A 15-year-old male patient was referred because of right lower limb atrophy, dysesthesia in the proximal third of the right lateral leg, mild ipsilateral pain and weak right foot eversion lasting for 9 months. He was regularly squatting while studying (Fig. 1). Tinel sign was possitive.
MR imaging was performed with a 1.5T scanner using standard sequences. The axial and coronal T1-w images showed increased signal intensity at the lateral compartment muscles in keeping with fatty infiltration (Fig. 2). Neither mass nor oedema was shown on STIR images (Fig. 3). The muscles were atrophic, in keeping with chronic denervation of the common peroneal nerve (CPN).
Peroneal neuropathy is the most common mononeuropathy of the lower extremity [1, 2, 3, 4]. It results commonly from direct trauma, traction injuries, and surgery. Rarely, vascular lesions, diabetes, external compression (casts, immobilization) and space occupying lesions including aberrant muscle, scar, schwannoma, neurofibroma, osteochondroma, ganglion cyst, osteophytes, and fracture fragments.
Entrapment neuropathies (compression of a short segment of a nerve) may result from mechanical pressure due to mass occupying lesions or dynamically due to limb positioning [1-4].The most common site of CPN entrapment is the fibular neck, as at this site, the nerve is covered only by skin and could be easily compressed [5]. The second site of entrapment is at the fibrous channel underneath the peroneus longus muscle (drawing).
Limb positioning associated to CPN palsy includes habitual cross legging as in this case, with direct pressure on the peroneus longus muscle and/or the fibular neck (Fig.1, drawing), and prolonged immobilization [6, 7] . Prolonged squatting and extended lithotomy position due to childbirth or obstetric surgery usually produce bilateral CPN entrapment [2].
Until recently the diagnosis was based on clinical (foot drop, positive Tinnel sign, loss of sensation at the distribution of CPN) and neurophysiological studies [7]. Early diagnosis is related to the degree of functional recovery as treatment depends on the cause and the site of entrapment.
Imaging strategy includes plain x-ray to exclude osseous abnormalities, and ultrasonography and MR imaging for direct assessment of the nerve and the causative pathology [3]. Muscle denervation at the acute stage is demonstrated with high signal intensity of the affected muscles on fluid sensitive sequences and normal signal and morphology on T1-w images [4-6]. In the chronic stage, there is muscle atrophy with fatty infiltration and loss of volume of the affected muscles which is depicted as high signal intensity on T1-w MR images and lack of any abnormal signal on fluid sensitive sequences.
The treatment depends upon the underlying cause of CPN paresis. It includes conservative therapy (NSAIDS, rest, physiotherapy), local injections of steroids and if symptoms persist, surgical decompression. Mass lesions are also treated surgically. In our patient the treatment was conservative as the injury to the CPN was chronic with non-reversible denervation muscle atrophy.
Peroneal nerve entrapment neuropathy due to habitual leg crossing
According to the provided MRI images and the patient's clinical information, there is a noticeable reduction in the volume of the lateral muscle group of the right lower leg (muscle atrophy). On T1-weighted sequences, these muscles exhibit high signal intensity, suggesting fatty infiltration. This finding commonly appears in chronic nerve innervation disorders resulting in target muscle atrophy. The lesion primarily involves the area innervated by the common peroneal nerve around the fibular neck. No obvious mass or compressive lesion is observed, nor is there any clear fracture or heterotopic ossification. The imaging findings align with the clinical presentation (weakness in the distal right lower limb, sensory abnormalities, and decreased strength in foot eversion).
Considering the patient's history of prolonged cross-legged or compressive posture, the MRI findings of atrophy and fatty replacement in the lateral muscles of the right lower leg, and local clinical signs (positive Tinel’s sign, weakened foot eversion, sensory abnormalities), the most likely diagnosis is “Chronic Compressive Lesion of the Right Common Peroneal Nerve.”
(1) Treatment Strategy:
(2) Rehabilitation and Exercise Prescription (FITT-VP Principle):
Given the existing muscle atrophy, a gradual, individualized plan is necessary.
Patients should also be advised to correct their posture in daily life, avoiding prolonged knee flexion or compression at the fibular neck. Should there be any notable increase in pain or worsening of neurological symptoms during exercise or daily activities, prompt medical follow-up is recommended.
This report is based solely on the provided medical history and imaging data and is intended for reference only. It cannot replace an in-person consult or advice from a professional healthcare provider. Should you have any concerns or experience any change in your condition, please promptly seek evaluation and treatment at a qualified medical institution or by a specialist physician.
Peroneal nerve entrapment neuropathy due to habitual leg crossing
