Sural nerve schwannoma

A male patient presents with a history of slow-growing painless swelling at the posterior aspect of the left knee with a history of localised pain referring to the medial and posterior region of the knee, calf and ankle. On clinical examination, Tinel’s sign was positive.

Ultrasound showed a well-demarcated, oval, hypoechoic lesion in the deep subcutaneous plane of the posterior aspect of the knee, measuring ~1.58x1.25 cm. A few well-defined internal cystic areas were also seen (red arrows in Figure 1). The lesion was seen inferior to the popliteal crease in the midline, lying along the course of the medial sural cutaneous nerve (Figure 2). Tibial nerve and popliteal vessels were seen separately (Figure 3). The lesion demonstrated arterial flow on Doppler ultrasonography (Figure 4).

On MRI, T1-weighted, coronal image (Figure 5) shows a sharply demarcated, homogeneously iso to a hypoechoic lesion (related to skeletal muscle) in the deep subcutaneous plane in the posterior aspect of the knee in between medial and lateral head of the gastrocnemius muscle. T2-weighted, fat-sat, sagittal image (Figure 6) shows an oval, mildly heterogenous, hyperintense lesion, located in posterior aspect of knee, underneath the deep fascia. STIR coronal image (Figure 7) shows no suppression of signals (suggests no fatty components). T2-weighted GRE sequence shows no significant blooming artefact. T1 post-contrast axial (Figure 8a) and coronal (Figure 8b), fat sat images show intense homogenous contrast enhancement of the lesion.

Schwannomas are benign tumours of nerves which arise from nerve sheaths, exclusively from Schwann cells. Peak incidence occurs in the fifth to sixth decade without any gender predilection. It often occurs in the head, neck and upper extremities, with 1–3% incidence involving peripheral nerves [1]. They can be found as sporadic and associated with syndromes such as neurofibromatosis. The aetiology of sporadic schwannomas is not well understood. Patients present with slowly growing painless swelling; however, large swellings can produce compression of adjacent structures, resulting in pain in lower extremity most commonly sciatic and tibial nerves are involved [2].

Radiographing features of schwannoma are nonspecific. USG shows a well-defined hypo-echoic lesion with internal cystic and fatty changes. Haemorrhage can be seen infrequently. The larger the tumour, the more likely it will degenerate. Internal vascularity was demonstrated on Doppler ultrasound. MRI shows iso to low signal lesions (compared to skeletal muscle) on T1-weighted MRI with high signals on T2-weighted images. Post-contrast T1-weighted MRI shows intense contrast enhancement [3].

It is difficult to distinguish schwannoma from neurofibroma based on imaging characteristics. This also applies to malignant peripheral nerve sheath tumours (MPNSTs).

Distinction between schwannomas, neurofibromas, and MPNSTs can be challenging. A peripheral location with respect to the nerve axis is typical of schwannomas. The “target sign” (hypoechoic centre with an echogenic rim) is highly suggestive of neurofibroma, but it can also be seen in schwannomas and MPNSTs [4].

Angioleiomyomas are common soft tissue tumours of extremities arising from the subcutaneous plane and constitute an important differential diagnosis. These lesions are small, slow growing, firm masses, which are iso to slightly hyperintense on T1-weighted MRI than skeletal muscle. They exhibit heterogeneous high signals on T2-weighted MRI with variable contrast enhancement [5]. They may have peripheral low signal intensity rim corresponding to a fibrous pseudo capsule with adjacent vascular structures.

Monitoring is required in cases of small asymptomatic lesions by MRI every few months to check the growth of a tumour. Surgical enucleation is the treatment of choice in cases of symptomatic and rapidly growing tumours with follow-up on MRI. Recurrence of the lesion is uncommon.

Take home message

The possibility of sural nerve schwannomas should be kept in mind whenever dealing with small, subcutaneous lesions of the back of the knee. Clinical history is helpful, demonstrating sensory abnormality on the back of the knee and ankle.