Abstract

Objective: To identify the proportion of patients in a large idiopathic normal pressure hydrocephalus (INPH) cohort with large head circumference (HC) who presumably have congenital hydrocephalus that has not become clinically apparent until late in life.

Methods: HC was measured in 158 patients diagnosed with communicating INPH and assigned to HC percentile range adjusted for height and sex. The proportion of patients with INPH and HC above the 97th, 90th, or 50th percentile was compared to the proportion expected in a normal population.

Results: The proportion of patients with HC>90th percentile is significantly larger than would be expected in a normal distribution (19.6% vs 10%, p=0.0001), as is the proportion of patients with HC >97th percentile (8.9% vs 3%, p=0.0001). The relative association between INPH and HC >97th percentile was nearly tripled (Relative Association 2.95; Confidence Interval 1.36 – 6.41), but the relative association between INPH and HC >50th percentile was not significantly higher than predicted (Relative Association 1.13; Confidence Interval 0.95 – 1.34).

Conclusion: A significantly larger proportion of elderly adults with INPH have a HC greater than the 90th or 97th percentile than predicted by population norms, supporting the concept that compensated congenital hydrocephalus that does not become symptomatic until late in life is one etiology of INPH, but is not responsible for all INPH. The mechanism that leads to the development of INPH in most patients remains elusive.