.  A similar nerve irritation occurs in the wrist in the more commonly known carpal tunnel syndrome.  The posterior tibial nerve allows for sensation to the entire bottom of the foot, as well as the inner side of the foot and heel.  It can be sensitive to irritation from either a narrowing of the tarsal tunnel through injury or chronic stress (such as seen in people with flat feet), or by cysts, varicose vein branches, or other small masses that physically push on the nerve.

When this nerve is irritated by tissue impinging on it in the tarsal tunnel, the result can be abnormal nerve function.  In particular, the bottom of the foot and the heel can feel numb, or there can be a sharp, burning, or tingling pain present.  Much of the pain is felt directly on the inner side of the heel.  However, early tarsal tunnel syndrome can feel strictly as if the pain is in the bottom of the heel.  This is due to referred pain, or pain that is sensed at the end of the nerve but is generated or referred from the point where the nerve is actually irritated, which is closer to the ankle.  The pain is felt more with activity, like in plantar fasciitis.  However, unlike fasciitis, the problem is not limited to the arch, and the pain can remain in the foot even during rest if the tarsal tunnel is irritated enough.  In addition to the nerve in the tarsal tunnel itself, there are several branches of this nerve that come off of it before and after the tarsal tunnel that can cause similar symptoms if they are inflamed by injury or foot strain from abnormal foot structure.

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