Anterior cutaneous nerve entrapment syndrome (ACNES) is a condition characterized by the entrapment of one of the cutaneous branches of the intercostal nerves in the abdominal fascia [1]. This entrapment leads to ischaemia of this nerve, which results in severe pain symptoms [2]. It is a commonly overlooked cause of abdominal pain, which means the cause of abdominal pain is often wrongly attributed to intra-abdominal disorders [3,4], resulting in unnecessary, costly consultation and potentially harmful testing and even surgery [5,6]. There is often a considerable delay before an ACNES diagnosis is made [7-10]. ACNES can be established in patients following a thorough patient history and physical examination [11].

The classic presentation of an ACNES patient is a young female who complains of severe, constant pain localized to an area of approximately one fingertip [12]. A positive Carnett’s and Pinch test increases the likelihood of the ACNES diagnosis [13,14]. A good response to trigger point infiltration using local lidocaine injection has both a diagnostic and therapeutic purpose [15] and can be performed freehand or ultrasound guided [16]. If this therapy provides inadequate pain relief, an ultrasound guided transverse abdominis block [17,18], pulsed radio frequency [19,20] or a surgical neurectomy should be discussed with the patient [21,22], offering an effective but potentially harmful treatment.

A new treatment modality, offering higher effectiveness than trigger point infiltration but with fewer complications than surgical neurectomy, is highly desirable. In this retrospective study we investigate the analgesic effect of a peripheral nerve block catheter in patients suffering from ACNES. The rationale behind this therapy is that it effectively serves as a continuous rectus sheath block. In this novel therapy, a nerve block catheter is placed percutaneously in the region corresponding to the expected course of the nerve thought to be responsible for the pain.

1. Study design

This single centre retrospective study selected patients treated in the period February 1 2022 to May 31 2022 at Laurentius Hospital in Roermond, the Netherlands. Inclusion criteria consisted of being > 18 years old and having a confirmed diagnosis of ACNES. All patients had consistent clinical signs of ACNES and had all previously been treated at least twice using ultrasound guided local infiltration with local anaesthetics (20 cc Bupivacaïne 5 mg/ml), resulting in short-term pain relief of less than one week. Exclusion criteria consisted of having an alternative or concomitant abdominal nerve pathology, allergy to local anaesthetics and patients who were not deemed suitable for an invasive procedure under local anaesthesia. The pain severity at each timepoint was documented using the Visual Analogue Scale (VAS) pain score from 0 (no pain) to 10 (most excruciating imaginable pain). We defined patients as being pain free if they gave a pain score of less than three. The requirement for informed consent was waived by the Institutional Review Board.

2. Intervention

Under ultrasound guidance and local anaesthesia using 20 cc Xylocaïne, a TuohySono 19 G cannula (Pajunk Holding GmbH, Geisingen, Germany) was placed percutaneously in the region corresponding to the expected course of the intercostal nerve thought to be responsible for the pathology. The tip of the cannula was placed between the posterior border of the rectus muscle and the posterior rectus sheath after hydrodissection of this plane using 5 cc Xylocaïne. Through this hollow needle, a peripheral nerve block catheter (Pajunk Infiltralong catheter with 15 holes over the last 4 cm) was advanced until it was located between the posterior rectus sheath and the abdominal rectus muscle. An image of the cannula and catheter set can be seen in Fig. 1, an image of the ultrasound guided procedure can be seen in Fig. 2. An initial bolus of 20 cc Ropivacaïne 7.5 mg/ml (Naropin, Aspen Pharma Trading Limited, Dublin) was injected through the catheter. Then, an elastomeric pump (FuserPump, Pajunk Holding GmbH, Geisingen, Germany) was filled with 350 ml of a Ropivacaïne 2 mg/ml solution and continuous infiltration was initiated at a 5 ml/hour rate. After three hours, evaluation of the current pain was performed. If the pain score was higher than three on a Numeric Rating Scale from 0 to 10, the anaesthetic effect of the peripheral nerve block catheter was considered insufficient. In that case, the flow rate was increased to 8 ml/hour leading to adequate anaesthetic effect in all patients. Patients were discharged the same day. All had a pain score that was considered acceptable. When the elastomeric pump was empty (normally after three days), patients removed the catheter themselves and discarded the catheter and pump. Evaluation of effectiveness was performed after 14 days and after six months. The primary outcome of this study was the level of pain reduction seen three hours after the introduction of the nerve block catheter. The secondary outcome was the level of pain reduction at 14 days and at six months.

Figure 1. Intraoperative picture of ultrasound guided percutaneous placement of the pain catheter. The needle can well be visualised, positioning the catheter between the dorsal rectus sheath and the dorsal border of the rectus muscle at the area where the affected and entrapped intercostal nerve is expected.

Figure 2. Ultrasound image of the catheter placement. The hollow needle is placed a few cm cranio-lateral from the intended point of maximum pain. Under ultrasound guidence, the needle advances till it is placed between the rectus muscle and the posterior rectus sheath. There, local anesthetic will be infiltrated, resulting in hydrodissection between muscle and posterior rectus sheath. The catheter can than easily be placed in the optimal position.

In the specified four month period, a total of 10 patients underwent this therapy. The mean age of the patients was 30 years and nine out of the ten patients were female. The median pre-operative pain score (measured on a Numerical Rating Score or NRS) was 7. The median post-operative score was 0, the median score on day 14 was 2.

The primary outcome parameter was the pain three hours after insertion of the catheter. All patients demonstrated a significant reduction in pain post-procedure. Nine patients reported being (almost) pain free, having a pain score of less than three.

The secondary outcome was the pain score 14 days after the procedure. Six patients reported being still free of significant pain, negating the need for surgical neurectomy. The other four patients derived insufficient benefit from the therapy and thus underwent a neurectomy shortly after the nerve block catheter therapy.

After six months, we performed a telephone interview. Of the six patients that had not been operated upon, five were still totally free of pain. The sixth patient experienced mild discomfort VAS 3, considered too mild to use pain medication or warrant surgery.

Results are shown in Table 1.

Table 1 . Patient characteristics.

Patient	Age	Pre-op*	Post-op*	14d*	Result	6 months*
1	77	6	1	5	Neurectomy required	0
2	64	6	0	0	No further treatment	0
3	44	7	2	5	Neurectomy required	0
4	21	4	0	0	No further treatment	0
5	20	5	0	5	Neurectomy required	0
6	19	8	0	8	Neurectomy required	7
7	27	7	0	0	No further treatment	0
8	32	7	0	0	No further treatment	3
9	32	6.5	2.5	2	No further treatment	0
10	19	8	5	1	No further treatment	0
Median	30	7	0	2

*Pain score, measured as a numeric rating scale ranging from 0 (no pain) to 10 (the most excruciating pain imaginable)..

The goal of our study was to assess the short-term analgesic effect of a peripheral nerve block catheter in patients suffering from ACNES. These data demonstrate that nine out of ten patients who underwent this novel therapy were pain free immediately post-procedure, and that six out of ten patients remained sufficiently pain free after 14 days. We find these results to be quite satisfactory, given the fact that all these patients had been convincingly diagnosed with ACNES. They had all been treated using regular pain medication and at least twice with infiltration therapy with local anaesthetics.

This therapy seems preferable to surgery for a number of reasons. The therapy is less invasive and thus the post-operative pain is less and the recovery time is far shorter. Aesthetically unpleasant scars can be avoided in these (often young) patients. The risk of rare but disturbing complications such as seroma and wound infection is absent. Catheter placement was well tolerated in all patients under local anaesthesia, where the majority of surgical neurectomies is performed under general anaesthesia. Therapy can therefore take place in an outpatient setting.

Unfortunately, it is not clear why some patients responded well to therapy while others did not. All patients were assessed by a surgeon with extensive experience in ACNES treatment in a referral hospital for ACNES surgery. All patients had a good but short-term result on infiltration with local anaesthetics at least twice, suggesting the diagnosis of ACNES was made correctly. Despite this strict patient selection, ACNES remains a clinical diagnosis and it is impossible to prove that no other underlying disorder might have caused the symptoms in some of these patients.

Long-term results are not yet known for this novel therapy. More research and preferably a well-designed and adequately powered Randomized Controlled Trial are needed to enable us to assess mid-term and long-term efficacy, enabling better selection of patients for their most effective and least invasive treatment.

No potential conflict of interest relevant to this article was reported.