Distinct Imaging Features of Peripheral Nerve Sheath Tumours in NF2‐Related Schwannomatosis: A Case Report

Introduction Peripheral nerve sheath tumours (PNST) are an important feature of the NF2 ‐related schwannomatosis. These constitute primarily schwannomas which are characterized as nodular, solitary benign tumours of single fascicles. We here describe a pronounced, extensive, multifascicular enlargement of the right ulnar nerve and bilateral brachial plexus in a teenage patient with NF2 ‐related schwannomatosis. Neither their imaging nor the clinical characterization were consistent with classic schwannomas. Methods Within a multimodal work‐up, detailed and standardized high‐resolution nerve ultrasound was correlated to clinical, electroneurographic, and MRI findings. Follow‐up data over 2 years were provided. Results The clinical presentation of a slowly progressive, predominantly motor axonal neuropathy of the right ulnar nerve prompted a comprehensive nerve ultrasound examination, initiated by this clinically and neurophysiologically defined “sentinel” finding. Ultrasound imaging revealed an extensive, fusiform enlargement of the right ulnar nerve, as well as bilateral involvement of the brachial plexus, the latter without corresponding clinical symptoms. Subsequent genetic analysis confirmed the diagnosis of NF2‐related schwannomatosis. Discussion Signs of a mononeuropathy are a frequent manifestation of NF2 ‐related schwannomatosis in childhood. The here presented nerve imaging characteristics, however, did not correspond to classic schwannomas. The fusiform, long‐extended, multifascicular enlargement with mixed hyper‐ and hypoechogenic components on ultrasound, as well as markedly T2‐hyperintense and T1‐isointense appearance with diffuse gadolinium enhancement on MRI, is consistent with perineurioma. These mesenchymal nerve tumours have rarely been described in NF2 ‐related schwannomatosis. This association is further supported by the present case and highlights the necessity for additional peripheral nerve imaging studies in NF2‐related schwannomatosis, including the application of high‐resolution ultrasound.