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A midline posterior-fossa tumour (most often a medulloblastoma) in children typically compresses the fourth ventricle, producing obstructive hydrocephalus with headache, vomiting and papilledema, and its vermian/cerebellar location causes gait instability or truncal ataxia; diplopia can result from a VI-nerve palsy due to raised intracranial pressure, fitting all the findings here [Hung, 1990, PMID 2184531; Yazigi-Rivard, 2008, PMID 18995998].