A 2-year-old male presented with progressive head enlargement noticed since 6 months of age. Examination: alert, only able to say 'dad' and 'mom', unsteady standing, enlarged head with dilated scalp veins, upward gaze palsy. Head CT shows marked enlargement of bilateral lateral and third ventricles with midline structures in place, normal fourth ventricle size/shape. MRI reveals aqueductal stenosis. Which factor does NOT cause communicating hydrocephalus?