meningitis, including fever; therefore, the diagnosis may depend entirely upon examination of CSF and careful observation If a shunt reservoir is present, then CSF may be obtained with a shunt tap As mentioned in a prior section of the chapter, a CT scan or MRI of the brain should be performed prior to lumbar puncture Lumbar puncture in the backdrop of unrecognized hydrocephalus or mass lesion may risk a potentially fatal herniation syndrome The manifestations of postoperative meningitis are often much more subtle than those of the typical pneumococcal or meningococcal variety If signs of meningeal irritation should occur in isolation or in association with any other changes, neurologic or metabolic, examination of the CSF is mandatory before any antibiotics are administered Because cell count, glucose concentration, and protein concentration are abnormal after craniotomy, an absolute diagnosis must await the result of CSF culture or the demonstration of bacteria on Gram stain Empiric treatment with broad-spectrum intravenous antibiotics should be started immediately following LP and directed at gram-positive cocci and gram-negative organisms, as described in the previous section The antibiotic regimen should then be tailored once the final culture results and sensitivities have been obtained Ventriculitis The clinical picture of ventriculitis differs little from that of meningitis, although the presentation is usually much more subtle Meningeal symptoms may be minimal and fever variable, whereas alteration in mental status and neurologic function predominate Both meningitis and ventriculitis tend to occur in the postoperative period more than days after violation and contamination of the subarachnoid or ventricular space The only diagnostic test is microscopic and bacteriologic examination of the ventricular fluid As with meningitis, broad-spectrum antibiotics should be initiated pending Gram stain and culture results Abscess Brain abscess, or its immediate precursor, cerebritis, is relatively rare in the postoperative period If an abscess does not communicate with the ventricular or subarachnoid space, meningeal signs will usually be absent The development of meningeal signs or infected CSF in the face of focal deficits must heighten the clinician’s suspicion for abscess However, in 95% of cases of cerebral abscess, the CSF may be completely normal and