Both compression of the 6th cervical nerve root (C6 radiculopathy) and compression of the median nerve at the wrist (carpal tunnel syndrome) can cause painful dysesthesias in the thumb. How can these two conditions be differentiated?
Both a C6 radiculopathy and carpal tunnel syndrome are compression neuropathies – physical pressure on a nerve interfering with its function -- and can present with very similar symptoms. For example, the thumb is innervated by branches of the median nerve, which itself has fibers from the C6 nerve root. Thus, compression of either the C6 root or the median nerve itself may be the cause of thumb symptoms.
Cervical radiculopathy (in layman’s terms, a pinched nerve) is a clinical condition in which dysesthesias and motor deficits are caused by compression of a cervical nerve root in its foramen, most likely from degenerative changes or disc disease (Figure 1).
The cervical nerves are numbered by the vertebral body below the nerve: for example, the C6 spinal nerve exits between the C5 and C6 vertebral bodies. A more accurate but inelegant way of stating that might be “the cervical nerves are named by adding one to the number of the vertebral body above the nerve.”
(This rule will for us to account for the existence of a “C8” nerve, which exits between C7 and T1, though there is no C8 vertebral body.)
A C6 radiculopathy thus is usually caused by compression of the foramen between the C5 and C6 vertebral bodies.
The levels of cervical root compression and their characteristic patterns of dysesthesias and muscle weakness is shown in Table 1 and Figure 2.
Carpal tunnel syndrome and its relation to radiculopathies
Carpal tunnel syndrome is caused by compression of the median nerve as it passes under the transverse carpal ligament at the wrist. This syndrome can present with similar symptoms to a C6 radiculopathy. Certain defining characteristics can help differentiate carpal tunnel syndrome and C6 radiculopathy:
Findings suggesting C6 radiculopathy
- Because degenerative joint disease and degenerative disc disease are the usual underlying causes of cervical radiculopathy, degenerative findings increase the likelihood that arm symptoms are radicular in origin. Such findings include neck pain; impaired cervical motion, especially loss of extension; a plain x-ray showing degenerative (spondolytic) changes; or an MRI showing either osteophytes or soft tissue structures (e.g., disc) causing narrowing of the neural foramen.)
- Pain proximal to the wrist is more likely due to a cervical compression–though it is possible for compression in the carpal tunnel to cause pain in the forearm.
- Likewise, motor weakness, if present, will include the elbow flexors and wrist extensors (proximal to the wrist).
- Spurling’s maneuver will exacerbate C6 radicular pain, but not median nerve entrapment. Spurling’s maneuver is performed by applying downward pressure to the top of the patient's head while the patient extends the neck and rotates the head toward the affected side; this combination of extension, rotation, and external compression decreases the size of the neural foramen and increases compression on the nerve root, thereby inducing symptoms. (Because Spurling’s maneuver produces only symptoms, not signs, it may be helpful to have patients perform this on themselves and report what they feel. This way, excess pressure will not be encountered and there is no objectivity lost).
- Having patients place their forearm on the top of their head will relieve radiculopathy symptoms but not those from peripheral compression.
- Biceps and/or brachioradialis hyporeflexia.
Findings suggesting carpal tunnel syndrome
- Symptoms that are affected by the position of the hand (extremes of flexion or extension) suggest carpal tunnel syndrome.
- Hand pain during the night or pain felt just before waking that is relieved by “shaking out” of the hand.
- Weakness of the thumb abductor (a late finding in carpal tunnel syndrome).
- Producing or exacerbating pain or paresthesias in the median nerve distribution with any of these provocative tests:
- Tinel’s test: Examiner repetitively taps the transverse carpal ligament to percuss the median nerve.
- Phalen’s test: The patient is asked to hold the wrists in a position of maximal flexion for 30 seconds. (This maneuver can moderately increase the pressure in the carpal tunnel.)
- Durkan’s test: Examiner continuously compresses the carpal tunnel with a thumb for 30 second.
- Nerve conduction studies [NCS] showing slower transmission as median nerve transverses carpal tunnel.
Additional Points to Consider
- The veterinary axiom “a dog can have lice and fleas” reminds us that two similar conditions can be present at once. In that regard, it is possible that a patient has nerve compression at the neck and at the wrist, that, has both cervical spine radiculopathy and carpal tunnel syndrome.
- Along those lines, in 1973, Upton and McComas (Upton ARM, McComas AJ. The double crush in nerve entrapment syndromes. Lancet. 1973;2:359–362. doi: 10.1016/S0140-6736(73)93196-6 https://pubmed.ncbi.nlm.nih.gov/4124532/) described their "double crush hypothesis," that because of “serial constraints of axoplasmic flow,” compression of an axon in a cervical nerve root makes a peripheral nerve more likely to be impaired by compression in the arm. According to this hypothesis, two small compressions, neither or which is large enough to produce symptoms, might in combination produce them. Although this hypothesis has not been proven, many refer to it as the “double crush phenomenon.”