A 35-year-old male roofer presented with a 7-month history of a right foot drop associated with pain in the right calf and foot, and paraesthesia in the foot and toes. An MRI examination of the right knee was performed.
A 35-year-old male roofer presented with a 7-month history of a right foot drop associated with pain in the right calf and foot, and paraesthesia in the foot and toes. An MRI examination of the right knee was performed, and axial and coronal spin echo T1-weighted and fat suppressed FSE T2-weighted images were obtained. These showed the presence of a well-defined, bilobed mass of an increased signal intensity on T2-weighted images and a low-to-intermediate signal on T1-weighted images, approximately measuring 2 x 2 cm centred immediately anterior to the neck of the fibula (Fig. 1). Based on this, the MRI diagnosis made was that of a ganglion cyst. A high signal track on T2-weighted images could be followed cranially from this lesion towards the superior tibio-fibular joint, suggesting a communication between the lesion and the joint (Fig. 1). The abnormal signal was returned from all muscles of the anterior and peroneal compartments of the lower leg, except the peroneus tertius, with increased signal on the T2-weighted images indicating a diffuse oedema (Fig. 2). In addition, there were streaky areas of an increased signal intensity on the T1-weighted sequence, indicating fatty infiltration (Fig. 3). This implied that the ganglion cyst was compressing the common peroneal nerve with resulting muscle denervation. At surgery, this diagnosis was confirmed, and the lesion was dissected free from the common peroneal nerve and excised. A histological assessment done confirmed the diagnosis of a ganglion. Post-operatively, there was a gradual improvement with return to normal sensation and absence of pain or paraesthesia. The patient’s foot drop persisted at three months at the time of discharge.
Ganglia are a recognised cause of muscle denervation when they develop in close proximity to a peripheral nerve. Sporadic descriptions of such ganglia arising close to the common peroneal nerve have been made in the past (1) and are typically associated with a triad of pain, paraesthesia and weakness in the supplied region (2). Lesions have also been documented in the supraglenoid notch of the scapula and within Guyon’s canal of the wrist (3) where they compress the suprascapular nerve and the ulnar nerve, respectively. These ganglia are often only suspected when EMG confirms muscle denervation, the lump itself remaining impalpable. In such situations, MRI allows visualisation of the nature of the compressive lesion and its anatomical relations. MRI also offers an opportunity to image and document muscle denervation. Specific MRI features may give an indication of the chronicity of denervation (4, 5). Acutely, muscle denervation is followed by muscle oedema, which will increase the muscle signal on T2-weighted sequences. Chronically, fatty infiltration will be seen as an increased signal on T1-weighted sequences, a finding that implies an element of irreversibility. The ability to map areas of irreversible muscle denervation pre-operatively could help plan tendon transfer surgery (4) if needed. In this case, fatty atrophy was demonstrated on the T1-weighted sequence suggesting a degree of irreversibility to the muscle atrophy. This corresponded to the clinical findings, where muscle strength was not restored at 3 months despite the return of normal sensation, which confirmed that the nerve was intact. As irreversible muscle atrophy was suggested on doing a pre-operative MRI, the patient was informed that a full recovery of muscle strength could not be expected when he was consenting surgery. The importance of this case report is threefold. First, the report stresses that the MR appearances of denervated muscles are characteristic in chronic situations, and that it may be unnecessary to conduct EMG studies. Second, MRI can often demonstrate the underlying cause of compression thereby allowing a specific diagnosis. Third, MRI provides prognostic information by indicating the presence of irreversible muscle denervation non-invasively. This can be useful in patient counselling and pre-operative planning. In short, MRI offers a ‘one-stop shop’ for the evaluation of a patient with clinical signs of compressive peripheral neuropathy.
Peroneal nerve palsy caused by a ganglion cyst.
Based on the provided MRI images and the examination results of the patient's right knee, the following main radiological features can be observed:
Considering the patient’s clinical presentation (right foot drop, pain, numbness, and sensory abnormalities) and the cystic lesion adjacent to the common peroneal nerve shown on imaging, the following diagnoses should be considered:
Combining the patient’s age, occupation (roofer, possibly subject to repetitive strain or local compression), clinical symptoms (foot drop, sensory changes), and MRI findings (a cystic lesion near the common peroneal nerve with associated fat infiltration in the affected muscles), the most probable diagnosis is:
Common peroneal nerve compression by a cyst (ganglion cyst/articular cyst) leading to chronic denervation.
To further clarify the nature of the lesion, ultrasound-guided aspiration or surgical exploration with histopathological examination may be considered.
Note: Actual implementation of these exercises should be performed under the guidance of a professional rehabilitation therapist or physician to ensure safety and efficacy.
This report is based solely on the provided case information and imaging results, serving as a preliminary and reference medical analysis. It cannot replace an in-person consultation or the opinion of a specialist. If there are further questions or worsening of symptoms, it is recommended to seek timely medical attention or a professional healthcare team for evaluation and treatment.
Peroneal nerve palsy caused by a ganglion cyst.
