When one becomes many: a pediatric case of extensive multilevel spinal extradural arachnoid cysts challenging diagnostic paradigms. Illustrative case.

BACKGROUND: Spinal extradural arachnoid cysts (SEACs) are rare, typically solitary lesions. Multilevel SEACs are exceptionally uncommon. This report highlights a pediatric case of extensive thoracic SEACs, emphasizing their potential for rapid neurological decline and the importance of early recognition in atypical spinal cord compression.

OBSERVATIONS: A 13-year-old female presented with progressive myelopathy due to multilevel SEACs (C7-L2), challenging the notion of SEACs as usually solitary. Imaging revealed severe cord compression (flattened to < 2 mm at T4-7). Surgical intervention (laminotomy, cyst excision, dural repair) led to significant recovery. Histopathological analysis confirmed the diagnosis, demonstrating hypocellular fibrous cyst walls without neoplastic or inflammatory features, supporting a developmental origin and differentiating SEACs from mimics. This case expands the understanding of SEACs as potentially extensive, high-risk lesions in children.

LESSONS: This case underscores the need to consider SEACs in pediatric progressive myelopathy, even with atypical symptoms. Histopathological confirmation is critical to exclude mimics and guide management. Rapidly progressive decline necessitates timely surgery when cord compression is evident. Successful surgical outcomes set a precedent for complex SEACs. The rarity of multilevel SEACs raises questions about underlying connective tissue or genetic predispositions. This report advances the literature on multifocal SEACs and refines diagnostic and management approaches. https://thejns.org/doi/10.3171/CASE25312.