Mechanism of CSF drainage benefits unclear in normal pressure hydrocephalus

By Eleanor McDermid

The beneficial effect of removing cerebrospinal fluid (CSF) from patients with normal pressure hydrocephalus (NPH) is not mediated by reduction of ventricle volume, research suggests.

This would be an obvious mechanism, say Niklas Lenfeldt (Umeå University, Sweden) and colleagues, but clinical improvement does not seem to correlate with ventricular reduction after shunting. In this study, to avoid the difficulties of quantifying CSF removal during shunting, the team assessed the effect of external lumber drainage (ELD), often used as a test of whether NPH patients will respond to a shunt.

"A clinical aspect of ELD is that the initial effects on symptoms, if present, occur quite quickly, similar to the reaction to tap tests. This result makes it plausible that it is the effect from the individual withdrawals, not the accumulated effect, that is relevant for improvement," say the researchers.

"This view is also supported by our finding that total drain volume is not connected to clinical features."

They suggest that the temporary period of reduced pressure after each extraction may allow time for neurologic recovery.

The team removed an average 415 mL of CSF from 15 patients with NPH. Fluid was removed every 3 hours to a target volume of 25 mL each time and a total target volume of 500 mL. This was associated with a reduction in ventricular size of 3.7 mL on average, or a 4.2% contraction.

Ventricular volume reduction and total drainage volume did not correlate; the seven patients who had drainage volumes closest to the 500 mL target had ventricular volume reductions ranging from 1.3 to 7.5 mL.

Half of the patients had clinical improvements after ELD. The researchers witnessed improved gait in seven, faster walking speed in five, and also saw improvements in measures of cognition and executive function. However, neither CSF drainage volume nor any measure of ventricular change was associated with clinical improvements.

Lenfeldt et al note that although ventricular contraction may not reflect overall CSF volume reduction, no study has yet assessed changes in the spinal CSF compartment.

"Thus, the complete picture in terms of postdrain changes of the CSF system is not yet in place," they write in Neurology. "Further research in imaging the entire craniospinal CSF system before and after both shunting and ELD, including assessment of pressure, is vital to get a holistic view of how CSF volume changes affect clinical improvement and why ELD in some cases fails to predict shunt responsiveness in patients with NPH."

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