I recently saw this middle aged lady with unilateral carpal tunnel syndrome. She presented with a four month history of right sided carpal tunnel syndrome type symptoms. Four years earlier she was diagnosed with the same and received a steroid injection which completely settled her symptoms.
Clinically there was certainly a mild carpal tunnel syndrome and this fitted with the history of waking at night with numbness in the hand. Carpal tunnel provocation tests were mildly positive.Musculoskeletal ultrasound showed a bifid median nerve (as seen above) . Lanz et al (1977) were the first to describe anatomical variations of the median nerve in the carpal tunnel, there thought are outline din this paper by Mizia et al (2011). Propeck et al (2000) were the first to discuss imaging of the bifid median nerve. Bayrak et al (2008) looked into the prevalence of bifid median nerves in those with carpal tunnel syndrome vs normal controls and bifid median nerve was more common in CTS patients with 32 (19%) of 170 patients having bifid median nerves vs 11 (9%) of 120 controls.
As you can see in the images above, the cross sectional area of the median nerve at the carpal tunnel inlet measured 12mmsq compared to only 9mmsq of the non-symptomatic site (see below). Please see the article by Klauser et al (2008) for further information.
As this lady previously had such good benefit from steroid injection she again opted for this after I had first explained risks and side effects. I opted to inject the carpal tunnel via the transverse approach where the needle is guided adjacent to the median nerve. This is less invasive and generally less uncomfortable for patients than landmark guided steroid injections which are traditionally given deep into the carpal tunnel. For more detailed information on how to administer ultrasound guided injections in the transverse plane please read this free text article from Smith et al (2008) . Please note that as the needle is in close approximation to the median nerve during the procedure this method is only recommended for skilled and experienced operators as it is extremely important to visualise the needle tip throughout the entire procedure.
Given the apparently very low risks of landmark guided injection it is not been clearly shown why steroid injections should be done ultrasound guided. However the following image from www.carpaltunnel.net...demonstrates some food for thought...
(reproduced from: www.carpaltunnel.net)
... where the gap between the ulnar artery and the nearest of the median nerve branches (marked A) is only 3.4 mm and where the median nerve itself has two branches spread across a horizontal distance of about a further centimetre. This provides strong encouragement to take a look inside the wrist to assess the anatomy before inserting a needle. Particularly as we all have slight anatomical variations!
I personally prefer the radial transverse approach as it minimises the risk of injecting into the ulnar artery which as described above can in some people be very close to the median nerve. The only negative with this approach is that the trajectory of the needle has to be superficial to the flexor carpi radialis tendon which sits very much more anterior from the carpal tunnel. This therefore requires a very superficial approach, as you can see in the image below.
There also needs to be alot of ultrasound gel standoff with the needle being just below the skin and penetrating the transverse ligament being reasonably parallel to the carpal tunnel.
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