{'fraction_non_alphanumeric': 0.01820388349514563, 'fraction_numerical': 0.0024271844660194173, 'mean_word_length': 5.301223241590214, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 1, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Hydrodissection for treatment of carpal tunnel syndrome

In this issue, Evers et al. demonstrate that, in a cadaver preparation, infiltration of saline into the carpal tunnel can reduce the resistance to longitudinal sliding of the median nerve. This experiment was prompted by recent interest in the concept of “hydrodissection” for the treatment of carpal tunnel syndrome (CTS). This can be traced back to a 2008 paper describing an ultrasound-guided approach to the familiar procedure of corticosteroid injection for CTS in which the needle is inserted from the ulnar border of the wrist transversely into the carpal tunnel. The authors speculated that the infiltration of injectate between the median nerve and transverse carpal ligament and between the median nerve and underlying tendons “may disrupt adhesions.” Since then, enthusiasts for ultrasound-guided carpal tunnel injection have adopted the idea that they are breaking adhesions between the nerve and surrounding structures as an established fact. The term hydrodissection has become commonplace in descriptions of ultrasound-guided carpal tunnel injections, and the technique has even been described in a recent textbook as being useful for disrupting adhesions, especially after surgery. The technique has been combined with multiple needle fenestration of the transverse carpal ligament in an attempt to weaken it, and it has been used to treat stiff hands resulting from scleroderma. There is, thus, a burgeoning industry in performing procedures for CTS that elaborate the comparatively simple and quick anatomically guided injection of corticosteroids into a technologically impressive pseudosurgical procedure accompanied by a superficially plausible, scientific sounding explanation for why it might be effective. This narrative must be challenged. Among the above-mentioned papers, it is notable that, in every case, the hydrodissection procedure is carried out with an injectate containing between 40 and 80 mg of methylprednisolone, or an equivalent amount of an alternative steroid. Corticosteroid injection is known to be markedly effective in the short term as a treatment for CTS, with approximately 80% of patients reporting benefit from injection, even when the injection is not ultrasound guided and no attempt is made at hydrodissection. It should, therefore, be incumbent on the proponents of hydrodissection to demonstrate that their procedure confers additional benefit over and above corticosteroid injection without hydrodissection or, alternatively, that hydrodissection alone with saline has a therapeutic effect superior to that of a placebo injection, in which saline would be injected just proximal to the carpal tunnel with the same full panoply of ultrasound guidance. There have been trials claiming superior outcomes for ultrasound-guided injection, irrespective of any attempts to hydrodissect, but this has not been observed in all studies. There are, however, problems with these. All are small, and none of them have satisfactory controls and blinding designed to allow for the placebo effect of the use of ultrasound guidance. Even when ultrasound imaging is used to aid in the diagnosis of CTS, most patients are startled, intrigued, and impressed that such detail of the carpal tunnel and real-time movement of the tendons and nerve can be so clearly displayed. The potential for this to enhance the apparent effect of injection is profound. Two unproven assumptions underlie the reports in this literature. The first is that adhesions between the median nerve and surrounding structures have some pathogenic role in CTS. The second is that corticosteroids must be placed directly in the carpal tunnel or immediately surrounding the nerve to be effective. It has long been known that both longitudinal and transverse movements of the median nerve within the carpal tunnel are reduced in patients with CTS compared with normal controls. The idea that this is due to adhesions, however, is far from proven, and an alternative explanation can be found in the now Correspondence to: J. D. P. Bland; jeremy.bland@nhs.net