Article by Professor John Corbett
MB, BS (HONS), FRCP (UK), FRACP, MA (Oxford), D. Phil (Oxford), MACLM, Professor of Medicine (Griffith)
All General Practitioners would be familiar with common peripheral-nerve disorders, such as carpal tunnel syndrome, ulnar nerve compression at the elbow and superficial or common peroneal compromise of these nerves in their superficial course over the fibular head.
However, some GPs may be less familiar with a condition known as “Double Crush Syndrome”, where the axons of the same peripheral nerve are compromised by excessive pressure at more than one site.
Sensory symptoms of such mononeuropathies include pins & needles, numbness, burning and other types of pain. Common motor symptoms include weakness and – in severe cases – muscle-wasting.
When considering a diagnosis of peripheral nerve compression (commonly known as a “pinched nerve”), it is imperative to determine the exact site of compression, as this will have a material influence on the best choice of treatment. Moreover, any treatment will commonly be ineffective if it does not address the location of the clinically-significant nerve entrapment.
Let’s first consider carpal tunnel syndrome, as this is one of the most common mononeuropathies. Most often, the presenting complaint will be tingling and/or pain or numbness in the ventro-lateral hand – particularly at night, early in the morning or when driving. If the symptoms are severe, they will occasionally radiate proximally and the distribution of sensory loss may involve cutaneous areas innervated by the C6 nerve-root.
Every GP will know that the C6 nerve-root (which emerges between C5 and C6 in the cervical spine) is commonly involved in degenerative spondylosis and cervical intervertebral disc injuries, in which the C6 nerve-root can become compressed. When this occurs, especially with major nerve-root compression at a lateral location, the patient will typically also complain of medial scapular pain, as well as radicular pain. Patients will often also complain of radiated pain in association with certain head and neck movements, such as lateral flexion. As the C6 nerve-root is responsible for innervation of the thumb, index finger (and certain more proximal muscles involved in flexion of the ipsilateral elbow (notably the brachioradialis)), the patient’s complaints may overlap with those of a patient presenting with carpal tunnel syndrome.
As always, there is no substitute for taking a careful history and conducting a detailed examination of the patient. However, this may not always provide sufficient clues for a definitive conclusion to be reached. Indeed, the patient may be experiencing overlapping sensory symptoms from two separate pathologies – carpal tunnel syndrome and a simultaneous C6 nerve-root compression.
What to Do?
The answer is first to establish whether median-nerve compression is indeed occurring at the carpal tunnel. A combination of motor and sensory nerve-conduction studies on the median nerve, usually with median-ulnar or median-radial nerve comparison studies, will normally answer this question. In addition, imaging the cervical spine with CT or MRI will delineate any cervical spinal pathology.
In addition, it is simple to carry out an EMG study of a C6-innervated muscle proximal to the carpal tunnel (e.g. the brachioradialis). If this muscle shows EMG evidence of denervation and if the more peripheral tests confirm carpal tunnel syndrome, the diagnosis of “double crush syndrome” is confirmed.
In such cases, the protocol followed at Corbett Neurophysiological Services includes performing computerised quantitative EMG motor unit analysis (QEMUA), whereby the recorded EMG data is replayed and re-analysed offline, subsequent to the initial testing. This allows for quantification of any EMG abnormalities, which may include abnormal duration, amplitudes, polyphasia or “turns” (of similar significance to polyphasia).
Conceptually similar issues with “double crush syndrome” can occur with the ulnar nerve at the elbow and the C8 nerve-root, as the ulnar nerve supplies the 5th digit and one-half of the 4th digit, which is also subserved by the C8 dermatome. In such cases, it can be very helpful to perform an EMG study on flexor muscles in the forearm, which also depend on the C8 nerve-root for innervation (these muscles are usually not involved in ulnar nerve compressive lesions at the elbow, as the motor nerve fibres to these flexor muscles normally branch from the main ulnar trunk proximal to the elbow).
A “double crush” presentation of ulnar nerve problems can also occur from a combination of ulnar nerve compression at retrocondylar groove posterior to the elbow and at Guyon’s canal, just distal to the medial aspect of the wrist crease.
Other examples of “double crush” can occur, the most common being a combination of L5 nerve-root compression in the lumbar spine and peroneal nerve compression at the fibular head. The latter nerve is subserved by the L5 nerve-root and the diagnosis is often clarified by studying a more proximal L5 muscle innervated by the L5 nerve-root proximal (e.g. the tensor fascia lata muscle in the ipsilateral thigh). Again, CT or MRI imaging of the lumbar spine will often clarify a lumbar pathology.
For more information on neurophysiology testing including nerve conduction studies (NCS), electromyography (EMG) and quantitative EMG motor unit analysis (QEMUA) or to refer your patients for testing visit www.corbett.com.au or phone 07 5503 2499.