• Eunice You, Rakan Bokhari, and Denis Sirhan.
• Faculty of Medicine, McGill University, Montreal, Quebec, Canada.
• World Neurosurg. 2019 Nov 1; 131: 275-280.e1.

BackgroundEpidermoid cysts are slow-growing extra-axial lesions that account for approximately 1% of all brain tumors. They rarely occur in the brain stem, and those possessing both intra-axial as well as extra-axial components represent an even smaller subset.Case DescriptionWe report the unusual manifestation of 2 adult male patients, whose recurrent posterior fossa epidermoid cysts progressively invaded the anterior surface of the pons. Over the years, the patients would present with various complaints of ataxia, diplopia, facial spasms, and hearing loss and underwent several interventions for recurrence. Eventually, the epidermoid cysts progressed to completely split the pons, forming a surgical corridor that was amenable to a posterior approach. We describe the patients' favorable clinical courses, pathologic and neuroradiologic findings, as well as review the literature for similar such cases.ConclusionsTo the best of our knowledge, there are only 8 other cases of brainstem epidermoid lesions reported in the literature that are presumed to have originated extrinsically and thereafter transgressed the pons. However, we are the first to provide radiographic documentation confirming the progression of the lesions from extra-axial to intraparenchymal. Furthermore, although clinically subtle, we have decided to coin the term "split-pons syndrome" to better define this radiologic entity. Based on the evolution of the epidermoid's growth pattern, the authors propose a delayed posterior approach (telo-velo-tonsillar) to minimize morbidity.Copyright © 2019. Published by Elsevier Inc.

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