Introduction
Between 10% and 30% of patients presenting to gastroenterologists with chronic abdominal pain have chronic abdominal wall pain (CAWP). These patients often undergo unnecessary procedures and tests while their diagnosis is delayed by 2 years on average, adding more than $6700 in direct healthcare costs. An appropriate diagnosis and correct treatment of abdominal wall pain can often be curative. The abdominal wall is supplied by the T7–T12 nerve roots, and the nerves can be affected at the level of the nerve roots or at the distal branches where they make a right-angle turn at the lateral edge of the rectus abdominis muscle. The distal branches are particularly prone to causing pain, either from surgical changes or scar tissue. Applegate coined the term abdominal anterior cutaneous nerve entrapment syndrome (ACNES) for this pathology. It is usually felt as a sharp, localized pain, worse with activity, and is independent of eating or bowel habits. , Diagnosis is notoriously difficult and relies almost exclusively on history and physical findings. CAWP can be seen at any age, but predominantly affects women. Carnett’s test is often the most consistent physical exam finding to aid diagnosis. Interventions for CAWP include transversus abdominis plane (TAP) block, ACNES block, neuromodulation, and surgical neurectomy, which are preferable to long-term medical management. ,
Etiology and Pathogenesis
The abdominal wall consists of several muscle layers. The paired rectus muscles are found in the midline. Laterally, from superficial to deep, lie the external abdominal oblique, the internal abdominal oblique, and the transversus abdominis muscles. The abdominal wall is innervated by the anterior branches of the intercostal nerves of T7–T12. These nerves travel in a plane between the internal oblique and the transversus abdominis, where they are a target for the TAP block. They then pierce the posterior rectus sheath and make a 90-degrees turn through a fibrous sheath to become the anterior cutaneous nerves, and subsequently make another 90 degrees° turn beneath the skin.
The differential diagnosis of CAWP is extensive and includes peripheral nerve entrapment or traumatic injury, referred pain from the abdominal or thoracic viscera, T7–T12 radicular lesions, herpes zoster, painful rib, myofascial trigger points, and rectus sheath hematoma. The most commonly discussed cause of CAWP is anterior cutaneous nerve entrapment syndrome (ACNES), which is caused by the entrapment of a cutaneous branch of the lower intercostal nerve (T7–T12). Many patients previously diagnosed with functional abdominal pain syndromes have a component of abdominal wall pain.
Peripheral nerve entrapment of the anterior cutaneous branches of the lower intercostal nerves can be caused by surgical trauma or anatomical variations of the anterior intercostal neurovascular bundle and surrounding structures in the abdominal wall. This can occur when a surgical incision directly damages the cutaneous nerve, or when scar tissue formation (or suture) impinges on a cutaneous nerve causing iatrogenic ACNES. In patients who have not had abdominal surgery, the nerve is most commonly entrapped at the lateral border of the rectus muscle. Here, the neurovascular bundle travels through a fibrous ring in the rectus sheath that can compress the structures and produce the symptoms of ACNES.
Clinical Presentation
Pain is the dominant finding in CAWP, and the specific presentation is variable. Patients typically report 1–3 months of well-localized pain. The pain may range from mild to debilitating, and the pain itself may be aching, burning, or dull. Patients will often be able to point to the exact area of pain (as opposed to patients with visceral abdominal pain, which is poorly localized), and may report a history of pain improving with manual application of pressure to the painful area. The pain can also radiate posteriorly over a thoracic dermatome. Increased abdominal pressure or conditions such as obesity exacerbate the pain. A history of prior abdominal surgery may also point to a diagnosis of CAWP, as cutaneous nerve entrapment may occur due to the formation of scar tissue or suture placement. ,
Diagnosis
In one survey, only 26% of internists were able to diagnose and choose the appropriate diagnostic step when given a classic vignette of chronic abdominal wall pain. Patients who present to their primary care provider or gastroenterologist for abdominal pain may have diagnostic workup completed to rule out any structural abnormalities or malignancies. Labs can include comprehensive metabolic panel, complete blood count, and liver enzymes and diagnostic imaging may include abdominal X-ray and computed tomography of abdomen and pelvis.
The diagnosis of CAWP is based on history and physical exam. The first step in diagnosis in CAWP is differentiating it from visceral abdominal pain. Although this differentiation can be difficult, in general, visceral abdominal pain is diffuse and poorly localized, while abdominal wall pain is well localized by the patient. If there is confusion, diagnostic injections may clarify the diagnosis. , This includes infiltration of a local anesthetic directly into the point of tenderness, as well as abdominal wall blocks such as TAP blocks, rectus sheath blocks, erector spinae plane blocks, and paravertebral nerve blocks. Improvement of alleviation of the pain with a diagnostic block points to the abdominal wall as the cause of the pain.
A differential epidural block has also been described to help differentiate between visceral, central, and abdominal wall pain. This involves the placement of a thoracic epidural that is initially injected with saline (as a placebo) and then incremental doses of local anesthetics. The physiologic basis for this exam relies on the theory that sympathetic and visceral afferent nerves are more sensitive to local anesthetic than large cutaneous sensory fibers. Therefore, the time point at which the patient’s pain is relieved by the local anesthetic (or returns after the local anesthetic boluses are discontinued) can help elucidate the source of the pain. Furthermore, if the patient’s abdominal pain is not relieved with any dose of local anesthetic, this may indicate a centrally mediated pain syndrome. However, the interpretation of this block is very subjective, and the interaction between local anesthetic and nerve fibers is unpredictable and nonstandard between patients. Therefore, this test is not recommended as an initial step in the diagnostic workup.
Physical Exam Findings
Several physical exam findings are pathognomonic for abdominal wall pain. Carnett’s test is the most well-described physical exam finding. , To perform this test, the patient is placed in the supine position with the knees flexed to relax the abdominal wall. In the relaxed position, the painful area is initially palpated. Then, the patient is asked to tighten the musculature of the abdominal wall by lifting the head and shoulders off the bed. The test is positive if there is increased tenderness with increased abdominal wall tension (indicating that the pain arises from the abdominal wall). If the pain is intraabdominal in origin, it is more likely that pain will decrease with increased abdominal wall tension, as the tensed abdominal muscles guard the abdominal viscera.
Additional physical exam findings include a fixed location of tenderness, superficial tenderness, or point tenderness over the abdominal wall with less than 2.5 cm of diameter. An allodynic response may occur when pinching the skin overlying the painful area, which is known as the “pinch test.”
Treatment
There are no evidence-based guidelines regarding the treatment of CAWP, and there are few high-quality studies. Therefore, most of the treatments described are based on expert opinion or chronic pain interventions studied for other etiologies of chronic pain. The treatment of CAWP can include pharmacologic interventions, injections, advanced procedures, surgical interventions, as well as physical therapy.
Pharmacologic
Systemic pharmacotherapy is usually first-line for most pain syndromes, but in ACNES this is not the case. NSAIDs, acetaminophen, weak opioids, anticonvulsants, and antidepressants provide partial benefit. Therefore, it would be reasonable to try conservative therapy with standard neuropathic medications. This would include anticonvulsant (i.e., gabapentin, pregabalin) and antidepressant (serotonin and norepinephrine reuptake inhibitors, tricyclic antidepressants) medications.
Injections
Injection of local anesthetic with steroids directly into the localized area of pain may provide significant long-term relief for CAWP. One study demonstrated that 91% of patients suspected to have CAWP experienced greater than 50% pain relief after a local anesthetic-steroid injection. More recent reports of ultrasound-guided cutaneous nerve injections describe the technique of the procedure, lasting up to 10 months after two injections. The target of an ultrasound-guided cutaneous nerve injection is between the rectus abdominis and the linea semilunaris, approximately 1 cm medial to the linea semilunaris. If the area of pain is more difficult to localize, abdominal wall blocks such as transversus abdominis plane blocks (TAP), rectus sheath blocks, and paravertebral nerve blocks may also have diagnostic as well as a therapeutic benefit in the treatment of CAWP.
A TAP block anesthetizes the nerves more proximally, between the internal oblique and transversus abdominal muscles. It reliably provides analgesia below the umbilicus, though it may not adequately cover above the umbilicus. It can be performed laterally, posteriorly, or subcostally (see Figs. 16.1 and 16.2 ). This block may be used for diagnostic or therapeutic purposes in the setting of CAWP if more focal injections are not helpful.
