Chronic abdominal wall pain should be suspected when chronic abdominal pain is narrowly confined to a small area. The most common cause is an entrapped anterior branch of one of the thoracic nerves but it may also result from surgical scars and hernias. Diagnosis is made by patient history, physical examination, especially Carnett test, and positive reaction to local anesthetic/steroid injection. Intra-abdominal disease is infrequently missed if patients are closely followed after an initial diagnosis is made, but chronic abdominal wall pain may co-exist with intra-abdominal disease processes. The diagnosis of chronic abdominal wall pain is uncommonly made by physicians, despite its relative frequency in general practice. The condition should be considered as one of the possibilities in a patient with chronic abdominal pain.
In 1926 Carnett, a surgeon, published the seminal article recognizing the syndrome that he believed was caused by intercostal neuralgia. Carnett’s test is a simple clinical test in which abdominal tenderness is evaluated while the patient tenses the abdominal muscles. It is useful for differentiating abdominal wall pain from intra-abdominal pain. When pain arises from an intra abdominal source, the tensed muscles in the abdominal wall guard the underlying bowel, thus reducing the discomfort (negative test). However, when the pain arises from the abdominal wall, the muscle contraction will accentuate the pain (positive test).
The most important cause of chronic abdominal wall pain is entrapment of a anterior cutaneous branch of one of the lower (T7-T12) intercostal nerves in its tortuous course through the abdominal wall muscle. After turning at a 90 degree angle, the nerve passes from the posterior sheath of the rectus abdominis muscle through a fibrous opening and then branches at right angles while passing through its anterior sheath. It has been thought that the underlying problem is nerve compression with resulting ischemia or lack of blood supply, explained by the nerve’s course through the muscle. Applegate termed the condition as “anterior cutaneous nerve entrapment syndrome” and suggested the entrapped nerve may also be pushed by intra- or extra-abdominal pressure or pulled by a scar causing pain in the abdominal wall.
Pain in the abdominal wall may arise from three or more sources, including:
Entrapment of the anterior cutaneous nerve appears to be the most common cause of chronic abdominal pain. Since 1972 Applegate and several other authors have hypothesized that the underlying lesion is nerve compression with resulting ischemia, explained by the nerve’ s course in relationship to the musculature. Other lesions affecting the nerve, including herpes zoster, tumors, or traumatic radiculitis, might cause similar findings by different mechanisms. Myofascial trigger points seem to be a less frequent explanation than nerve entrapment. Occasionally rectus sheath hematomas or small Spigelian (lineasemilunaris) or incisional hernias may be implicated. Epigastric hernias and“painful rib” (“ rib tip” or “ slipped rib” ) sometimes account for upper quadrant pain.
Chronic abdominal pain may initially be sharp, followed by a dull persistent ache. It may range from mild to excruciating, continuous to intermittent, with complete remissions lasting for months or years. Radiation may be widespread, sometimes extending posteriorly over the dermatome.
An important diagnostic clue is that pain may be so sharply localized that the patient indicates that a fingertip can entirely cover the area. This almost always indicates that the pain originates in the abdominal wall; visceral pain cannot be so precisely localized because of the wide spinal cord overlap of the viscerosensory representation. However, when severe, the pain may radiate diffusely. It may be brought on by conditions that increase abdominal pressure or cause nerve traction, such as tight clothing, obesity, or, as previously noted, postoperative scarring. Temporary relief may be obtained by sitting, lying, or, relatively frequently, by hand-splinting the affected area. Patients may identify specific circumstances that precipitate or aggravate pain, such as standing, lifting, walking, stretching, laughing, coughing, or sneezing. Nausea, bloating, overeating, menstruation, and urinary bladder distension have also been implicated, possibly by accentuating nerve entrapment because of associated vascular congestion. Oral contraceptives and pregnancy have also been reported to accentuate abdominal wall pain as well as other entrapment syndromes, perhaps from tissue edema due to estrogen and progesterone.
Characteristically a localized site of maximum tenderness is identified with the patient in the supine position. This may be at or close to a surgical scar, or in its absence, tenderness is most often at the lateral edge of the rectus abdominis, the linea semilunaris. However, the tenderness may be anywhere. The area of most severe tenderness is often no more than 2 cm in diameter, although lesser degrees of discomfort may be more dispersed. It is superficial but may seem to be deep in obese individuals. Hershfield described the “ hover sign” in which the patient aggressively guards the area from light touch, sometimes by seizing the examiner’ s hand. Allodynia (pain produced by a stimulus, typically mechanical, that normally is not painful) may be manifested on light stroking or pinching the overlying skin. Lesser degrees of tenderness and allodynia are sometimes found over the distribution of the corresponding dermatome and, occasionally with radiculopathy, over the transverse process and vertebral body.
Physician should have a high suspicion index of abdominal wall pain in all patients presenting with chronic abdominal pain. The diagnosis should be suspected from history and physical findings. Gallegos and Hobsley devised an algorithm for evaluation of abdominal wall pain depending on the results of the Carnett test. If the test is positive and at or near a surgical scar and there is no hernia, local anesthetic injection is carried out. The wall is implicated implicated if pain is abolished. When the pain is unrelated to a scar and is increased by spinal movement, radicular pain is suspected. On the other hand, if pain is not accentuated by back motion and tenderness is localized to the abdomen, the wall is suspected. Depending on the specific location and exclusion of hernia, nerve entrapment or painful rib syndrome is diagnosed. The Carnett test alone has 78% sensitivity and 88% specificity.
Substantial pain relief after an accurately placed anterior cutaneous nerve block or trigger point anesthetic injection is considered confirmatory. Sharpstone and Colin Jones concluded that a successful injection of local anesthetic after a positive Carnett sign “ must be one of the most cost effective procedures in gastroenterology.” When it is properly carried out, moderate to excellent pain alleviation occurs in 60% to over 90% of patients.
Although it is generally assumed that response to a somatic nerve block or trigger point anesthetic injection differentiates somatic from visceral pain, it is acknowledged that this may not always be true and a placebo effect probably sometimes occurs. However, responses to placebos usually result less frequently (averaging about 30%) than the 70% or greater response rate after anesthetic injection.
The accurate diagnosis of prolonged pain of uncertain etiology, which sometimes carries invidious implications of “psychosomatic” origin, may in itself be a positive intervention. If the abdominal wall pain does not interfere with quality of life, avoidance, reduction, or modification of identified precipitating activities may be sufficient. An abdominal binder is often useful if gentle hand pressure alleviates the pain. During acute pain exacerbations local heat or cold may provide transient relief. Although nonspecific treatments, such as nonsteroidal anti-inflammatory drugs, analgesics, and antidepressants are commonly used, we have not been impressed by their effectiveness in chronic abdominal wall pain.
Local nerve blocks or myofascial trigger point anesthetic/ corticosteroid injections may be needed for pain that intrudes on the patient’ s quality of life. In a large majority of these patients the pain alleviation by the injections frequently lasts for prolonged periods, sometimes permanently. Anesthetics are thought to “ break” a chronic pain cycle, but the reasons for long term relief are unclear. Frequently corticosteroids are used in conjunction with the local anesthetic because they seem to enhance the anesthetic effect, perhaps by “ membrane stabilization.” They also have been found to reduce ectopic discharges from experimental neuromas.
Ultrasound can reliably be used for infiltration of the abdominal cutaneous nerves. This will improve the safety as well as diagnostic utility of the procedure.
At Pain Spa Dr Krishna is very experienced in interventional treatments. Dr Krishna always performs abdominal cutaneous nerve blocks under ultrasound guidance for greater accuracy and improved safety. Ultrasound gives the added advantage of visualizing the surrounding structures, which can be contributory to pain.
Pulsed radiofrequency treatment can be applied to the abdominal cutaneous nerves for patients who get a positive response to local anaesthetic blocks. Pulsed radiofrequency can provide sustained pain relief in patients with chronic abdominal wall pain or abdominal cutaneous nerve entrapment syndrome (ACNES).