Because ultrasound has become more widely available and accessible, clinicians have used the improved resolution to increase accuracy in their procedures. They also have used this technology to create novel procedures. One of the growing areas of musculoskeletal ultrasound to take advantage of this imaging modality is in the treatment of peripheral nerve entrapments. Ultrasound has been used both to diagnose and treat a variety of common conditions such as carpal tunnel syndrome (CTS) and cubital tunnel syndrome. One frequently referenced technique while treating peripheral nerves is hydrodissection. The purpose of this article is to evaluate the available literature on hydrodissection and determine its level of effectiveness and safety.
Definition of hydrodissection
The definition of hydrodissection varies widely depending on the procedure or surgery it is being used for. It has been described in reoperative therapy for adhesiolysis by introducing saline under pressure into planes of dissection. This then helps to remove adhesions by creating previously nonexistent surgical planes (2). Hydrodissection has been used to preserve perforating arteries in breast reconstruction surgery (17). Hydrodissection has been discussed also in the ophthalmological literature to establish surgical planes in the eye (12). More applicable for our discussion, Smith et al. (15) describes a process of hydrodissection in the treatment of CTS for delivering injectate around the nerve. The purpose of the hydrodissection in this article was to peel the median nerve from the surrounding retinaculum and connective tissue. The primary goal of the hydrodissection is to avoid inadvertent direct injection and injury to the nerve. The secondary goal for many clinicians with this technique is to separate a potential soft tissue adhesion or obstruction from the nerve that could be causing an entrapment.
A note on ultrasound injection techniques
To better understand the terminology used in the articles reviewed, it is important to define the different injection techniques. There are typically two groups, in-plane injections and out-of-plane injections. With in-plane injections, the needle enters the skin parallel to the long axis of the transducer (Fig. 1). With this approach, the needle tip is completely visualized as it reaches its target. With the out-of-plane approach, the needle enters the skin perpendicular to the long axis of the transducer (Fig. 2). With this approach, the needle appears as a bright dot as it enters the scanning area of the probe. The in-plane approach is most often used, as the needle tip can reliably be visualized entering the target. With the out-of-plane approach, the hyperechoic dot will still be present, even if the tip of the needle pierces through the intended target. Despite its limitations, the out-of-plane approach is still used with difficult and small anatomy where the needle or probe cannot lie in-plane.
A literature search for “nerve hydrodissection” in PubMed and other search engines yielded few articles with hydrodissection in the title. In an attempt to look for more evidence, search terms such as “ultrasound nerve injections” and specific nerve injections were used, for example, “ulnar nerve ultrasound injections.” The methods were then thoroughly reviewed to see if a hydrodissection technique, such as the description by Smith et al. (15), was used.
Specific Nerve Studies
Because of the prevalence of CTS, the median nerve has been evaluated and studied as a candidate for ultrasound-guided injections. DeLea et al. (6) performed a prospective study of patients with CTS receiving ultrasound-guided median nerve injections after hydrodissection. Pain and vasomotor changes were significantly reduced with this approach, and there were no adverse outcomes. It should be noted that in this study, there was not a control group with which to compare. Lee et al. (10) also evaluated patients with CTS receiving a corticosteroid injection. They randomized their patients to three groups, in-plane ultrasound injection, out-of-plane ultrasound injections, and landmark-based injections. The ultrasound groups received hydrodissection to “peel the nerve off the overlying flexor retinaculum.” The in-plane ultrasound-guided injection had statistically improved pain and functional scores compared with the out-of-plane group and landmark group. The in-plane group also had no nerve or vessel injury. Function and pain improved in all three groups. Only one of the studies that were reviewed randomized patients to a hydrodissection group or an injection-only group (7). Patients received a median nerve block before elbow surgery and were randomized to hydrodissection with D5W or to a block with lidocaine alone. Their main outcome measure was time to anesthesia, which showed no statistical change. Unfortunately, this study did not look at safety or adverse events, so one cannot draw conclusions in regard to decreased nerve injury with hydrodissection from this study.
Since cubital tunnel syndrome is another common peripheral neuropathy, the ulnar nerve has been researched with ultrasound-guided injections using nerve hydrodissection as well. A cadaver study by Kim et al. (9) demonstrated that an ultrasound-guided hydrodissection technique could separate the ulnar nerve from the medial epicondyle and the adjacent connective tissues. A small pilot study of 10 prospective patients with cubital tunnel syndrome used this approach and showed improvement in pain, decreased cross-sectional area, and improved electrophysiological measurements (4). They also sustained no neurological injuries.
Lateral femoral cutaneous nerve
In a prospective study of 20 consecutive patients with verified electrophysiological meralgia paresthetica, the patients received a perineural injection of anesthetic with corticosteroid. This study described a hydrodissection technique where the nerve was floated away from the adjacent structures. Sixteen of the original 20 patients had statistical improvement in pain and function. The other four received another injection. After 2 months, all of the patient’s symptoms disappeared completely (16). Similar results were obtained in a case study of a 45-year-old man with chronic meralgia paresthetica. Using a similar in-plane injection, the patient remained symptom free at an 18-month follow-up (13).
In a retrospective study of patients with chronic medial knee pain after total knee replacement, the infrapatellar branch of their saphenous nerve was subjected to injection. They used a hydrodissection technique followed by a corticosteroid injection. It was a small study of 16 people. Of the subjects, 75% improved their Visual Analog Scale (VAS) pain score to less than 3 to 4 from a baseline of 8 out of 10. There was no blinding and no mention of adverse outcomes. Because of the size of the group, no statistical analysis was performed (5).
At the time of this article, there are no studies that evaluate the safety of the hydrodissection technique for nerve injections. The most readily available literature focuses on safety of intraneural injections and the rate of accidental injections with ultrasound.
To determine the rate of unintentional intraneural injections, Liu et al. (11) recorded ultrasound on 257 patients receiving an interscalene or supraclavicular block before shoulder arthroscopy. Two blinded anesthesiologists then reviewed the video and found a 17% incidence of intraneural trespass. No patient experienced postoperative neurological complications. Their study mentions that hydrodissection was left at the discretion of the anesthesiologist, and their Results section does not mention how many times it was used or its correlation with intraneural injections. Hara et al. (8) found a similar incidence of intraneural injection on patients receiving a subgluteal sciatic nerve block while undergoing knee arthroscopy. Using similar methods and video reviews, they found a 16.3% rate of injection. Again, out of 325 patients, they reported no postoperative complications. Their study does not reference the use of hydrodissection but would indicate that hydrodissection had been done because of a halo appearance of injectate around the nerve.
A systematic review by Brull et al. (3) calculated the rate of neuropathy after a peripheral nerve block to be less than 3 in 100. Out of 16 studies evaluating the rate of permanent neurological injury after a nerve block, they found only one case.
There are fewer studies that evaluate long-term effects of inadvertent nerve injections. A case study by Russon and Blanco (14) shows a review of an unintended axillary plexus block in a patient undergoing wrist surgery. On a video review, they determined that the musculocutaneous nerve was inadvertently injected. At a 6-month follow-up visit, the patient exhibited no neurological complaints or deficits.
Bigeleisen (1) studied 26 patients undergoing an axillary plexus block for thumb surgery over a course of 6 months. They had sensory and motor testing before and 5 and 20 min after block placement and 6 months after their nerve block. Of the 26 patients, 22 had inadvertent injection of at least one nerve. At the 6-month follow-up visit, their sensory and motor testing was unchanged compared with their baseline.
Despite the widespread use of the hydrodissection technique, which is referenced often in articles, very limited high-quality data exists to determine its effectiveness. Most of the available research consists of case reports and retrospective studies. This low-level evidence does demonstrate that this technique could be effective. However, there is often lack of randomization and blinding, leading to inevitable selection bias. None of the studies reviewed evaluated an ultrasound-guided steroid injection with and without hydrodissection and with a control group. This would seem to be essential to determine if this is a necessary part of the procedure or if it is only necessary to deliver injectate around the nerve.
A major rationale for using the hydrodissection technique is to decrease the risk of inadvertent nerve injection and subsequent injury during these procedures. The current literature would indicate a possible 16% to 17% inadvertent injection rate with ultrasound guidance (8,11). However, neither of these two studies indicated the role of hydrodissection in its outcomes or results. All of the studies reviewed in the anesthesiology literature show little to no adverse effect of inadvertent nerve injection, short or long term. One could make the argument from this evidence that this may be a relatively safe procedure.
With the changing landscape of medicine, especially in our payer systems and value-based care, it is crucial that we establish effectiveness for treatments that many of us feel add to patient care. Many who use ultrasound for musculoskeletal medicine, this author included, note an improvement in our practice and care of our patients with this technology. Hopefully this article can encourage others to publish what they are doing and encourage those with the means to do more high-level research in the field.
The author declares no conflict of interest and does not have any financial disclosures.
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