Evidence that congenital hydrocephalus is a precursor to idiopathic normal pressure hydrocephalus in only a subset of patients

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 centile range adjusted for height and sex. The proportion of patients with INPH and HC above the 97th, 90th or 50th centiles was compared with the proportion expected in a normal population.

Results

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

Conclusion

A significantly larger proportion of elderly adults with INPH have a HC greater than the 90th or 97th centile than predicted by population norms, supporting the concept that compensated congenital hydrocephalus that does not become symptomatic until late in life is one aetiology of INPH, but is not responsible for all INPH. The mechanism that leads to the development of INPH in most patients remains elusive.Idiopathic normal pressure hydrocephalus (INPH) in older people can lead to a disabling syndrome affecting gait and balance, cognition and urinary continence.¹ Although INPH is usually considered an acquired form of hydrocephalus because the onset of symptoms is not until late in life, there is evidence that a subset of patients with INPH may actually have congenital hydrocephalus or a history of benign external hydrocephalus (BEH) of childhood leading to moderate ventriculomegaly. In 1989, Graff‐Radford observed that several patients with INPH had a large head circumference (HC), suggesting they had asymptomatic congenital communicating hydrocephalus that became symptomatic with ageing.² A larger study of patients with clinically diagnosed INPH also demonstrated disproportionate numbers with a large HC.³ Based on the finding that patients with clinically suspected INPH have an average intracranial volume greater than that of controls, it has been proposed that BEH is the precursor to INPH for all patients.^4,5We previously found that 15% of patients with the syndrome of hydrocephalus in young and middle‐aged adults (SHYMA) had congenital hydrocephalus, defined as HC >97th centile; however, 65% of patients with SHYMA had obstructive hydrocephalus, which INPH is typically considered not to include, and thus the SHYMA data cannot be extrapolated to the INPH population.⁶ We have diagnosed INPH for over 10 years using a protocol of spinal catheter insertion for controlled cerebrospinal fluid (CSF) drainage, CSF pressure monitoring, or both, which is considered the most accurate prognostic test by the INPH Consensus Guidelines,^7,8 and we routinely measure HC in our patients. The purpose of this study is to identify the proportion of patients with a large HC in a INPH cohort who presumably have congenital hydrocephalus that has not become clinically apparent until late in life.