Background
Among entrapment neuropathies, superficial peroneal nerve (SPN) entrapment is relatively
rare [[1],[2],[3],[4],[5],[6],[7],[8]] and only a few bilateral cases have been reported in the literature [[9],[10]].
Severe weight loss, as a result of anorexia nervosa, associated with common peroneal
nerve entrapment is very rare [[11],[12],[13],[14],[15],[16],[17]] and SPN involvement alone has not been described in the literature published in
English. Bilateral presentation is always related to systemic cause rather than local
mechanical compression.
Herein we report a case of severe weight loss secondary to anorexia nervosa causing
bilateral SPN entrapment in a young female patient who was treated successfully by
bilateral surgical decompression.
Case presentation
A 20-year-old, female university student presented to our outpatient orthopaedic clinic
with a two month history of vague pain on the outer border of both legs, and numbness
over the dorsum of the feet and big toes. Her symptoms were exacerbated by walking
and running and partially relieved by elevation. She had to stop to rest after 30
minutes of walking because of intolerable pain.
There was neither history of trauma or surgery to the lower limb nor history of lower
back problems. There was, however, a history of severe weight loss of (30 kg) during
the previous six months and the patient was diagnosed with anorexia nervosa using
criteria from the American Psychiatric Association’s Diagnostic and Statistical Manual
of Mental Disorders (DSM-IV-TR) and the World Health Organization’s International
Statistical Classification of Diseases and Related Health Problems (ICD).
Physical examination revealed bilateral tender points approximately 11 cm proximal
to the ankle joint on the outer surface of the leg, Tinel sign was also positive bilaterally.
There were sensory deficits on the dorsum of both big toes but no muscle weakness
or abnormal reflexes.
Examination of the lumbar spine and lower limbs revealed no clinical abnormalities
in the joints and there was neither suspicion of nerve root compression at the level
of the lumbar spine nor nerve entrapment at the neck of the fibula.
Radiographic examination of the lumbar spine, legs and feet were normal and EMG studies
were positive for bilateral entrapment neuropathy of the SPN proximal to the ankle
joint with no abnormality of the common peroneal nerves or of the proximal nerve roots.
After preoperative assessment, the patient was admitted for surgical treatment with
the diagnosis of SPN entrapment. The operation was done under general anaesthesia,
using pneumatic tourniquet. Bilateral explorations of the site of tenderness revealed
adhesions of both SPNs to the fascia with perineural fibrosis. Careful dissections
were done to free the nerves and neurolysis was successfully performed ([Figure 1]). The nerves were freed distally and proximally by splitting the overlying fascia
for a few centimetres above and below the site of entrapment.
Figure 1
Photograph at operation showing the superficial peroneal nerve.
Symptoms of bilateral peroneal nerve entrapment were relieved immediately and completely
in the postoperative period. Physiotherapy was started immediately to prevent postoperative
adhesions. No recurrence was observed in the first year following the operation.
Discussion
Superficial peroneal nerve syndrome is an entrapment neuropathy that usually results
from mechanical compression of the nerve at or near the point where the nerve pierces
the fascia to travel within the subcutaneous tissue.
A thorough and accurate knowledge of the course of the SPN and its relationships is
essential to understand the pathophysiology, and a thorough and careful physical examination
is important for diagnosing this condition. Stephens et al. described a physical sign
to identify the distal subcutaneous course of the SPN below the skin, primarily by
means of plantar flexion and inversion of the ankle and foot and, secondarily by a
passive flexion of the fourth toe [[1]].
In his study Styf, described 3 provocative tests for nerve compression at rest at
rest following exercise [[2]]. In the first test, pressure is applied over the anterior intermuscular septum
while the patient actively dorsiflexes the ankle. In the second test, the foot is
passively plantar flexed and inverted at the ankle. In the third test, while the patient
maintains the passive stretch, gentle percussion is applied over the course of the
nerve. These tests are useful in competitive athletes who have symptoms suggestive
of exercise-induced compartment syndrome.
Electrophysiological studies are helpful for the diagnosis, however, normal conduction
velocity may be found especially at rest which does not exclude compression of the
superficial peroneal nerve [[2]].
Injection of the nerve with lidocaine or Marcaine just above the site of involvement
may be the most valuable diagnostic tool. The patient can define the extent of relief
obtained from such an injection, which can be helpful in defining the zone of injury
and expected relief from surgical release or excision.
Entrapment of the superficial peroneal nerve has traumatic and non traumatic causes.
Local trauma and compression are the most common causes of nerve entrapment. This
may be due to recurrent stretch injuries or certain positions like prolonged kneeling
and squatting, which cause perineural fibrosis [[17],[18]]. Oedema after trauma may result in a mini compartment syndrome which may occur
when the tunnel was fibrotic, of low compliance and longer than 3 cm [[2]]. Chronic or exertional lateral compartment syndrome can also cause compression
of the superficial peroneal nerve, particularly in athletes [[19],[20]]. Fasciotomy of the anterior compartment for chronic anterior compartment syndrome
may also cause compression of the SPN nerve [[19]].
Nontraumatic causes of SPN entrapment are commonly due to anatomical variations such
as fascial defects, with or without muscle herniation about the lateral lower leg,
where the nerve is entrapped as it emerges into the subcutaneous tissue or a short
peroneal tunnel proximally. Nerve compression in patients with fascial defects is
explained by the normal increase in muscle relaxation pressure and intramuscular pressure
at rest during and after exercise. This increase is sufficient to cause herniated
muscle tissue and this can impinge upon or compresses the nerve [[20]].
Lowdon reported a case of an abnormally long course of the SPN nerve through the deep
fascia which was thought to have caused compression. Exercise may have exacerbated
the symptoms by producing mechanical irritation or by raising the pressure in the
peroneal compartment and thus increasing compression of the nerve [[3]].
Conclusion
In our case, the bilateral involvement forced us to think about a systemic cause of
SPN entrapment. The patient had severe loss of weight in a period of few months due
to previously undiagnosed anorexia nervosa which may have caused changes in the subcutaneous
tissues that led to adhesions and perineural fibrosis. Although the exact cause is
unknown; SPN entrapment should be kept in mind especially in patients with severe
weight loss and changes in body habits.
Competing interests
The authors declare that they have no competing interests.
