Discussion

Schwannomas are slow growing painless benign peripheral nerve sheath tumours that are rarely seen to occur in the lower limbs. Clinical presentation is often misleading and ambiguous. The schwannoma of a tibial or sciatic nerve may cause radicular leg pain, which leads to a diagnosis of lumbosacral radiculopathy, chronic regional pain syndrome, tarsal tunnel syndrome, entrapment neuropathies or diabetic neuropathy.1 The diagnosis in the index case was overlooked as sonographic evaluation suspected it to be a Baker's cyst—the most common popliteal swelling—which can occasionally produce compression neuropathy of the tibial nerve.

On sonography, schwannomas appear as well-defined, ovoid, heterogeneous masses in conjunction with cystic degeneration, eccentric location and posterior acoustic enhancement.2 MRI is another preferred tool that can define the nerve–tumour relationship. The lesion presents as a well-defined mass, usually fusiform in shape, located within the nerve, isointense to surrounding muscles on T1-weighted images and hyperintense on T2-weighted images.3 Ultrasonography, CT and MRI are all useful modalities in the preoperative diagnostic work up of this nerve tumour, however, each of these imaging studies has limitations in distinguishing between the different types of nerve sheath tumours aside from problems differentiating between benign and malignant lesions. Sometimes, long-standing slow growing nerve tumours are known to exhibit degenerative features as seen in the index case. The term ‘ancient schwannoma’ is used to describe such a rare subset, which can occasionally be misinterpreted as sarcomatous changes. Surgical excision ensures prompt symptomatic relief with a favourable prognosis.