Chicken and egg: peripheral nerve entrapment or myofascial trigger point?

Abstract

We appreciate your interest in our work and the comments on our article Successful treatment of abdominal cutaneous entrapment syndrome (ACES) using ultrasound guided in the KJP. Pain physicians always face the challenge of discriminating myofascial pain syndrome (MPS) and nerve entrapment as the primary cause of pain. However, as you pointed out, controversies regarding the sequence of onset of MPS and nerve entrapment are still unresolved - Chicken and Egg. The diagnosis of ACES is based on the finding of a constant site of tenderness that is superficially located, with a small area of maximal tenderness that can be localized with a fingertip (trigger point). A small area with somatosensory alterations surrounding the trigger point is often found in ACES, possibly enabling discrimination between a myofascial and a radicular cause of pain [1]. According to previous reports, MPS and radiculopathy are less frequent causes of abdominal wall pain [2,3]. Owing to the special anatomical features of the abdominal wall itself, there is a greater likelihood of pain due to nerve entrapment than due to MPS. ACES is thought to be an ischemic neuropathy caused by entrapment of lateral or medial anterior cutaneous branch of intercostal nerves 7-12. The cutaneous nerve branches through the rectus abdominisis so variable and abrupt that ischemia can easily develop under circumstances such as increased abdominal pressure, which can cause entrapment of the nerves in the muscular foramen [4]. Furthermore, even though MPS is associated with the abdominal wall, it is very difficult to find the taut band that is essential to diagnose MPS. Locating it mainly depends on physician's ability and experience in clinical practice. However, even experts may find it difficult, if the taut band is small and exists deep within the abdominal muscle [5]. ACES diagnosis is confirmed by local subfascial anesthetic injection of an anesthetic agent. This injection has both diagnostic and therapeutic value in these patients. It has been hypothesized that the injected volume leading to release of an entrapped nerve [6]. This technique could be called rather than nerve block or trigger point injection (TPI). A previous finding on attenuation of pain levels in patients following saline injection may support this suggested mechanism [7]. Under ultrasound guidance, we injected 10 ml of a local anesthetic agent, and confirmed the hydro-dissection of suspected nerve entrapment area via the image. In general, we are in agreement with your comments regarding the Chicken and Egg question; however, for management of chronic abdominal pain, we suggest that pain physicians administer a new kind of concept of injection, so called under ultrasound guidance. We hope that the techniques used in our study aid pain physicians in gaining more knowledge of ACES: an intriguing syndrome that is very common, but largely unrecognized.