A 64-year-old woman complains of gait imbalance, headache and deterioration of mental status over the past several months. Her vision is normal. A CT scan reveals hydrocephalus, but the lumbar puncture pressure is unexpectedly low. What does she have?

Normal-pressure hydrocephalus is a condition seen in the elderly in which there is symmetrical enlargement of the entire ventricular system. When patients with this condition are studied by lumbar puncture, it is found that despite ventriculomegaly, the ICP is abnormally low. This syndrome presents with a characteristic triad of symptoms - dementia, ataxia, and urinary incontinence.