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).
Figure 1: Sagittal and axial views on CT showing osteophytic overgrowth causing stenosis of the neural foramen. (Credit: Wikipedia)
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.
Table 1: Patterns of nerve root compression
Figure 2: Dermatomes of the hand and their corresponding spinal nerve roots.
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 more likely to due to C6 radiculopathy
- Because degenerative joint disease and degenerative disc disease are the usual underlying causes of cervical radiculopathy, findings suggestive of degenerative disease increase the likelihood of radiculopathy:
- neck pain and impaired range of motion of the cervical spine, especially in extension, (likely from facet degenerative disease)
- a plain x-ray showing degenerative (spondolytic) changes
- an MRI showing either osteophytes or soft tissue structures (e.g., disc) compressing the nerve as it exits the foramen.
- Pain proximal to the wrist is more likely due to a proximal compression.
- Likewise, motor weakness, if present, will include the elbow flexors and wrist extensors.
- 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).
- Biceps and/or brachioradialis hyporeflexia.
Findings more likely to due to carpal tunnel syndrome
- Symptoms that are affected by the position of the hand (extremes of flexion or extension) suggest carpal tunnel syndrome.
- Anecdotally, hand pain during the night or just before waking that is relieved by “shaking out” of the hand.
- Weakness of the thumb abductor is a late finding in carpal tunnel syndrome.
- Exacerbation of symptoms with Tinel’s test and/or Phalen’s test at the wrist.
- Nerve conduction studies [NCS] showing slower transmission as median nerve traverses 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.”