In adults, with a typical cranial CSF volume of at least 150 mL,26 it has been suggested that collection of up to 30 mL is safe8; CSF is synthesised at a rate around 0.35 mL/min,16 and so this volume will be replaced within a few hours at most.

By contrast, the total cranial CSF volume of neonates is around 40 mL at term,27 rapidly increasing in the first 1–2 years of life.26 There are no studies examining the maximum volume of CSF which can safely be taken from a neonate or infant, outside the context of hydrocephalus.

Beyond the age of around 2 years, MRI studies suggest cranial CSF volume is similar to adults,26 and thus collection of 30 mL is unlikely to be problematic.

IIH is a condition with raised ICP without a structural cause (eg, tumour or hydrocephalus) and normal CSF composition and can be defined by Friedman’s diagnostic criteria. It is also known as pseudotumour cerebri syndrome and was previously called benign intracranial hypertension. Patients typically present with headaches and may have visual symptoms and signs on examination, typically including papilloedema, with an otherwise normal neurological examination.19

When performing an LP for diagnosis of IIH, if CSF pressure is found to be raised, aim for a closing pressure within the normal range (eg, between 20 cmH2O and 25 cmH2O in most children above the age of 1 year).19

Some clinicians suggest not reducing pressure to less than 30 cmH2O if CSF pressure is exceptionally high (ie, greater than 60 cmH2O) to avoid risks of post-LP headaches, but this is not evidence-based.7 There is no agreed position on the management of such patients who are symptomatic for IIH but are found to have normal opening pressure.

CSF, cerebrospinal fluid; ICP, intracranial pressure; IIH, idiopathic intracranial hypertension; LP, lumbar puncture.