>Abdominal ACNES: anterior cutaneous nerve entrapment syndrome and trigger point injections in the ED

Abdominal ACNES: anterior cutaneous nerve entrapment syndrome and trigger point injections in the ED

Resident Clinical Pearl (RCP) March 2019

Devon Webster – PGY1 FMEM Dalhousie University, Saint John NB

Reviewed and edited by Renee Amiro and  Dr. David Lewis.


 

Case:

A 32 year old woman with a history of chronic abdominal pain has been sitting in RAZ, presenting with, predictably, lower abdominal pain. She has been investigated multiple times over, with comprehensive labs, ultrasounds, pelvic exams and a previous CT, all of which have been normal. She carries with her a myriad of diagnoses; chronic abdominal and pelvic pain, IBS, fibromyalgia, depression and anxiety.

On history she reports near constant, left lower quadrant pain over the past 4 months. It is worse when sitting up and lying on her left side. The pain is sharp and she is able to localize the pain with a single fingertip. On history, you elicit no red flags for an intra-abdominal source of her pain. You ask her to lay down on the examination bed and hold your finger over the area of maximal pain. You feel no mass or abdominal wall defects. You apply light pressure, which triggers the pain, and ask her to lift her legs up. She yelps in pain, noting significant worsening to the site after tensing her abdominal muscles.

While you think of your differential for abdominal wall pain, you are highly suspicious of anterior cutaneous nerve entrapment syndrome (ACNES)…

What is ACNES?

  • Anterior cutaneous nerve entrapment syndrome (ACNES) is one of the most frequent causes of chronic abdominal wall pain and often goes undiagnosed. It is caused by entrapment of the anterior cutaneous abdominal nerves as they pass through the fibrous abdominal fascia.
  • This common condition can be treated rapidly and effectively by local trigger point injection of lidocaine and long acting steroid in the emergency department.

Pathophysiology:

  • The cutaneous branches of the sensory nerves arising from T7-T12 must make two 90* turns, traversing through channels within the abdominal fascia at the linea semilunaris (lateral border of the rectus muscles) in order to innervate the cutaneous surface of the abdomen.
  • While the neurovascular bundle should be protected from impingement by fat, it is susceptible to entrapment due to the tight passageway through the fibrous channels and sharp angulation.

Risk factors:

  • There are multiple risk factors for entrapment, and subsequent pain: tight clothing or belts, intra or extra-abdominal pressure, scarring and obesity. Pregnant women and those taking OCPs may also be at higher risk.
  • 4x more common in women, particularly those between ages 30-50 years of age.

 

 Clinical features on history:

  • Patients may describe chronic abdominal pain with maximal tenderness over a small area of the abdomen, typically <2cm
  • Pain is typically at the lateral edge of the rectus abdominis muscles and has a predilection for the right side although, the pain may be anywhere over the abdomen and may be in multiple locations.
  • Pain tends to be sharp in nature, positional and aggravated by activities that tense the abdominal muscles. Pain is generally better supine and worse when sitting or lying on the side.
  • There should be no red flags associated with the history suggestive of a more nefarious source of pain (e.g. GI bleeding, change in bowel function).

 

Physical exam:

  • Use a Q-tip to apply pressure as you move along the abdomen and try to locate the area of maximal tenderness. In most ACNES patients, you will find an area of allodynia or hyperalgesia corresponding to the area of nerve entrapment.
  • Look for a positive Carnett’s sign:
    • Ask the patient to either lift the head and shoulders or alternatively, lift their legs off of the bed while lying flat while you apply pressure over the area of pain on the abdomen.
    • Tightening of the rectus muscles should protect intra-abdominal pathology and pain will be reduced. In the case of abdominal wall pathology, including ACNES, pain will remain the same or be increased.
  • Understanding extra vs intra-abdominal pain:
    • There are 2 types of pain receptors: A-delta and C fibers.
      • A-delta: These fibers mediate sharp, sudden pain and innervate skin and muscles. Patient’s can localize this pain with a fingertip and this corresponds well with extra-abdominal wall pain, such as in ACNES
      • C fibers: Mediate dull ‘visceral’ pain that is often difficult to localize and results in pain over larger areas of the abdomen. These fibers innervate the viscera and parietal peritoneum.

 


Approach and Differential Diagnosis for Abdominal Wall Pain:

  • Look for ‘red flags’ (e.g. GI bleeding, abnormal labs, malnourished appearance) and rule out intra-abdominal sources of pain.
  • Once this has been ruled out, consider your differential for extra-abdominal wall pain which may include the following…

 

 

Diagnosis:

  • ACNES can be diagnosed on the basis of 3 criteria:

 1) Well localized abdominal pain

 2) Positive Carnett’s sign

 3) Response to trigger point injection of local anesthetic and steroid

 

 Treatment

  • Trigger point injections:
    • Act as both a source of treatment and diagnosis.
    • Provides immediate relief of symptoms to 83-91% of patients.
    • Injections can be repeated q-monthly.
    • Works through immediate anesthetization of the nerve, steroidal thinning of surrounding connective tissue and hydrodissection.
  • If the pain returns after trigger point injections, after considering other diagnoses, patient’s can be referred for chemical neurolysis (alcohol injections) or in some instances, surgical neurectomy.
  • Conservative treatment may include activity modification (e.g. avoid stomach crunches) and physical therapy

 

Technique for trigger point injections:

  1. Mark the site of maximal tenderness
  2. Inject 1-3 mL of 1% lidocaine and 1 mL of a long acting steroid using a 1.5 inch 26 gauge needle. Insert the needle until the tender area is reached (pt will let you know)
  3. Pain should resolve within 5 minutes.

 

  • US guidance may be useful for increasing the precision of the injection and can be used to visualize the passage of the nerve through the abdominal fascia.

Video guided review of ACNES:

https://www.youtube.com/watch?v=bDyX3myA0Gw&t=163s

 


References:

  1. Meyer, G, et al. “Anterior cutaneous nerve entrapment syndrome.” Uptodate. Accessed March 8, 2019. URL: https://www.uptodate.com/contents/anterior-cutaneous-nerve-entrapment-syndrome
  2. Suleiman, S, Johnston, D. “The Abdominal Wall: An Overlooked Source of Pain” American Family Physician. August 2001.
  3. Kanakarajan, S., et al. “Chronic Abdominal Wall Pain and Ultrasound-Guided Abdominal Cutaneous Nerve Infiltration: A Case Series.” Pain Medicine, volume 12, Issue 3, 1 March 2011, Pages 382-386.
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