Chronic Abdominal Pain

Anterior Cutaneous Nerve Entrapment and Myofascial Pain Syndromes

An estimated 10% to 30% of patients presenting to a gastroenterologist with a complaint of chronic abdominal pain have chronic abdominal wall pain (CAWP) as a cause of their symptoms. Anterior cutaneous nerve entrapment syndrome (ACNES) and myofascial pain syndrome (MFPS) are common causes of CAWP. These syndromes share clinical, diagnostic, and treatment characteristics; the importance of recognizing them rests in providing the patient with an accurate diagnosis and effective treatment, as well as avoiding further expensive investigation and unnecessary surgical intervention. The abdominal wall should be suspected as the cause of symptoms when there is a complaint of chronic and unremitting abdominal pain that is unrelated to eating or bowel function but clearly related to movement. Patients with CAWP are often overweight, and the disorder is more common in women than men.

Although ACNES was initially described in the 1970s, it remains a frequently underrecognized, overlooked, and misdiagnosed cause of chronic abdominal pain. In ACNES, the pain is believed to occur when there is entrapment of a cutaneous branch of a sensory nerve that is derived from a neurovascular bundle emanating from spinal levels T7 to T12. The nerve entrapment may be related to pressure from an intra- or extra-abdominal lesion or to another localized process such as fat, fibrosis, or edema. Pain emanating from the abdominal wall is discrete and localized, in contrast to pain originating from an intra-abdominal source, which is diffuse and poorly localized. Anatomic considerations and mechanisms of nerve entrapment have been detailed elsewhere. Patients usually point to the location of their pain with one finger, and the examiner can often localize the area of maximal tenderness to a region less than 2 cm in diameter. During physical examination, the patient often guards the affected area from the examiner’s hands ( hover sign ). Patients often note that activities associated with tightening of the abdominal musculature result in an exacerbation of pain, and during physical examination, the clinician will note increased localized tenderness to palpation when the patient tenses the abdominal muscles ( Carnett sign ).

In MFPS, pain emanates from myofascial trigger points in skeletal muscle. Causative factors include musculoskeletal trauma, vertebral column disease, intervertebral disk disease, osteoarthritis, overuse, psychological distress, and relative immobility. The exact pathophysiology of pain in MFPS remains unclear. CAWP may occur in patients with MFPS. Pain may be referred from another site, and identification of trigger points is a useful physical finding. When attempting to identify a trigger point, the examiner uses a single finger to palpate a tender area. This is most often located in the central portion of a muscle belly, which may feel indurated or taut to palpation, and elicits a jump sign . This finding refers to a patient’s response by wincing, jerking away, or crying out as the myofascial trigger point is detected. Less commonly, trigger points may be located at sites like the xiphoid process, costochondral junctions, or ligamentous and tendinous insertions.

Successful treatment of ACNES and MFPS not only improves symptoms, but also confirms the diagnosis. Treatment strategy depends on the symptom severity. With mild and intermittent symptoms that are reproducibly precipitated by certain movements, simple reassurance and a recommendation to avoid such movements may suffice. Non-narcotic analgesics, NSAIDs, and heat applications can be used during exacerbations. Physical therapy may be beneficial, although no randomized studies have supported this approach. For severe and persistent symptoms, injection therapy with a local anesthetic, with or without a glucocorticoid, is recommended. Various techniques of injection have been described. A retrospective survey-based study with follow-up of 6 to 12 months revealed that ultrasound-guided trigger-point injection with an anesthetic plus a glucocorticoid resulted in significant symptom improvement in over one third of patients. Not unexpectedly, somatization was inversely related to treatment success. In a study of 136 patients in whom the history and physical examination suggested abdominal wall pain, and in whom benefit was noted with injection therapy, the diagnosis remained unchanged in 97% of cases after a mean follow-up of 4 years. In carefully selected patients with symptoms refractory to injection therapy, a prospective nonrandomized investigation suggested that diagnostic laparoscopy with open exploration of abdominal trigger points may be beneficial. In this study, after intra-abdominal adhesions in close proximity to a trigger point were lysed, subcutaneous nerve resection was performed. After a median postoperative follow-up of 37 months, an impressive 23 of 24 patients (96%) believed that this approach was beneficial in managing their previously intractable pain. A retrospective observational study and a double-blind, randomized, controlled trial from the same investigators also showed long-term benefit from anterior neurectomy in patients with symptoms refractory to more conservative therapy.