Anterior cutaneous nerve entrapment syndrome (ACNES) is increasingly being considered to be a common cause of abdominal pain. It is estimated that 2% of patients who present to the emergency department with abdominal pain suffer from ACNES [1]. Unfortunately, ACNES is frequently overlooked and misdiagnosed resulting in unnecessary investigations, futile diagnostic procedures, repeat admissions, and significant morbidity.

The entrapment of the anterior cutaneous branch of the thoracoabdominal (T7-11) and subcostal (T12) nerves results in the development of ACNES [2],[3]. By piercing the posterior rectus sheath at 90° through a fibrous ring at the lateral border of rectus abdominus the nerve becomes susceptible to compression [2]. Entrapment of the nerve may result from increases in intra- and extra-abdominal pressure, ischemia, herniation, or scaring [4].

A thorough history and examination is essential to diagnose ACNES, in doing so excluding organic causes of abdominal pain. Anterior cutaneous nerve entrapment syndrome is a clinical diagnosis requiring the presence of well-localized abdominal pain, increased tenderness to palpation during muscle tensing on physical examination (Carnett’s sign), somatosensory disturbances of the surrounding skin, and response to a trigger point injection with local anesthetic [4].

The goal of treatment is to alleviate patient’s symptoms. Current recommendations include modification of physical activity, simple analgesia, trigger point injections with local anesthetic alone or in combination with a glucocorticoid, chemical neurolysis, pulsed radiofrequency, and anterior neurectomy [5],[6]. Despite multiple treatment strategies and modalities, it is evident that a majority of patients will only experience temporary pain relief [4]. We report two consecutive cases in which an ultrasound-guided transverse abdominal plane (TAP) block has provided long-term relief for patients suffering from ACNES.

Case 1: A 45-year-old female presented to the emergency department with left upper quadrant pain for one year. The pain was described as well-localized to the lateral border of rectus abdominus. She experienced no associated symptoms. On examination a positive Carnett’s sign was elicited. Her laboratory results were unremarkable. Extensive imaging including an abdominal ultrasound, computed tomography (CT), hepatobiliary iminodiacetic acid (HIDA) scan as well as a colonoscopy and gastroscopy excluded organic causes of abdominal pain. Subsequently a trigger point injection using 10 mL of lidocaine 1% with adrenaline was performed. The patient demonstrated partial resolution of pain within 15 minutes of the procedure confirming the diagnosis of ACNES. A decision was made to proceed with an ultrasound-guided TAP block lateral to the rectus sheath for treatment of her pain. The procedure was performed under sedation. A 21-gauge 110 mm needle was used to inject 20 mL of 0.5% Marcaine with adrenaline and 4 mg of Dexamethasone into the neurovascular plane (Figure 1). She required a repeat TAP at six weeks and has been symptom free for six months.

Case 2: A 52-year-old male presented to clinic with a 3-month history of well-localized left upper quadrant pain located at the lateral border of the rectus muscle. The patient had no associated symptoms. Carnett’s sign was demonstrated on physical examination. The diagnosis of ACNES was confirmed with the transient resolution of symptoms 24 hours post-trigger point injection. An ultrasound-guided TAP block was performed in an attempt to relieve his symptoms. Intravenous sedation was used prior to injecting 20 mL of 0.25% Marcaine and 4 mg of Dexamethasone into the neurovascular plane using a 21-gauge 110 mm needle. Complete resolution of the patient’s symptoms was observed at six months follow-up.

By injecting local anesthetic in a plane between internal oblique and transverse abdominus one is able to block the neuronal afferent fibers of thoracoabdominal (T7-11) subcostal (T12) nerves (Figure 2) [7],[8]. Transverse abdominal plane blocks have become technically easier and safer given the ubiquity of ultrasound and skilled practitioners who are qualified to perform the procedure. It provides a safe and viable option for the management of abdominal wall pain. Despite its increasing use for acute pain management in abdominal and pelvic procedures minimal research has been performed assessing its efficacy in the treatment of somatic pain syndromes such as ACNES.

To date there are only two case reports demonstrating the efficacy of TAP blocks for the treatment of ACNES [9],[10]. Imajo et al. highlighted the case of a 56-year-old male diagnosed with ACNES following a cholecystectomy who was initially managed with an ultrasound-guided rectus sheath block to partial effect [9]. To further alleviate his pain an ultrasound-guided bilateral TAP block with 20 mL of 0.5% mepivacaine was used successfully [9]. Sahoo et al. were also able to demonstrate the successful implementation of an ultrasound-guided TAP block for the treatment of ACNES in two patients [10].

Current treatment strategies for the management of ACNES have demonstrated poor long-term outcomes. Trigger point injections have become the gold standard for the diagnosis and treatment of ACNES. Local anesthetic (1–3 mL of lidocaine) alone or combined with a glucocorticoid (40 mg of triamcinolone) is injected into the subcutaneous tissue of the affected site. Immediate pain relief is observed in up to 91% of cases; however, commonly reoccurs in 2–3 hours [4]. Chemical neurolysis is recommended in patients who have failed treatment following a trigger point injection. A cohort of 44 patients who underwent chemical neurolysis demonstrated complete, partial, and no response in 54%, 40%, and 6% of patients [11]. Patients with partial and no response may be suitable for an anterior neurectomy. A 50% reduction in pain intensity was observed ion 61% of patients with a mean follow-up period of 32 months [12]. Limited data are available regarding the efficacy of pulsed radiofrequency, modification of physical activity, and the use of nonsteroidal anti-inflammatory medications.

Despite the multitude of treatment modalities, it is evident that none provide an effective long-term resolution to the management of ACNES. Hence, one must reconsider the treatment algorithm for the management of ACNES. We have demonstrated ultrasound-guided TAP block as a potential cost effective, efficacious, and safe procedure in the hands of a skilled physician for the treatment of ACNES. Therefore, we suggest that this case series prompts further research for the use of TAP blocks for the management of ACNES.

Despite its prevalence ACNES is quite often overlooked as a diagnosis for abdominal pain. An index of suspicion is imperative to avoid unnecessary investigations and procedures. Once diagnosed, there are a myriad of treatment strategies that can be used to treat ACNES. We provide evidence supporting the use of an ultrasound-guided TAP block for long-term symptomatic relief in patients with ACNES. However further research is required to assess its efficacy.