Melanotic schwannoma of peripheral nerve root

A 29-year-old female presented with low back pain radiating to the left leg, associated with weakness in the left lower limb.

Multiplanar MRI of the spine was done, including screening sagittal T1 and T2 sequences, and axial T1 and T2 at the site of the lesion. A round to oval mass was seen on the left side at L5–S1 inter-vertebral level, along the S1 nerve root, coursing through the left sacral foramina. The mass had an intrinsically high T1 signal (Figure 2a) and low T2 signal (Figure 2b) on MRI. There was no intradural component or involvement of cauda equina (Figure 1).

Background

Schwannomas are peripheral nerve sheath tumours of neural crest origin. They are usually solitary, well-encapsulated, and are slow growing. Melanotic schwannomas are composed of neoplastic Schwann cells, from which melanin is synthesised. They account for less than 1% of peripheral nerve sheath tumours. The melanotic nature and intramedullary location are exceedingly rare for these lesions. Posterior spinal nerve roots and sympathetic ganglia are common locations for melanotic schwannoma. When compared to conventional schwannoma, these lesions present in younger patients and affect patients in the age group of 30–40 years. Patients present with varied symptoms of pain, sensory deficits, muscle weakness, and sometimes paralysis. Melanotic schwannomas are also encapsulated tumours but with a more aggressive nature than conventional schwannomas. The malignant potential of these tumours is approximately 10%, and the local recurrence rate is up to 35% [1–3].

Clinical Perspective

Melanotic schwannomas present most often in the age group of the third to fifth decade. Patients with Carney complex present younger. The clinical presentation depends on the location of the tumour. These lesions have a predilection for spinal nerves, causing cord compression or focal neurology. Both sensory and motor deficits are encountered at presentation. Patients with larger peripheral lesions present with palpable swelling or lump. Since most of these lesions are benign, complete resection of the tumour is done for curative intent. Some of these tumours also show malignant potential, and a significant minority of patients have shown metastases. Metastasis most commonly occurs in the lungs and pleura [2,4,5].

Imaging Perspective

MRI is the investigation of choice for the evaluation of these tumours. These lesions are typically hyperintense on T1 and hypointense on T2-weighted images. These signal characteristics are due to paramagnetic free radicals present in the melanin. On the other hand, non-melanotic schwannomas or conventional schwannomas show a hypointense signal on T1 and a hyperintense signal on T2-weighted images. Both these lesions show post-contrast enhancement, thus differentiating them from spinal cord oedema [6,7].