The Canterbury NCS Severity Scale for CTS
This method of combining several different neurophysiological measurements into an overall rating of CTS severity has been in use in my own department since 1992. It is based on the principle that changes in sensory conduction tend to occur before changes in motor conduction and that changes in conduction velocity tend to precede changes in the amplitude of recorded nerve and muscle potentials. It has since become apparent that there is a motor study which shows abnormalities very early in CTS (the lumbrical/interosseous distal motor latency comparison test) but this has not seriously impaired the overall usefulness of the scale. The other design principle was that, so far as possible the scale would be usable by neurophysiology laboratories that use different methods to my own but that, even so, a patient attending two different laboratories should normally end up evaluated as the same severity grading, thus allowing some comparability of studies between laboratories.
If you have a copy of your own nerve conduction studies to hand you may be able, even as a patient, to translate the result to this grading if the details given are adequate. If you are about to attend for testing you can take the following guide to grading to the laboratory and ask them what grade your results fall into. (We will make a more printer friendly form of this available in the future)
The scale grades neurophysiological severity of CTS from 0 - no neurophysiological abnormality, to 6 - extremely severe CTS. It requires as a bare minimum that the laboratory records the median distal motor latency from the wrist to the abductor pollicis brevis muscle (APB) and a sensory nerve action potential between a median innervated finger and the wrist. With one exception the actual measurements recorded should be assessed as being normal or abnormal by that laboratory's own standards/normal ranges. Finally, if these two measurements are normal then the laboratory must perform at least one of the more sensitive comparative tests for CTS which have been described in the literature, though exactly which does not matter. This requirement is according to AANEM guidelines in any case.
It is easiest to apply the grading in reverse when looking at a set of results. Done this way, then as soon as your results meet a criterion you do not need to look any further. Note that all of these assume that other nerves have been tested and are normal - ie we have already made a diagnosis of CTS - this is merely about evaluating severity:
a) If the surface motor potential from APB is unrecordable (technically defined as <0.2mV peak to peak amplitude) then it is GRADE 6. Stop here.
b) If the distal motor latency from wrist to APB is >6.5 msec then it is GRADE 5. Stop here (this is the one measurement which is absolute, ie not dependent on individual laboratory normal ranges, ideally this recording should be made with the stimulating electrode 6-8 cm from the recording point but because the recurrent motor branch of the median nerve does not run a straight course, distance measurements should be taken with a pinch of salt).
c) If a median sensory potential between a digit and the wrist is unrecordable (thumb, index or middle fingers) then look at the distal motor latency to APB - if it is delayed then the hand is GRADE 4, if the motor potential is of normal latency then the hand is GRADE 2 - Stop here
d) If the distal motor latency to APB is delayed then the hand is GRADE 3 - Stop here
e) If median sensory conduction between digit and wrist is slow (shown by either a latency measurement at a fixed distance outside normal range or by a slow calculated conduction velocity) then the hand is GRADE 2 - Stop here
f) If the hand does not meet the above criteria but is considered to show evidence of CTS using any of the more sensitive tests then it is GRADE 1. In my own laboratory I prefer to see two different sensitive tests both giving an abnormal result before I am wholly convinced of this.
APPENDIX - Permissible sensitive tests
- Lumbrical/Interosseous distal motor latency comparison
- 6cm transcarpal median/ulnar latency comparison
- Ring finger/wrist sensory conduction comparison (either looking for a ‘double peak’ at the wrist on finger stimulation or comparing latencies at the ring finger on wrist stimulation
- Any form of ‘inching’ study at the carpal tunnel
- Finger/palm - palm/wrist conduction velocity comparison
- Median/radial sensory nerve conduction velocity comparison (short segment)
- Combined sensory index (Which adds together three different sensory measures - any laboratory using this will know what it is! - the source reference is Robinson 1998)
Revision date - 20th December 2011