The theory behind injections into the carpal tunnel is that they decrease swelling and inflammation in the carpal tunnel (around the tendons that run alongside the nerve in the carpal tunnel) making more room for the median nerve so that it is no longer compressed by them.

Corticosteroid, or cortisone, injections for carpal tunnel syndrome is a powerful anti-inflammatory agent that can be injected into the carpal tunnel. Although these injections often relieve painful symptoms or help to calm a flare up of symptoms, their effect is sometimes only temporary. A cortisone injection may also be used by your doctor to help diagnose your carpal tunnel syndrome.

Cortisone can reduce edema and inflammation around the nerve and can temporary improve symptoms for carpal tunnel. In my experience, the injections are rarely curative. They can help predict response to surgery and so in that regard can be useful for prognostication prior to surgery.

Cortisone injections work by reducing inflammation and swelling around the nerve thus relieving pressure.

There are mixed feelings in the hand surgery community on steroid injection for carpal tunnel. I feel they are beneficial and i think they work by decreasing inflammation in the tunnel and alleviating pressure on the median nerve. There are studies to support they can solve the problem in those patients that have symptoms less than 6 months. I am concerned that you have had symptoms more than 5 years, have you had a nerve conduction /emg test yet? May be best to seek treatment from a hand surgeon to get an idea of options.
https://www.assh.org/handcare/hand-arm-conditions/carpal-tunnel/

Cortisone is a potent anti inflammatory. After injected, the carpal tunnel has a bigger cross sectional diameter...temporarily. In my practice, I don’t recommend cortisone for symptoms of greater than one year.
I hope that helps.

Harrison Solomon, M.D.

Cortisone would decrease the swelling of the tendons that surround the median nerve. This could decrease the pressure on the median nerve within the carpal tunnel. This can relieve symptoms; however, depending upon the severity of your carpal tunnel syndrome, symptoms oftentimes recur after a cortisone injection. In addition, as you have had symptoms for 5 years, it is somewhat unlikely that a cortisone injection(s) will "cure" your carpal tunnel syndrome.