Nerve conduction studies and Carpal Tunnel Syndrome
Because of varying techniques and patient selection, sensitivities of manoeuvres putting pressure on the carpal tunnel (Phalen’s and Tinel’s signs) have a wide range of sensitivity and are not totally reliable. This is partly because compression of a normal median nerve can also produce sensory symptoms. This makes it difficult to define an abnormal response with provocative clinical testing.
Assessment with Nerve Conduction Studies
Objective testing of sensory nerve function involves stimulating a digital nerve, e.g. with ring electrodes on the thumb, as seen in the figure below; and recording the response, e.g. with surface electrodes at the wrist, also seen in the figure below. The stimulus is a short lived electrical pulse which, although mildly uncomfortable, is tolerated by almost all patients without difficulty.
Value of Nerve Conduction Studies in Carpal Tunnel Syndrome
As expected, patients with definite Carpal tunnel syndrome are more likely to have abnormal nerve conduction studies (Witt et al, 2004). Abnormal nerve conduction studies indicative of carpal tunnel syndrome are associated with a greater chance of a successful surgical outcome (75%) compared to surgery done on clinical grounds alone in the presence of normal nerve conduction studies (51%) (Bland J, 2007).
Some argue that the value of nerve conduction studies lies in their ability to detect median nerve pathology in cases where patients have few major clinical symptoms and where the symptoms are atypical or equally perhaps refute the diagnosis where pain may be a predominant symptom without typical sensory complaints (Witt et al, 2004;Wilder Smith et al, 2006).
However, nerve conduction studies are the most definite diagnostic tests for carpal tunnel syndrome, with a high degree of sensitivity and specificity. They are performed to confirm the diagnosis, to determine the severity and exact site of nerve entrapment and to preclude alternative diagnoses that overlap with carpal tunnel syndrome in presentation.
Nerve conduction studies are the only objective way to determine and quantitate the improvement after carpal tunnel decompression. Comparative pre and postoperative studies enable assessment of inadequate decompression of the carpal tunnel or recurrence of entrapment over time (Tahririan M et al., 2012). This is of importance in patients who claim no clinical improvement after surgery or who later redevelop symptoms.
Normal Nerve Conduction Studies
Patients with apparent carpal tunnel syndrome with normal nerve conduction studies require careful decision making before surgery is considered. If a local steroid injection does not provide symptomatic relief then surgery may not be useful. There are, however, patients who have typical symptoms of carpal tunnel in whom nerve conduction studies are normal. Such patients in my clinical experience are those who typically have occasional night time symptoms with few daytime complaints, where symptoms are quickly reversible and not of sufficient severity to produce abnormal nerve conduction results. Nevertheless, patients without support for carpal tunnel syndrome on nerve conduction studies have a less satisfactory surgical outcome (Bland J, 2001).
Nerve conduction studies are less accurate in the early stages of carpal tunnel syndrome and in younger patients (Wilder-Smith E et al, 2006). This may be because nerve conduction studies identify nerve damage but not brief nerve symptoms, thus explaining negative results with symptoms of short duration.
There remains some question in the surgical literature as to whether nerve conduction studies for carpal tunnel syndrome improves diagnostic accuracy (Bland J, 2007). However, it is regarded as good clinical practice to perform nerve conduction studies prior to carpal tunnel surgery.
Abnormal Nerve Conduction Studies
The figure below shows sensory responses from digits with delayed/absent median sensory potentials compared to radial and ulnar sensory potentials. This principle forms the basis of electrodiagnosis with comparatively delayed median responses compared to other nerves or normal controls.
A 75% success rate in surgical release of the carpal tunnel relates to findings in a large number of patients who are regarded as much better or cured with follow-ups ranging from several weeks to several years. However, with surgical decompression 8% of patients are worse than previously, reflecting pooled results in more than 30,000 operations (Bland J, 2007).
Prognosis of carpal tunnel with surgery:
Complete cure or only minimal residual symptoms 75%
Slight improvement or no change 17%
Prognosis of carpal tunnel with splinting of the wrist:
Complete cure or only minimal residual symptoms 30%
Slight improvement or no change 17%
(Bland J, 2007).
Indicators of Poor Outcome
Reasons for poor outcome include:
- less able to manage normal activities of daily living
- constant symptoms
- atypical clinical features including symptoms in digit V
- co-existent diabetes or a neuropathy
- long duration of symptoms
- greater age
- heavy manual occupation
- industrial injury compensation claims
- objective neurological signs (a fixed median nerve sensory deficit and visible wasting of Abductor Pollicus Brevis).
- advanced disease on nerve conduction studies with an unrecordable surface motor action potential from Abductor Pollicis Brevis and denervation in Abductor Pollicus Brevis on EMG with subjective success rate of 47%. Even in patients with these features, troublesome paraesthesia and pain often respond well. Nevertheless, when there is irreversible nerve damage full relief of symptoms cannot be expected.
- normal nerve conduction studies
Evaluation of Failed Carpal Tunnel Release
Unsuccessful carpal tunnel decompression has been observed in patients with other neurological problems, including neuropathy, radiculopathy or spinal cord lesions. However, worsening nerve conduction studies should raise the question of failure to completely divide the transverse carpal ligament - the most common cause of surgical failure (Schrijver H et al, 2005; Bland J, 2007).
With persistent symptoms repeat nerve conduction studies may be useful.
Although clinical parameters and nerve conduction abnormalities improve significantly after carpal tunnel release, there is only a modest correlation between the two. However, nerve conduction studies do improve after surgery in the majority of patients. The change may not be apparent for some 3-6 months and nerve conduction studies may not return completely to normal. Nevertheless, pre and postoperative nerve conduction studies provide an objective measure to determine changes after decompression.
Bland J. Do nerve conduction studies predict the outcome of carpal tunnel decompression. Muscle Nerve 2001;24:935-940.
Bland J. Treatment of Carpal Tunnel syndrome. Muscle Nerve 2007;36:167-171.
Schrijver H, et al. Correlating Nerve conduction studies and clinical outcome measures on Carpal Tunnel syndrome: Lessons from a Randomised controlled trial. J Clin Neurophysiol 2005;22:216-221.
Tahririan M, et al. Changes in electrophysiological parameters after open carpal tunnel release. Adv Biomed Res 2012;1:46.
Wilder-Smith E, et al. Diagnosing Carpal Tunnel Syndrome – Clinical Criteria and Ancillary Tests. Nature Reviews Neurology 2006;2: 366-374.
Witt J, et al. Carpal Tunnel syndrome with normal Nerve conduction studies. Muscle Nerve 2004;29:515-522.