Bipolar pulsed radiofrequency neuromodulation of median nerve for treatment of carpal tunnel syndrome – a preliminary study

INT RO DU CT ION: The results of direct surgical release of the median nerve in patients with carpal tunnel syndrome are frequently far from being satisfactory. Aim of the presented study is to assess the early results of bipolar pulsed radiofrequency (PRF) neuromodulation of the median nerve (MN) for treatment of carpal tunnel syndrome (CTS). MAT ERIAL AND MET HODS : Fifteen adult patients with CTS (10 women and 5 men, mean age 58.5 years) were treated. The bipolar technique of neuromodulation was applied, with confirmation of the correct electrode position by neurophysiological examination. The study protocol included the Boston Carpal Tunnel Questionnaire (BCTQ), Numerical Rating Scale (NRS) for pain assessment and tip pinch strength assessment performed before neuromodulation as well as 4 and 12 weeks after treatment. Moreover, the sensory nerve conduction velocity (SNCV) was assessed before and 12 weeks after the intervention. RES ULT S: A significant improvement in symptom severity (33.53 ± 8.25 at the baseline vs. 18.33 ± 11.06 and 25.67 ± 12.39 at 4 and 12 weeks) and functional status (21.0 ± 5.79 vs. 12.07 ± 7.20 and 17.73 ± 9.09) BCTQ subscores, as well as tip pinch strength (3.39 ± 1.68 vs. 5.86 ± 1.98 and 4.93 ± 2.22) were observed. A reduction in pain and improvement in SNCV were also found, but did not reach statistical significance. CO NCL US IO NS: Bipolar PRF neuromodulation of MN is a promising therapeutic tool for patients with CTS and could potentially be an alternative to direct surgical release of MN. A longer follow-up period is required to assess the longevity of clinical improvement after the treatment.


INT RO D UCT IO N
Carpal tunnel syndrome (CTS) is the most frequent human compression neuropathy, with a prevalence of 3-5% in the general population [1,2]. The male/female ratio is estimated to be 1:3.6 [3]. If left untreated, compression of the median nerve (MN) can lead to chronic pain and paraesthesias, hand dysfunction and neurological deficits. Mild cases usually benefit from non-surgical treatment, while moderate and severe cases frequently require surgical interventionopen or endoscopic carpal tunnel release (CTR) [4]. Surgical treatment carries a risk of complications, occurring approximately in 25% of cases, with hand numbness, hematoma and wound infection being the most frequent [2]. Moreover, the pain persists in 12%, while 22% of patients develops a new type of pain one year after CTRso-called chronic postoperative pain [5]. Other, less invasive interventional modalities like pulsed radiofrequency (PRF) neuromodulation of MN could potentially fill the gap between conservative and surgical treatment. Unipolar pulsed radiofrequency of MN was first applied by Haider et al. [6] in a patient with recurrent CTS after previous surgery, in whom a 70% symptom reduction was achieved after PRF. In this particular case, the radiofrequency electrode was positioned on the median nerve at the level of the cubital fossa under ultrasound guidance. This site of PRF was dictated by post-surgical scarring at the wrist. A randomized clinical trial performed by Chen et al. [7] in patients with CTS showed significantly better results of unipolar PRF of MN in comparison with conservative treatment (wrist splinting). In the cited study the electrode was positioned at the level of the wrist. The purpose of our study was to assess the early results of bipolar PRF neuromodulation of the median nerve for the treatment of CTS.

MAT E RI AL AN D ME TH OD S
A prospective observational study was conducted with a single-group pretest/posttest design, according to the classification by Berger et al. [8]. Fifteen adults with CTS (10 women and 5 men, the mean age 58.53 ± 10.38 years) were treated. The bipolar technique of neuromodulation was applied, with neurophysiological confirmation of the correct electrode position. The procedure was performed with a G4 Radiofrequency Generator (Cosman Medical, Burlington, Mass., USA). Two cannulas (22G, length 50 mm, active tip 4 mm) were arranged in parallel, and introduced perpendicular to the skin surface, above the course of MN in the forearm, between the tendons of the flexor carpi radialis and the palmaris longus muscles. The first cannula was introduced 4 cm proximally from the first bracelet (Rascette) line, and the second was 1 cm proximal to the previous cannula. The cannulas were navigated towards MN under electrophysiological guidance; sensory (50 Hz, up to 0.45 V) and motor (2 Hz, up to 1 V) stimulation was performed via the RF electrode, for each cannula separately. After an injection of 0.5 ml 0.9% saline through each cannula, the bipolar PRF was performed (4 cycles, 2 min each, 2 Hz, 20 ms, 45 V, temperature not to exceed 42℃). After the procedure, the puncture sites were dressed in a sterile manner. The PRF procedures were performed by two surgeons (A. Krzywda and J.L. Słowiński).
The examination protocol consisted of the Boston Carpal Tunnel Questionnaire (BCTQ) with the Symptom Severity Scale (SSS) and Functional Status Scale (FSS) subscale scores, the Numerical Rating Scale (NRS) for pain intensity assessment, and tip pinch strengthall performed at 0, 4 and 12 weeks after treatment. A neurophysiological examination, including measurement of the sensory nerve conduction velocity (SNCV) in the median nerve was performed before and 12 weeks after the PRF procedure. Ethical approval of the project was obtained from the Bioethical Committee of the Medical University of Silesia, Katowice, Poland.

Statistical analysis
Continuous variables with normal distribution were presented as the mean and standard deviation, and those with non-normal distributionas the median as well as lower and upper quartiles. Categorical variables were expressed as counts and percentages. The differences between three consecutive time points were compared using the repeated measures analysis of variance or the Friedman test for normally or non-normally distributed data, respectively. Bearing in mind the aim of the study, we compared only the first and the last time point after repeated measures tests, using a paired samples t-test or the Wilcoxon signed-rank test for normally or non--normally distributed data, respectively, purposely without any corrections for multiple comparisons. The normality of the distribution was assessed using the Shapiro-Wilk test. A p-value less than 0.05 was considered statistically significant. Analyses were performed using SAS software, version 9.4 (SAS Institute Inc., Cary, NC, USA).

R ES ULT S
The results of this study are summarized in Table I. A significant improvement in the SSS and FSS BCTQ subscores and tip pinch strength were observed. A reduction in pain and improvement in SNCV were also found, but did not reach statistical significance. At baseline, one patient met the criteria of mild CTS, nine moderate and five severe CTS according to the neurophysiological grading elaborated by Padua et al. [9]. At the 12 week follow-up, the grade of three patients improved, but the difference was not statistically significant. There were no complications or side effects from the PRF procedure. Padua's et al. classification [9] (No. of cases is provided) mild n = 1 n/a n = 2 } 0.0710 moderate n = 9 n/a n = 10 severe n = 5 n/a n = 3 SNCV (m/s) 31.50 (0-40.50)** n/a 35.10 (28.00-42.20)** 0.2095 BCTQ -Boston Carpal Tunnel Questionnaire; SSS -Symptom Severity Scale; FSS -Functional Status Scale; CTS -carpal tunnel syndrome; NRS -Numerical Rating Scale for pain assessment; PRF -pulsed radiofrequency; SNCV -sensory nerve conduction velocity; SD -standard deviation; n/a -not applicable; * mean ± SD; ** median, lower and upper quartiles

DI S CU S SI O N
Pulsed radiofrequency was first applied in the interventional treatment of pain by Sluijter et al. [10] in the 1990s. Over the next two decades, a wide spectrum of clinical applications for PRF emerged, e.g. radicular pain, facet pain, trigeminal neuralgia, occipital neuralgia, shoulder pain, knee pain and other pain syndromes [11,12,13]. It was recently found that bipolar PRF could be more effective in pain treatment compared with unipolar PRF. This is probably due to the larger and denser electrical field between a closely spaced pair of electrodes [14]. A small number of patients was recruited and we did not perform a long-term follow-up. A longer-term follow-up will show whether the benefit of bipolar PRF is maintained over time.

CO NC L U SI O N S
The favourable early results of bipolar PRF of the median nerve in patients with CTS encourage a wider application of this approach by neurosurgeons, orthopaedists, hand surgeons or pain management specialists.
Importantly, as a minimally invasive procedure, PRF can be performed in an outpatient setting. Furthermore, PRF can be offered to patients awaiting surgical release of carpal tunnel. A longer follow-up and recruitment of a larger group of patients is needed to determine the longevity of clinical improvement after PRF.

Funding
The project was funded by the Medical University of Silesia, Katowice, Poland, grant No. KNW-1-205/N/7/K.

None declared
Author's contribution