Why CT before LP in Suspected Meningitis?

Traditional thinking was if there is a space occupying lesion, it may result in increased ICP. When you perform a LP, it will drop pressure at the site of the LP => creating a pressure gradient and result in cerebral herniation.

Hence it is very common to request computed tomography (CT) to rule out space occupying lesions with +/- mass effect before lumbar puncture (LP).

This has become a contentious subject. Many will argue that a normal CT doesn’t reliably rule out the risk of herniation post LP. Hence its diagnostic utility in patients without any signs of raised ICP (i.e Papilledema, focal neurologic signs, seizures, GCS <12, Cushing triad) or risk factors (i.e immunocompromise, history of CNS disease, seizures) is questionable. The clinical findings and patient history should always be considered when deciding on CT before LP in the setting of suspected meningitis.

RULE OF THUMB: you should never delay antibiotics to do a CT then LP in the setting of meningitis. Treat empirically!!!

References

1. Rodrigo Hasbun, M.D., James Abrahams, M.D., James Jekel, M.D., and Vincent J. Quagliarello, M.D. Computed Tomography of the Head before Lumbar Puncture in Adults with Suspected Meningitis. N Engl J Med 2001; 345:1727-1733.
2. Gopal, A et al. Cranial computed tomography before lumbar puncture: a prospective clinical evaluation. ARCH INTERN MED. Dec 1999; 159:2681-2685