Keywords
nerve compression syndrome - ultrasonography - diagnostic imaging
Palavras-chave
síndromes de compressão nervosa - ultrassonografia - diagnóstico por imagem
Introduction
Abdominal cutaneous nerve entrapment syndrome (ACNES) is a commonly misdiagnosed condition,
its prevalence among patients referred to general practitioners, abdominal surgeons,
or pain clinics for chronic abdominal pain ranges from 10 to 30%.[1]
[2] In an emergency department setting studied by van Assen et al,[3] it was found that ∼ 2% of the patients with acute abdominal pain had ACNES. The
most common symptom is a dull or burning pain with a sharp, well-localized component
in a restricted area of the abdominal wall. The pain can be chronic or acute. It generally
arises from lower intercostal nerves (T8–T11) probably caused by entrapment of the
cutaneous abdominal nerve at the lateral border of the rectus abdominis muscle (RAM),
specifically where this nerve enters the RAM channel through a fibrous ring. As the
nerve passes through the RAM, each nerve and its vessels are surrounded by fat, binding
the nerve, arteries and veins into a bundle. Separating the bundle from the muscle,
there is the fibrous ring situated about midway between the anterior surface of the
RAM and its posterior side, providing a smooth environment through which the bundle
can slide. This area is believed to be more susceptible to ischemia or to physical
compression of the nerve due to the juxtaposition of the soft neurovascular bundle
to the firm fibrous ring.[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
We present a case of ACNES seen at the Division of Neurosurgery of the Hospital Universitário
Gaffrée e Guinle of the Universidade Federal do Estado do Rio de Janeiro (HUGG – UNIRIO,
in the Portuguese acronym), calling attention to the clinical diagnosis, pointing
out its imaging characteristics, highlighting the value of high definition linear
ultrasound (HDLUS) as a valuable diagnostic method and showing that a noninvasive
treatment can be effective.
Case Report
A Caucasian male, 69 years old, presented with burning pain on the right anterolateral
abdominal wall in the region of the T9–T10 dermatomes. The pain had begun 4 months
earlier and started after lifting a heavy weight. The patient had diabetes mellitus
(DM), systemic arterial hypertension, and was overweight (29.1 kg/m2).
A general practitioner, a cardiologist, and a general surgeon successively evaluated
the patient. The cardiological tests, the thoracic abdominal and pelvic computed tomography
(CT), and the abdominal-pelvic magnetic resonance imaging (MRI) that were requested
for investigation were all considered inconclusive.
The patient was seen at the HUGG neurosurgery division due to the persistence of pain
with neuropathic characteristics. At the neurological evaluation, he reported intense
burning pain at the left T9–T10 dermatome area (visual analogue scale [VAS] 8), with
no changes at ectoscopy apart from the overweight. The pain was continuous without
remission periods and extremely worse during the night. The patient presented with
hypoesthesia and discriminative sensibility was reduced compared with the contralateral
abdominal wall in the T9–T10 area. The Carnett test was positive. The Valsalva maneuver
also enhanced the pain.
As the previous imaging exams of the patient were inconclusive and intercostal nerve
compromise was suspected, it was performed a HDLUS of the abdominal wall that demonstrated
the left RAM and its sheath lesion in a paramedian and supraumbilical topography with
cicatricial fibroadipose tissue. The muscle injury was associated with fibrocicatricial
thickening of the abdominal cutaneous nerve. Supraumbilical diastasis of the RAM could
also be noticed. ([Fig. 1])
Fig. 1 Ultrasound of abdomen; arrow shows fibrocicatricial thickening of the abdominal cutaneous
nerve (black arrow).
Other imaging studies (MRI and CT) were reanalyzed by the radiologist of our institution,
after the diagnosis with HDLUS. Some abnormalities were noticed with the reevaluation
of the images, such as high signal intensity on the MRI T2 image and decreased density
on the CT image around the RAM strain area. ([Fig. 2])
Fig. 2 On the right axial abdominal MRI (T2) and on the left axial abdominal CT; both demonstrate
the rectus abdominis strain area (white arrow).
With a clinical and imaging diagnosis of ACNES, the patient was oriented to nutritional
evaluation for weight control and pregabalin was administered at a dose of 75 mg 3
times a day in the 1st month. The patient was accompanied at the outpatient clinic and, after 2 months,
reported pain relief of 80% (VAS dropped from 8 to 3). Three months after the beginning
of the treatment, the patient reported a total relief of pain and, after 10 months,
he had lost 15kg and the pregabalin administration could be reduced to 75mg/day. The
last evaluation was 1 year after the diagnosis, the patient was on physiotherapy,
had a normal control of his DM and continued nutrition support. The pain did not return.
Discussion
Abdominal cutaneous nerve entrapment syndrome is a commonly misdiagnosed condition
that can have visceral pain as differential diagnosis, or the patient might even present
with a coexisting visceral condition that does not explain the pain by itself. Therefore,
it is important to recognize the clinical signs of ACNES and to look for the imaging
characteristics of the lesion.[3]
[4]
[5]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
Typically a unilateral neuropathic pain in the area of the T8–T11 dermatomes with
hypoesthesia in this same region, ACNES has an important clinical test, formulated
by Carnett,[16] to aid in the diagnosis. This test is based on the presumption that if the pain
has a visceral origin, the tensed abdominal muscles guard the underlying structures,
reducing tenderness, whereas continued tenderness during muscle contraction implicates
the abdominal wall as the source of the pain. In the presented case, the Carnett test
was positive and the Valsalva maneuver enhanced the pain, as it can increase the abdominal
pressure to the entrapped nerve.[2]
[3]
[4]
[5]
[6]
[11]
[16]
[17]
The next diagnostic step would be the imaging evaluation. Conventionally, to study
the abdomen, physicians can use CT, MRI, and ultrasound (US). Computed tomography
is the most commonly used in the emergency room in patients whose first clinical hypothesis
is a visceral pain, and that was the case of our patient, who also underwent an MRI
because of this suspicion. However, although the images showed indeed high signal
intensity on MRI T2 and a decreased density on CT at the RAM strain area ([Fig. 2]), probably indicating fibrocicatricial tissue, as ACNES was not a clinical suspicion
at the moment, these abnormities were only seen after the reevaluation of the imaging
exams in our institution.[12]
[13]
[14]
[18]
Nonetheless, although we could presume that the best imaging exam for these cases
would be MRI, given that the abnormities can be seen if followed by the clinical suspicion,
and that this exam is known to be one of the best to study muscles, we should not
forget that it is also one of the most expensive.[18] Therefore, neuromuscular HDLUS (18–5 MHz), used in the present case, is a great
contribution in the hands of an experienced examiner, revealing itself even more sensitive
to diagnose some neuromuscular lesions than MRI, despite being an examiner-dependent
technique. In addition, it is a relatively cheap and fast exam.[19]
Although Kanakarajan et al have already discussed the importance of US on ACNES as
a guidance to therapeutic injections, we suggest its use for diagnostic purposes.[8]
With the use of HDLUS on the abdominal wall, it is possible to differentiate fiber
discontinuity due to muscle strain of the RAM as decreased echogenicity areas and
the fibrocicatricial tissue can be localized as increased echogenicity areas, corroborating
the abnormalities found on the MRI and CT exams, imaging methods that demand more
time and are more expensive.[18] So, if the clinical evaluation indicates that that problem is on the abdominal wall
and not from a visceral origin, we suggest the use of HDLUS as the first imaging modality
exam.
Nowadays, it is recognized that a multimodal approach for such cases is paramount,
especially in chronic refractory cases. However, some authorities[14]
[15]
[20] try to confirm the diagnosis and use trigger point injection (lidocaine) without
mentioning any systemic drug (nonsteroidal anti-inflammatory drugs, weak opioids,
antiepileptics, and antidepressants) or other multimodal treatment.[4]
Weight control, physiotherapy, and systemic drug administration following the guidelines
for neuropathic pain therapy was the protocol established for the presented case.
The result was positive with this noninvasive management. It is important to point
out, as the current literature reports, that systemic drugs may be used as a first
step or as a complementary treatment for ACNES, considering the result with chronic
pain relief. However, these studies do not confirm if the administration of systemic
drugs alone can help patients with ACNES.[4]
If the use of systemic drugs fails to decrease the pain, the best option would be
lidocaine trigger point injection, preferably ultrasound-guided, not associated with
corticosteroids, as this combination is not certain to increase the success of this
therapy. As for refractory cases in which multiple trigger point injections associated
with a multimodal approach does not have success, the patient might need a more invasive
strategy, such as a neurectomy of the anterior cutaneous nerve endings. These two
types of management can lead to long-term pain relief in ∼ 75% of the patients with
ACNES.[4]
[14]
[15]
[20]
Other options described in the literature would be pulsed radio frequency treatment,
spinal cord stimulation of the dorsal root ganglion, and intraperitoneal onlay mesh
reinforcement. However, these methods still need further studies to confirm their
efficacy.[4]
[21]
[22]
An important bias of our study is the fact that HDLUS, used in the presented case,
is an examiner-dependent method, and that we compare only one case to the current
English published literature, which may lead to some disagreements that could be overcome
on a larger sample.
Conclusion
Patients who present on clinical evaluation with unilateral neuropathic pain in the
area of the T8–T11 dermatomes, hypoesthesia in this same region should be investigated
for ACNES with HDLUS performed by an experienced professional reducing the costs to
conclude a correct diagnose, and the noninvasive management might be helpful and effective.
