• CSF outflow

We read the paper by Kahlon et al with great interest.¹ Comparative studies about the use of different diagnostic techniques to predict the response to shunting in hydrocephalus are of great value as they are likely to form a landmark for future clinical practice. Therefore, it is of paramount importance that the procedures taken for comparison are methodologically sound.

Unfortunately, the interpretation of the lumbar infusion study given by the authors raises our concern. For unknown reasons, the authors have taken into account only the end equilibrium pressure obtained during a constant rate lumbar infusion and neglected the baseline CSF pressure. The authors presumed that this pressure was the same in everybody and equal to the value resulting from the mean taken from the whole cohort (11 mm Hg).

Interpretation of the infusion study can be based only partially on the resistance to CSF outflow (Rcsf), with other parameters describing CSF dynamics like the baseline pressure, elasticity, etc, taken into account.^2–4 The resistance to CSF outflow is, undoubtedly the most important parameter, about which a number of independent studies have been conducted in the past,^2,5 including a quite recent multicentre Dutch trial.⁶

The proper way to calculate Rcsf results from the well known Davson’s formula²:

ICP reached during infusion is equal …