Reference values for CSF outflow resistance and intracranial pressure in healthy elderly

Objective: The intracranial pressure (ICP) and CSF outflow resistance (R_out) are essential to describe the dynamics of the CSF system. R_out affects ICP, pulse amplitudes, CSF absorption, and the compliance of the system. The objective of this study was to determine the reference values in healthy elderly subjects.

Methods: Elderly people (60–82 years), who considered themselves healthy, were recruited through an advertisement in the local newspaper. All were evaluated with a 3-T MRI. Subjects were eligible if they did not have any psychiatric or neurologic disorder or signs of advanced atherosclerotic disease. CSF resting pressure (ICP) and R_out were determined by a constant pressure infusion method with the patient in the supine position. The study population consisted of 40 subjects (mean age 70 years; 23 women).

Results: The median ICP was 11.6 mm Hg (15.8 cmH₂O) and the reference interval was ICP 7.8–14.3 mm Hg (10.6–19.4 cmH₂O) (defined as 5th to 95th percentiles). The median R_out was 8.6 mm Hg/mL/min. The variation in R_out was large and not normally distributed. The 90th percentile of R_out was 17.4 mm Hg/mL/min.

Conclusions: This study reports reference values for ICP and R_out and should be used for comparison when investigating disorders with suspected CSF dynamic disturbances in the elderly. ICP was in the same range as that reported in the young and middle-aged. The upper limit of R_out was higher than previously believed to be the upper limit of normal for this age group.