Periclavicular multifocal, infiltrative extraneural soft tissue perineuriomas: illustrative case.

backgroundPerineuriomas are benign, typically solitary tumors with intraneural and extraneural subtypes presenting distinct clinical manifestations and symptomatology. Here, the authors report a unique case of multifocal, infiltrative extraneural soft tissue perineuriomas localized to the shoulder. OBSERVATIONS: A 33-year-old female presented with 1 year of right neck and shoulder pain accompanied by a new onset of numbness and tingling down her right arm. MRI of a mass apparent over her right trapezius and scapula revealed several mixed cystic and solid neoplasms encroaching on the C5-6 nerve roots and brachial plexus. The tumors were biopsied and confirmed by immunohistochemical staining to be extraneural soft tissue perineuriomas. Further excision was carried out to relieve the patient of her symptoms associated with mass effect on the brachial plexus. LESSONS: This case highlights the importance of a thorough workup of soft tissue and peripheral nerve sheath masses. There is extraordinary difficulty in diagnosing and treating extraneural perineuriomas, given their wide array of clinicopathological presentations, and it is crucial that these tumors are accurately identified to avoid overtreatment. https://thejns.org/doi/10.3171/CASE24388.