Carpal tunnel syndrome (CTS) refers to symptomatic compression of the median nerve in the wrist as it passes through the carpal tunnel, which is bounded by the carpal bones and the transverse carpal ligament. In CTS, a combination of compression and inflammation results in demyelination and eventually axonal injury of the median nerve. The resulting median neuropathy at the wrist is the most common mononeuropathy seen clinically. Hand paresthesias are one of the hallmark symptoms of CTS; weakness in median-nerve-innervated intrinsic hand muscles is seen in more severe cases. The palmar cutaneous branches of the median nerve arise prior to the carpal tunnel, and therefore it is only the digital branches going to the thumb, index, middle, and half of the ring finger that are affected in CTS. Despite this specific distribution of median nerve sensory changes, many patients are unable to localize their symptoms in the hand and may even describe paresthesia in the forearm where there is no median sensory innervation.
While there are multiple intrinsic hand muscles innervated by the median nerve after it passes through the carpal tunnel, the abductor pollicis brevis (APB) muscle is the most useful to assess clinically. The APB is easily inspected for atrophy and weakness: atrophy can be seen in the lateral portion of the thenar eminence, and weakness is seen with thumb abduction. When weakness is present, confirmation of the diagnosis with nerve conduction studies/electromyography (NCS/EMG) is appropriate. In patients with isolated paresthesias or numbness in a clear distribution of the median nerve but without weakness, a trial of conservative management is reasonable prior to performing further testing. This typically consists of a “cock-up” wrist splint at night, which places the wrist in mild extension and maximizes the space within the carpal tunnel, thereby decompressing the median nerve.
Definitive diagnosis of CTS requires electrophysiologic testing with EMG/NCS. In CTS, reduced median nerve sensory conduction across the wrist is frequently identified on a standard median sensory study from the index finger. However, the most sensitive finding for CTS is reduced mixed median nerve sensory conduction after palmar stimulation, which assesses median sensory conduction through the carpal tunnel across a smaller distance. To ensure that the reduced median conduction velocity is secondary to CTS and not a more diffuse process such as an axonal or demyelinating polyneuropathy, it should be compared to ulnar sensory conduction across the wrist. In more severe CTS, motor involvement will be apparent on EMG/NCS. For the median motor study, the median distal motor latency assesses the time required for motor conduction through the carpal tunnel to the APB muscle, which can be prolonged in moderate-severe CTS. Note that the median motor conduction velocity will remain normal in CTS because that velocity assesses median motor conduction in the forearm and not through the carpal tunnel.
The complete absence of responses on median sensory and motor NCS indicates median nerve pathology but does not reveal the location of the median nerve injury. In these instances, a more complex NCS can be done (such as an ulnar/median lumbrical comparison study) to more clearly localize the lesion. Additionally, needle examination of median innervated muscles proximal (e.g., flexor pollicis longus) and distal (e.g., APB) to the carpal tunnel can be informative.
In patients with mild to moderate CTS, intracarpal steroid injections are given with the goal of reducing inflammation affecting the median nerve. Such injections have demonstrated symptomatic benefit with a variable duration of relief. Unfortunately, most patients experience an eventual return of their symptoms.
Carpal tunnel release surgery involves incision of the transverse ligament and can be performed as an open or endoscopic procedure. Good symptom relief is seen in approximately three-quarters of patients after surgery.