Anterior Cutaneous Nerve Entrapment Syndrome (ACNES)

What is ACNES?

Abdominal cutaneous nerve entrapment syndrome (ACNES) is one of the now recognised causes of chronic abdominal pain. It remains an overwhelmingly underdiagnosed condition and consequently not readily managed or recognised. It is generally characterised by patients presenting with a severe, often refractory, chronic abdominal pain just lateral (next to) the midline. Often the pain can be pinpointed to a specific location on the abdomen. It is theorised that the cutaneous branches of the lower thoracoabdominal intercostal nerves are ‘trapped’ at the lateral border of the rectus abdominis muscle. The pain experienced will be in the distribution of one or more dermatomes as shown in the figure below.

What are the symptoms? 

Pain is experienced just next to the middle of the abdomen either on the right or the left side from the lower border of the ribs down to just above the groin. The pain can radiate around to the back, up and down the abdominal wall and is not related to any gastrointestinal symptoms such as diarrhoea, constipation or other altered bowel habit with or without weight loss. The prevalence of the syndrome ranges between 15% and 30% depending on the definition and the diagnostic criteria that is used. In adolescents, it is reported to be diagnosed in one out of eight cases of chronic abdominal pain. In the emergency department, the prevalence of ACNES in the patients presenting with acute abdominal pain has been reported to be a mere 2% of cases as usually there is the presence of a serious acute pathology in such patients.

Clinically when examining a patient, pain at the point of tenderness can be palpated when the abdomen is soft and if the patient is asked to ‘sit up’ and hold this position i.e tensing the abdominal muscles; if this makes the pain worse then this is a positive Carnett’s test and more in keeping with ACNES, if not then the test is negative.
It is important to exclude an underlying abdominal or gastrointestinal condition before the diagnosis of ACNES can be made.

In summary: Diagnosis can be made when a patient presents with the following as described below (although not all need to be present)

  • Unilateral locoregional pain at the abdominal wall lasting for at least 1 month
  • The presence of a unilateral tender spot at the abdominal wall (a trigger point of <2 cm2 fingertip area of maximal tenderness, localized at the lateral border of the rectus abdominis)
  • A positive Carnett’s test
  • A positive skin pinch test and/or altered skin perception to light touch and/or cold at the area of the most intense pain
  • Normal laboratory findings with no indication of inflammation or infection, and in the absence of any surgical cause of pain
  • Negative imaging of the abdominal wall
  • Temporary positive relief in pain response of at least by 50% after injecting a local anaesthetic (usually lidocaine) at the diagnostic trigger point.

Patients are scored on an 18-point questionnaire, which is ratified for the diagnosis of ACNES and surgical treatment is more effective if any individual scores at least 12 or more

Treatment options

Treatment initially is managed by a gastrointestinal specialist clinician to ensure that there is no other pathology present, which may account for the abdominal pain experienced; this may involve the use of endoscopic examinations, a CAT scan of the abdomen as well as blood tests. Injections into the muscles/nerves at the affected site of pain, can also aid in pain relief but this is generally used more so to diagnose the condition and assess as to whether surgery or other treatment such as pulsed radiofrequency will be useful or not?
Pain killers are used in a step wise fashion and rely on the consultation and expertise of a pain specialist. Some of the painkillers used may be quite strong and lead to a patient becoming constipated, so the use of laxatives is sometimes also required. Chemical treatment by using various ‘injections’ with in some cases stronger agents such as triamcinolone and botox have also shown good results.

Another method of treatment is called ‘pulsed radiofrequency’ and involves using ultrasound to place an electrical probe between the fascial planes in the abdominal wall where the nerves lie. Once the position is correct then a 2Hz frequency is applied for 6 minutes.

Surgery: is very straightforward and relies on the use of a small incision just at the point of the pain and dividing the nerves that pierce the abdominal wall to supply the overlying skin (cutaneous nerves). This procedure is undertaken with the patient asleep (under general anaesthetic) with only small risks associated with the operation such as infection, bleeding of the wound and in some cases a haematoma can form. The procedure will leave a scar on the abdominal wall about 5 or less cm in length, but it is a relatively safe operation, as it does not involve invading the abdominal cavity. The surgery is undertaken as a day case so there is usually no overnight stay required.

Managing the pain 

Once a diagnosis is made as described above this is followed by a carefully guided management plan. Initially injecting a local anaesthetic (usually lidocaine) at the diagnostic trigger point provides a temporary relief in pain response for at least by 50% or more individuals and can be helpful in making the diagnosis. Painkillers are also used and may include pain medication, which is specifically designed for nerve pain such as amitriptyline and gabapentin as well as the use of anti-inflammatory medication, such as non-steroidals and opiate based drugs. Your pain specialists assess and guide on the prescription of these medications as well closely following up how effective the medication is in controlling the pain experienced.

In summary, pain is managed in a multidisciplinary fashion, which is key for any individual presenting with ACNES and relies of the use of painkillers, injections (neurolysis) and sometimes eventually surgery.

Evidence/ Success

Large centres carrying out the operation for ACNES have shown good results and quote a 70% success rate at one year. Some studies have shown pain free results in 86% of patients with over 75% experiencing long-term success. About 2/3rd of patients do get better with an improvement in their quality of life as well a reduction in the use of as well as in many cases cessation of pain killers they are taking. Other recommended treatments include pulsed radiofrequency ablation and localised injections as described above.

Treatment description 

  1. Pain killers including morphine in some cases
  2. Trigger point injections into the site of pain
  3. Pulsed radiofrequency ablation
  4. ACNES surgery 

Recovery and rehabilitation 

It is important to ensure that you receive good rehabilitation advice from your treating clinician and if you have seen a physiotherapist in the past it is worth continuing with some physio and rehab after your treatment.

  1. Trigger point injections will lead to slightly more pain after the procedure with some redness possible – this will settle down in less than a week or a few days. The pain may become worse before an effect of reduced pain is noted. In addition the pain once resolved may recur and this can happen a week or two after the procedure or sometimes it can last longer. The procedure does not restrict any individual from carrying out their normal activities.
  2. Surgery – the small scar is a painful for a few days but this pain settles and pain killers are rarely required past four days. The wound can become inflamed and irritated, look red and also become infected (weepy or pus may be present). A few days course of antibiotics is all that is required if this occurs. Rarely the wound can fill up with fluid, this is called a seroma and resolves with pressure in time and if there is a small bleed associated with this then it is called a haematoma – very rarely a reoperation is required to clear the haematoma.