Abdominal cutaneous nerve entrapment syndrome (ACNES)

Abstract

Abdominal cutaneous nerve entrapment syndrome is caused by entrapment of an intercostal nerve in a fibrous ring in the rectus abdominis muscle and causes neuropathic pain. It remains an overlooked cause of chronic abdominal wall pain. Carnett’s test is useful to make a diagnosis. An injection of local anaesthetic and corticosteroid combination relieves pain and it is both diagnostic and treatment. This review article describes its pathophysiology, clinical diagnosis and its management. The databases Medline, and Google Scholar were searched using the terms chronic abdominal pain in general, surgical and gynaecological practice. Database were merged and duplicates were removed. The aim of the review is to update the knowledge on this topic in day to day clinical practice.

Full text

- 22 - Jana Medical Journal
Review Article
Abstract
Abdominal cutaneous nerve entrapment syndrome
is caused by entrapment of an intercostal nerve in
a brous ring in the rectus abdominis muscle and
causes neuropathic pain. It remains an overlooked
cause of chronic abdominal wall pain. Carnett’s
test is useful to make a diagnosis. An injection of
local anaesthetic and corticosteroid combination
relieves pain and it is both diagnostic and treatment.
This review article describes its pathophysiology,
clinical diagnosis and its management. The
databases Medline, and Google Scholar were
searched using the terms chronic abdominal pain
in general, surgical and gynaecological practice.
Database were merged and duplicates were
removed. The aim of the review is to update the
knowledge on this topic in day to day clinical
practice
Introduction
The chronic abdominal pain gives anxiety and
loss of work and economy to both to patients and
health care system. Therefore, it demands several
investigations and management modalities. The
dierential diagnoses of chronic abdominal pain
are intra-abdominal disorders such as irritable
bowel syndrome, spastic colon, and gastritis. When
a correct diagnosis is not arrived, they are given
a psychiatric diagnosis such as psychoneurosis,
depression, anxiety, hysteria and malingering.
There is an under recognized and underappreciated
cause of chronic abdominal pain, called abdominal
cutaneous nerve entrapment syndrome (ACNES).
If a patient presents with chronic abdominal pain
and no diagnosis is arrived, ACNES should be
considered as a most probable diagnosis. (1, 2)
Epidemiology
It is estimated that incidence of ACNES is 1 in
1800. Further, it is seen in up to 30% of the patients
with chronic abdominal pain who had negative
results of prior diagnostic work up. (2)
The peak incidence of the condition is seen among
the age group of 30-50 years and it is reported
in 12% of pediatric outpatients with chronic
abdominal pain.
Pathophysiology of ACNES
The anterior abdominal wall receives its sensory
supply via the anterior and lateral cutaneous
branches of the anterior rami of the 7th–12th
thoracic nerves.
The infrasternal area is supplied by the T7; the
umbilicus is by T10; the suprapubic are by T12 and
L1 by the iliohypogastric and ilioinguinal nerve
(Figure 1).
Figure 1: The anterior abdominal wall
ABDOMINAL CUTANEOUS NERVE ENTRAPMENT SYNDROME (ACNES)
Markandu Thirukumar1; Thambawita H.R 2
1 Department of clinical sciences, FHCS, Eastern University
2Teaching Hospital Batticaloa
Corresponding Author : Markandu Thirukumar, dr.thiru10@yahoo.com, https://orcid.org/0000-0001-8499-7175
This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International License, which permits unrestricted
use, distribution and reproduction in any medium provided the original author and source are credited
DOI: http://doi.org/10.4038/jmj.v31i1.62

Vol.31, No.1, July 2019 - 2 3 -
The cutaneous sensory nerves and vascular bundles
lie in the plane between the internal oblique
and transverses abdominis muscles [figure 2].
They supply the skin after passing towards the
posterior wall of the rectus sheath and through the
neurovascular channel in the rectus muscle.
Figure 2: Anatomy of thoraco abdominal nerves.
These neurovascular channels freely mobile
within a brous ring in the rectus muscle [gure
3]. Entrapment and mechanical irritation occur
when they change the direction to enter a brous or
osseo-brous tunnel or when passes over a brous
or muscular band. (3)
Figure 3: The course of the sensory nerves of the
anterior abdominal wall.
Fibrous ring is the most susceptible for entrapment
and a site of nerve compression and ischaemia
which produces the symptoms of ACNES. Nerve
traction and compression is also caused by rectus
muscle contraction.
Localised swelling due to the irritation may
directly injure the nerve or compromise the nerve’s
circulation. Valleix phenomenon explains that the
tenderness of the main nerve trunk may be found
proximally or distally to the affected portion.
Proximal tenderness may result from vascular
spasm or from unnatural traction on the nerve trunk
against the point of entrapment. In ACNES, all
these mechanisms can be at work. (4)
Clinical presentation
Patients often present with abdominal wall pain
mainly at the right side lateral edge of the rectus
abdominis muscle, but can be in multiple locations.
Also the pain radiates to the aected dermatome.
It is sharply localized to a small (<2cm) area of that
is always felt in the same place and is usually dull
or stabbing type. There are features of neuropathic
pain such as retrograde radiation (Valleix
phenomenon) due to entrapment neuropathy.
The pain gets aggravated when the patient lies on
the aected side or sits. Tight clothing, sneezing,
coughing, laughing, and physical exercise are other
aggravating factors. Even though patients often do
not feel “sick,” their quality of life can be impaired.
Pain is felt horizontally in the upper abdomen and
more oblique in the lower abdomen due to the course
of nerves responsible for ACNES in these regions.
When the radiation occurs only with movement, it
suggests the entrapment within the muscle. When
the cutaneous nerve branches entrapped in scar
tissue following abdominal surgeries, the direction
of pain radiation shows the dermatomal distribution
of the particular nerve entrapped.
There are recognized risk factors for ACNES; such
as previous laparotomy and laparoscopic surgery
and rectus muscle strengthening exercises. In
addition, obesity, pregnancy and oral contraceptive
use are also other risk factors for ACNES.
Physical examination
The physical examination is performed when the
patient is in supine position and it is important to
arrive at a diagnosis of ACNES. Pain is exactly
localized with a ngertip at the linea semilunaris,
i.e., the lateral border of the rectus abdominis
muscle in most of the patients.

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Carnett’s test
When pressing on the point of greatest pain with the
nger tip, the pain gets worsened when the anterior
abdominal wall is being contracted (Positive
Carnett’s test). But pain does not always become
worse during the examination.
When the pain originates from the abdominal
viscera, pain is less marked during the examination
(Negative Carnett’s test). Adequate voluntary
contraction of the anterior abdominal musculature
is essential for the proper examination.
Neurovascular channel is constricted when the
rectus muscle contracts and worsens the symptoms
of neuropathy. Abdominal hernias, abdominal wall
haematomas, and rib tip syndrome also produce
positive Carnett’s sign on examination. (5)
Pressure over the nerve at the anterior openings in
the rectus sheath will cause pain (Positive Hover
sign).
Hypaesthesia, hyperalgesia, or allodynia around
the area of pain also supports a diagnosis of ACNES
and it has been reported by 75% of the patients with
ACNES. (6)
The “pinch test” is useful if the origin of the site of
the pain is not identied. This test is picking up the
patient’s skin with the subcutaneous fat between
the thumb and index nger, rst on one side of the
midline of the abdomen and then on the other side.
The patient will state whether one side hurts more
than the other. Cotton and pinprick technique can
be used to check for hypoesthesia or hyperesthesia
around the pain site. (7)
Management
Health education about ACNES and the rectus
muscle stretching exercises should be given to
all patients. Though the ecacies of non-specic
pharmacological therapies are unclear, heat or cold
application, abdominal binders and transcutaneous
electrical nerve stimulation are useful.
For instance, paracetamol, non-steroid anti-
inammatory, anti-convulsant, anti-depressants,
topical treatments such as 5% lidocaine plasters,
and capsaicin cream and opioids.
Recommended Treatment for ACNES
First line therapy is generally injection of local
anaesthetic and corticosteroid combination
which relieves pain and reduce the herniation
of neurovascular bundle through the ring. A
local injection of an anesthetic agent completely
relieves the pain. The combine injection is the most
commonly used one to treat ACNES. And it is both
diagnostic and treatment.
0.5 to 1 ml of 2% Lidocaine is used and the length
of needle varies according to the thickness of
subcutaneous tissue. Usually 21 G or 22 G is used.
Spinal may be needed sometimes to reach the
injection point. (8)
Technique for inserting the needle
There are many techniques to identify the
landmark for injection. Palpation with fingers,
nerve stimulator to identify the nerve and ultra
sound guided injection. Ultrasound-guided local
anaesthetic injection is increasingly recommended
nowadays in the literature and it gives the median
duration of pain relief of 12 weeks. (9)
There are palpable depressions on the lateral
edge of the rectus muscle and this is the point of
injection where the needle is introduced through
skin, subcutaneous tissue, aponeurosis and to the
fatty plug surrounds the neurovascular bundle when
emerging from the brous channel.
When passing through the tissues, aponeurosis and
the fatty plug produce resistant to the needle and the
needle should not be inserted deeper than this level
as this will further increase the pressure within the
bro muscular channel.
The tip of the needle should be placed in front of
the brous ring just beneath the aponeurosis, and
the examiner should make sure the position of the
needle before the injection by pulling out the needle
into the subcutaneous tissue to insert again.
The injection is best given in patient standing and

Vol.31, No.1, July 2019 - 2 5 -
bearing down position. But can be given in lying
position if the patient is comfortable.
Position the needle with middle nger of one hand
in the aponeurotic opening and use the other hand
for cleaning the area with alcohol and inserting the
needle above the tip of the nger. The other hand
should not be taken o until the needle is being
situated in correct position and use the same hand
to stabilize the needle while injecting the drugs.
Patient should be explained not to breathe during
the injection.
Neuromodulation using pulse radiofrequency
lesion has been attempted prolong the pain relief.
There is an inflicting a second nerve injury
resulting with these injection procedures a small
but signicant risk. Surgical options are available
for ACNES. Suggest surgical neurectomy for
ACNES, but the long-term outcome is yet to be
known. (10, 11)
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