Introduction 3 3 3 4 19 1 5 9 14 The hydrodynamic concept of hydrocephalus Classification of hydrocephalus 9 14 14 Causes of decreased intracranial compliance Intracerebral pulse pressure theory of chronic hydrocephalus 14 9 10 12 14 Rationale for ETV in communicating hydrocephalus ETV versus shunting 8 16 18 14 14 2 6 8 15 17 18 20 Discussion 20 3 4 9 11 12 14 9 11 12 14 14 The theory may thus have identified decreased intra-cranial compliance as a new and potential universal cause of chronic hydrocephalus. Decreased compliance is common to all different types of chronic hydrocephalus whatever their etiology. If the theory is correct, there is hope for improved treatment of chronic hydrocephalus of unknown etiology. How come then that treatment failure rather is the rule than the exception of this devastating condition? The reason may be that shunting and ETV essentially represents symptomatic treatments of hydrocephalus. To improve therapy, we have to find treatments that are more effective in increasing compliance in cases with unknown etiology or are directed against the cause of the decreased compliance in cases with known etiology.