Congratulations Dr Bland

By all means link to the site. The platform it is built on is outdated and needs a a major re-write but we can't afford that at present and it still works. At present we use the questionnaire to 'pre-screen' referrals from primary care for NCS - ?CTS - diverting patients with diagnostic scores < 20% to other services and that has proved to be very successful in keeping the waiting lists under control. JB 

Dear Dr Bland,

Thank you for allowing me to create a link to your site. A penny for your thoughts as a carpal tunnel expert, I wonder if you can shed some light on a question for me. I am curious as to why the 2nd lumbrical seems to be remain recordable in extremely severe CTS, when the APB is unrecordable. Do you happen to know why this seems the case? I can only assume it is due to the internal architecture of the median nerve, ie, the fibres extending to the 2nd lumbrical must run deeper than the recurrent motor branch and are less prone to compression?

Even more peculiarly, the motor latency to the second lumbrical seems to become prolonged very early in the course of CTS (See Loscher WN, Auer-Grumbach M, Trinka E, et al. Comparison of second lumbrical and interosseus latencies with standard measures of median nerve function across the carpal tunnel: a prospective study of 450 hands. J Neurol 2000;247(7):530-34.) It's easy enough to imagine a 'protected' location within the nerve for that fascicle resulting in it being relatively spared but rather harder to square that with it starting to demyelinate early in the disease so the answer at present is 'I don't know' but it's possible that the early involvement may be purely techincal. When we are assessing the latency to APB we have to contend with the non-linear course of the recurrent motor branch which introduces uncertainty into the measurement and tends to mean that most departments set their normal values relatively leniently for APB, though I do know some departments who use a limit of 3.8 msec (which is very tight by my standards - the AANEM recommend 4.5 if you don't have your own detailed controls). In contrast, latency to the lumbrical is usually assessed by direct comparison with the ulnar latency to the interosseous which means no distance measurements are involved to introduce measurement errors and the comparative test is also largely self correcting for temperature so the 2-LI comparison can have a much tighter reference limit and that may be why we can pick up slowing there when we cannot technically detect it using APB. JB