The highest estimates of risk of lumbar puncture come from studies such as those of Rennick, who reported a 4 percent incidence of clinical worsening among 445 children undergoing lumbar puncture for the diagnosis of acute meningitis; most other series give a lower number. It must be pointed out that cerebellar tonsillar herniation may occur in fulminant meningitis independent of lumbar puncture; therefore the risk of the procedure may be even less than usually stated. In an attempt to determine the utility of the CT scan performed prior to a lumbar puncture, Hasbun and colleagues were able to identify several clinical characteristics that were likely to be associated with an abnormality on the scan in patients with suspected meningitis; these included a recent seizure, coma or confusion, gaze palsy, and other findings. The more salient findings from this study in our opinion was that only 2 percent of 235 patients had a focal mass lesion that was judged a risk for lumbar puncture and none had herniation after the procedure; many had CT findings of interest, including some with diffuse mass effect. This study does not entirely clarify the issue of the safety of lumbar puncture but it emphasizes that patients who lack major neurologic findings are unlikely to have findings on the scan that will preclude lumbar puncture.
Therefore, if there is clinical evidence of a focal lesion with increased intracranial pressure, then CT or MRI scanning of the head, looking for a mass lesion, is may at times be a prudent first step, but in most cases this is not necessary and should not delay the administration of antibiotics.
Only a sizable brain abscess or substantial brain swelling entirely interdicts a lumbar puncture in suspected bacterial meningitis. Furthermore, the fact that death results from cerebral herniation in many fatal cases of bacterial meningitis does not, of course, mean that lumbar puncture precipitated the demise. When there are signs of impending herniation or indications of a dangerous configuration on cerebral images, one may wish to draw blood cultures and and institute empiric treatment rather than take the small risk of precipitation herniation with a lumbar puncture. Any coagulopathy that is deemed a risk for hemorrhagic complication of lumbar puncture should be rapidly reversed if possible.
The spinal fluid pressure is so consistently elevated (above 180 mm H2O) that a normal pressure on the initial lumbar puncture in a patient with suspected bacterial meningitis suggests another diagnosis or raises the possibility that the needle is partially occluded or the spinal subarachnoid space is blocked. Pressures over approximately 350 mm H2O suggest the presence of brain swelling and the potential for cerebellar herniation. Some neurologists favor the administration of intravenous mannitol if the pressure is this high, but this practice does not provide assurance that herniation will be avoided.
A pleocytosis in the spinal fluid is diagnostic of meningitis. The number of leukocytes ranges from 250 to 100,000/mm3, but the usual number is from 1,000 to 10,000. Occasionally, in pneumococcal and influenzal meningitis, the CSF may contain a large number of bacteria but few, if any, neutrophils for the first few hours. Cell counts of more than 50,000/mm3 raise the possibility of a brain abscess having ruptured into a ventricle. Neutrophils predominate (85 to 95 percent of the total), but an increasing proportion of mononuclear cells is found as the infection continues for days, and especially in partially treated meningitis. In the early stages, careful cytologic examination may disclose that some of the mononuclear cells are myelocytes or young neutrophils. Later, as treatment takes effect, the proportions of lymphocytes, plasma cells, and histiocytes steadily increase. However, there are rare instances of low numbers or absent cells in the spinal fluid with meningitis, particularly in the context of pronounced neutropenia and forms of immunosuppression.