Rapid enlargement of a cervicomedullary junction neurenteric cyst with de novo fluid-fluid level formation

Background: Neurenteric cysts are rare congenital lesions that usually follow an indolent course. In cystic lesions, a fluid-fluid level generally indicates hemorrhage; in neurenteric cysts, it is seldom reported and poorly understood. Prior explanations rested largely on imaging and intraoperative inference. We report a cervicomedullary junction neurenteric cyst that enlarged rapidly after indolent growth, with a de novo fluid-fluid level in the absence of hemorrhage, and propose a histopathologically supported mechanism. Case Description: A 52-year-old man presented with posterior neck pain and dizziness. Magnetic resonance imaging demonstrated a 22-mm cystic lesion. Imaging 17 years earlier showed a 14-mm lesion. Over the next month, the cyst enlarged to 26 mm, developing a de novo fluid-fluid level and perilesional edema. Although suspected, hemorrhage was not confirmed at surgery or on histopathology; the cyst contained viscous proteinaceous fluid. The resected cyst wall showed foam cell clusters, interpreted as indirect evidence of prior wall disruption. Complete resection was achieved without neurological deterioration or recurrence at 8 months. Conclusion: A fluid-fluid level in a neurenteric cyst does not necessarily indicate hemorrhage. It may instead reflect a rise in intracystic pressure from chronic mucin secretion that progressively compromises the cyst wall, allowing osmotic influx of cerebrospinal fluid that drives rapid enlargement and dilutes the upper layer. To the best of our knowledge, this is the first report in which foam cell clusters provide histological support for such a non-hemorrhagic mechanism in a neurenteric cyst. Interval growth or perilesional edema may precede deterioration and should prompt consideration of timely intervention.