Nerve conduction velocity

An easy-to-use worksheet for interpreting electrodiagnostic studies is available at the end of this guideline. The worksheet should be used only when the main purpose of the study is to evaluate a patient for CTS. It is critical to conduct NCV testing in the following situation:

• 1.
  

  The diagnosis of CTS is being considered

  
  
• 2.
  

  Patient is on time loss for more than 2 weeks

  
  
• 3.
  

  Carpal tunnel decompression surgery is requested

Conceptually, validation of the clinical diagnosis of CTS depends on the finding of slowing of sensory and/or motor fibers of the median nerve across the carpal tunnel. The nerve conduction study methods used to test for slowing should not be affected by temperature (either the temperature should be maintained over 32°C or tests that are not influenced by temperature should be used). These electrodiagnostic studies (EDS) should have a high specificity, good sensitivity, and high degree of reliability. Such tests should also minimize the possibility of age or polyneuropathy creating a misleading or false-positive result; this can often be accomplished by comparing the median nerve to another nerve across the same distance across the wrist.

NCV tests are highly sensitive and specific for CTS. If the patient has a positive clinical picture of CTS but the NCV results are negative, the physician should investigate other competing clinical diagnoses such as pronator syndrome, cervical radiculopathy, or tendonitis. Less than 10% of patients with clinical CTS have normal NCV results. In these cases, the treating physician should be sure that the most sensitive and specific NCV tests are done. If not, a request for these tests should be made. In some cases of suspected CTS, the NCV tests can be repeated. However, unless there is a significant intervening event or a substantial change in the clinical assessment, there should be a delay of at least 1 year before repeating the NCV test, because it is unlikely that a difference will be seen at a shorter time interval.

NCV techniques along with their reference values and their upper limits of normal cut points used to corroborate a diagnosis of CTS include the following:

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  Median motor distal latency (8 cm)

  
  
  
  
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    Note: If median motor distal latency is abnormal, then ulnar motor distal latency at 8 cm must be within normal limits (≤3.9 ms)

    
    
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    Less than 4.5 ms

  
  
  
  
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  Median sensory distal latency 8 cm recorded (palm to wrist) or 14 cm recorded (index, long, or ring finger to wrist)

  
  
  
  
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    If either of these tests is used alone, at least one other sensory nerve in the ipsilateral hand should be normal

    
    
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    Less than 8 cm

    
    
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    Less than 14 cm

  
  
  
  
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  Median − ulnar motor latency difference (abductor pollicis brevis [APB] vs abductor digiti minimi [ADM]) (8 cm)

  
  
  
  
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    Less than 1.6 ms

  
  
  
  
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  Median − ulnar sensory latency difference to digits (14 cm), index or long finger compared to ulnar recorded at the small finger, or median − ulnar difference recorded at the ring finger

  
  
  
  
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    Less than 0.5 ms

  
  
  
  
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  Median − ulnar sensory latency difference across the palm (8 cm)

  
  
  
  
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    Less than 0.3 ms

  
  
  
  
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  Median − radial sensory latency difference to the thumb (10 cm)

  
  
  
  
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    Less than 0.6 ms

  
  
  
  
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  Combined sensory index (CSI)

  
  
  
  
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    The CSI is calculated by adding the 3 latency differences above: CSI = (median latency at 14 cm − ulnar latency at 14 cm) + (median latency at 8 cm across palm − ulnar latency at 8 cm across palm) + (median latency to thumb at 10 cm − radial latency to thumb at 10 cm)

    
    
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    Less than 0.9 ms

  
  

These upper limit cut points are derived from published literature. The limits for sensory latencies are chosen for high specificity (ie, few false-positives).

In all cases, and particularly in cases with borderline NCV results, control for skin temperature should be documented. In general, the above-referenced values hold for skin temperature in the range of 30°C to 34°C. Lower temperatures are associated with falsely slowed NCV results.

The department’s (Washington Department of Labor and Industries) policy on EDS follows that of the American Association of Neuromuscular and Electrodiagnostic Medicine. The department does not cover portable NCV tests.

Needle electromyography

Needle electromyography (EMG) sometimes has a role in the electrodiagnostic evaluation of CTS. If the clinical presentation is classic for CTS symptoms and no other signs and/or symptoms, and the result of nerve conduction study is entirely normal, no needle EMG or only limited EMG studies are acceptable. However, there are circumstances in which it would be reasonable to do needle EMG during an evaluation of CTS:

• 1.
  

  Results of nerve conduction studies are abnormal in a manner indicating CTS and the patient demonstrates wasting or clinical weakness of the thenar muscles, or the median motor nerve conduction study is significantly abnormal

  
  
• 2.
  

  The electromyographer suspects another possible diagnosis or a neuropathic process other than, or in addition to, CTS (eg, diabetes)

  
  
• 3.
  

  There is a history of an acute crush injury or other major trauma to the distal upper extremity

  
  
• 4.
  

  There are proximal symptoms (eg, neck stiffness, radiating pain) that suggest cervical radiculopathy may be present.

Quantitative sensory testing

The department does not cover quantitative sensory tests. Several tests of sensory function (vibration, temperature, pressure) have been reported in the scientific literature to be useful in investigational settings to differentiate between patients with and without neuropathy. However, because these techniques cannot localize peripheral nerve lesions, they are not useful for diagnosing specific entrapment neuropathies.