A midline prepontine cyst: Serial magnetic resonance imaging over 20 years shows very slow growth after its rapid shrinkage.

An otherwise healthy 22-month-old boy suffered high fever, irritability, nausea, dysphagia, dysarthria and right hemiparesis. Magnetic resonance imaging showed a cystic mass, 15 mm in diameter, with surrounding oedema in the base of the lower pons. The symptoms subsided in about 10 days after onset, followed by a rapid decrease of the cyst size to 5 mm. Thereafter, the patient's psychomotor growth has been normal. Annual follow-up magnetic resonance imaging scans showed very gradual enlargement of the cyst located on the ventral surface of the pontomedullary junction, reaching 16 mm in diameter in 21 years after onset. It was hyperintense on T1-weighted and isointense on T2-weighted magnetic resonance imaging. No haemosiderin deposition or gadolinium enhancement was seen. This is a rare report of a two decade longitudinal follow-up of a midline prepontine cyst showing asymptomatic and very slow growth. The possible nature of the cyst includes neurenteric, dermoid and epidermoid cyst.