Rectus sheath pulsed radiofrequency treatment for anterior cutaneous nerve entrapment syndrome: A case report

Introduction

Anterior cutaneous nerve entrapment syndrome (ACNES) is pain caused by compression of the anterior cutaneous branch of the intercostal nerve as it passes from the posterior sheath of the rectus abdominis muscle through the rectus abdominis muscle. In cases of chronic abdominal pain with no abnormalities on blood tests or imaging studies, it is necessary to consider this syndrome in the differential diagnosis. In this report, a case in which pulsed radiofrequency (PRF) treatment was applied to the sheath of the rectus abdominis muscle for ACNES that did not improve with frequent trigger point injections (TPIs) into the rectus abdominis muscle is presented.

Case report

The patient was a 27-year-old man with no significant medical history. Four months earlier, he became aware of pain in the right abdominal wall and visited his previous physician. After abdominal computed tomography and blood tests showed no abnormalities, he was prescribed acetaminophen 600 mg/day, probiotics, and polycarbophil calcium tablets 1500 mg/day, but his symptoms did not improve, and he returned to his previous physician. He was diagnosed with ACNES by his previous physician based on a single localized tender point on the right abdominal wall and a positive Carnett’s test. After receiving TPI (lidocaine, 10 ml) from the previous doctor, the pain improved, but the symptoms recurred 4 months later. Two more TPI were performed every 2 days, but there was no improvement in the pain, and the patient was referred to our pain center.

At the time of initial examination, the patient had tenderness with a numerical rating scale (NRS) score of 7/10 for pain in the region of the right abdominal wall, Th9 right intercostal nerve anterior cutaneous branch. Carnett’s test was positive, and there were no sensory abnormalities. The pain had forced him to take a leave of absence from his printing business. A questionnaire showed mild depressive tendencies with a score of 52 on a self-rated depressive scale. Abdominal X-ray and abdominal wall ultrasound examinations showed no abnormalities. Therefore, we too diagnosed the patient with ACNES.

At the initial visit, the patient was started on tramadol hydrochloride 50 mg/day, but after 2 days, rectus sheath block (RSB) and PRF treatment were performed on the posterior layer of the rectus abdominis muscle at the right Th9 level under ultrasound guidance because of a lack of improvement (NRS score 6/10; Figure 1). For ultrasound, a linear probe (13–6 MHz) of Sonosite Edge II (FUJIFILM Sonosite, Inc., Bothell, WA, USA) was used (Figure 2). For PRF treatment, the rectus abdominis sheath was first stimulated with 0.3 mA before injecting local anesthetic into the RSB. Then, the site where the most reproducible pain was observed was determined to be the site of strangulation of the percutaneous branch of the intercostal nerve, and PRF treatment was performed at a temperature of 42 °C, a voltage of 45 V, and a frequency of 2 Hz for 360 s. After PRF treatment, 20 mL of 0.375% ropivacaine was injected. His medication was changed to tramadol hydrochloride/acetaminophen, two tablets per day, and adjusted accordingly.

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Figure 1.

Pulsed radiofrequency applied to the sheath of the rectus abdominis muscle. Actual images of PRF treatment performed are shown; the RSB is performed in the sagittal plane under real-time ultrasound guidance. The tip of the needle is positioned below the rectus abdominis sheath.

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Figure 2.

Ultrasound image of rectus sheath block. The rectus abdominis muscle is shown on the ultrasound image. Arrows indicate the needle.

After PRF treatment, the pain improved temporarily to NRS score 0/10, but 12 days after PRF treatment, the pain had flared up to NRS score 4/10. Lower gastrointestinal endoscopy and abdominal MRI, which were performed to exclude internal organ disease, showed no abnormal findings. After two more PRF treatment sessions at an outpatient clinic, 4 and 6 weeks after the initial visit, the pain completely resolved, and the patient has been pain-free for 1 year.

Discussion

The anterior cutaneous branch of the T7–T12 spinal nerve, a sensory nerve of the abdominal wall, runs between the internal oblique and transversus abdominis muscles and proceeds to the posterior wall of the rectus abdominis muscle. It reaches the subcutis through the fibrous ring, a neurovascular channel of the rectus abdominis muscle. The mechanism of ACNES is thought to be that the percutaneous branch of the spinal nerve generates pain due to strangulation at the entrance or exit of the annulus fibrosus ¹ or herniation of the fatty tissue surrounding the annulus fibrosus. ² The annual incidence of ACNES averages 22/100,000; it accounts for ~1 of every 50 patients visiting the emergency room due to abdominal pain, ³ and 13% of patients aged 10–18 years with chronic abdominal pain were reported to have ACNES. ⁴ The most common cause of onset is idiopathic (57%), and a history of abdominal surgery (28%), accident or sports trauma, and pregnancy are also considered. ⁵ A positive Carnett’s test at the tender point is useful because it suggests that the pain originates from the abdominal wall. ⁶

Treatment options include acetaminophen, NSAIDs, antiepileptic drugs, antidepressants, and oral opioids, but their efficacy is unknown. Lidocaine cream and capsaicin ointment are also options, but evidence is lacking. ¹ If oral and topical medications do not improve pain, nerve blocks should be performed. TPI or RSB is the first choice. In fact, cases of improvement with a single RSB ⁷ and frequent RSB ⁸ have been reported. If RSB does not result in pain control, PRF treatment or surgical neurectomy may be performed. ⁸

PRF treatment has attracted attention for its safety and efficacy, based on the theory that the use of pulsed, high-voltage, high-frequency currents avoids the temperature increase that destroys the nerves. PRF treatment can be applied to treat peripheral nerves such as dorsal root ganglia and intercostal nerves at all spinal levels in a variety of pain syndromes, including nerve root pain, discogenic pain, arthralgia, postherpetic neuralgia, and intercostal neuralgia. ⁹ In a retrospective study of 26 adult patients with idiopathic ACNES followed up for the effects of PRF treatment, 25% of patients were still experiencing benefit after 1 year, and 8% were completely pain-free. ¹⁰ PRF treatment may be indicated in patients with a short duration of RSB. Surgical neurectomy reduced pain by more than half in 79.8% of cases, but it tends to be less effective in patients with a history of abdominal surgery, tenderness in the paraspinal region, or failure to respond to RSB. ¹¹ Neurectomy is more invasive and is a last resort for patients with persistent pain after PRF treatment.

In a randomized controlled trial of 66 patients diagnosed with ACNES to evaluate the treatment efficacy of PRF treatment versus neurectomy, 38% of the PRF treatment group and 61% of the neurectomy group showed >50% pain relief at 8 weeks after treatment. ¹² Although PRF treatment may be less effective in improving pain than neurectomy, it may be worth an early trial after considering the potential complications associated with neurectomy. In this case, TPI administered by the previous physician was effective for 4 months only after the first treatment, but the analgesic effect was insufficient after the second treatment. Therefore, PRF treatment, which is expected to have a greater analgesic effect, was selected. Although neurectomy was considered, PRF treatment was selected as a minimally invasive alternative to surgical intervention, because the involvement of psychosocial factors and a low pain threshold due to persistent pain could not be ruled out, and the surgical wound could cause additional pain.

Conclusion

Neurectomy is considered for ACNES, that is, difficult to treat even with frequent nerve blocks, but as a preliminary step, PRF treatment would be a less invasive and more effective treatment.