Idiopathic chronic adult hydrocephalus (ICAH) is due to expansion of the fluid-filled cavities in the brain. The clinical symptoms are gait disturbance, mental decline and incontinence. Treatment involves installing a ventriculoperitoneal shunt which is known to be able to induce regression of the symptoms in many cases meaning that ICAH is a classic, curable cause of dementia. Diagnosis relies on comparing symptoms before and after depleting cerebrospinal fluid (CSF) via a lumbar puncture (LP). In practice, the situation is complicated: improvement is often incomplete and there is no consensus on either how to assess the symptoms or how they change after CSF depletion. In consequence, the decision whether not to undertake surgery often depends on the neurosurgeon's clinical impression.

Over recent years, the cognitive profile of patients with ICAH has become better characterised and reproducible, objective techniques have been developed to assess motor function and CSF flow in the brain.

The investigators project aims to define the value of these new investigative techniques in the positive diagnosis of ICAH, in comparison to current decision-making tools.

Inclusion Criteria:

• Adult 65 years or older
• Hospitalized patients (at the University Hospital of Tours) requiring the assessment of chronic hydrocephalus in adults, suspected clinically
• Patient undergoing Lumbar Puncture subtractive
• Informed consent form signed
• Affiliated to a medical insurance

Exclusion Criteria:

• Contraindications to MRI
• Inability to walk or to stand before having lumbar puncture
• Patient under trusteeship and guardianship, or judicial protection