Nerve conduction studies and Carpal Tunnel Syndrome
Perhaps 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, in part because of compression of a normal median nerve can produce sensory symptoms. This makes it difficult to define an abnormal response with provocative clinical testing.
VALUE OF NERVE CONDUCTION STUDIES IN CARPAL TUNNEL SYNDROME
As expected, patients with definite CTS are more likely to have abnormal NCS. (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 NCS 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 symptoms (Witt et al, 2004;Wilder Smith et al, 2006).
NORMAL NERVE CONDUCTION STUDIES
It would seem that patients with carpal tunnel syndrome with normal nerve conduction studies require careful selection before surgery and if a local steroid injection did not provide symptomatic relief then surgery may not be useful. There are however, patients who have typical symptoms of Carpal tunnel in whom NCS are normal. Such patients in my clinical experience are those who typically have occasional night time symptoms with few daytime complaints where symptoms are presumably quickly reversible and not of sufficient severity to produce changes on nerve conduction. Nevertheless patients without abnormalities on NCS for CTS 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 conductions identify permanent nerve damage but not intermittent nerve damage - explaining negative results in early carpal tunnel syndrome.
However in carefully selected cases in an expert hand unit, a success rate of 90% has been reported in nerve conduction negative cases. There remain continuing questions in the literature as to whether nerve conduction studies as part of a pre-surgical work-up to carpal tunnel syndrome improves accuracy (Bland J, 2007). However it is regarded as good clinical practice to perform nerve conduction studies prior to carpal tunnel surgery.
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 previously. These results reflect 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 5,
• 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 APB).
• advanced disease on nerve conduction studies with an unrecordable surface motor action potential from abductor pollicis brevis and denervation in APB on EMG with subjective success rate of 47%. Nevertheless even in patients with these features troublesome paraesthesia and pain often respond well. However when there is, to some extent, irreversible nerve damage full relief of symptoms cannot be expected.
• normal nerve conduction studies
EVALUATION OF FAILED CT RELEASE
Although clinical parameters and nerve conduction abnormalities improve significantly after carpal tunnel release there is only a modest correlation between nerve conduction abnormalities and clinical outcome measures . The value of post operative nerve conduction studies may be in the improved nerve conduction despite persisting complaints which should alert the physician to a diagnosis other than carpal tunnel syndrome. Unsuccessful carpal tunnel decompression has been observed in patients with other neurological problems including neuropathy, radiculopathy or 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 error (Schrijver H et al, 2005; Bland J, 2007)
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 36:167-171 2007
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
Wilder-Smith E et al., Diagnosing Carpal Tunnel Syndrome – Clinical Criteria and Ancillary Tests Neurology July ’06 Volume 2 No 7
Witt J et al.,Carpal Tunnel syndrome with normal Nerve conduction studies Muscle Nerve 2004,29:515-522