In patients with bacterial meningitis, chest films are essential because they may disclose an area of pneumonia or abscess. Sinus and skull films may provide clues to the presence of cranial osteomyelitis, paranasal sinusitis, mastoiditis, or cranial osteomyelitis, but these structures are better visualized on CT scans, which have supplanted conventional films in most cases. The CT scan is particularly useful in detecting lesions that erode the skull or spine and provide a route for bacterial invasion, such as tumors or sinus wall defects, as well as in demonstrating a brain abscess or subdural empyema. MRI with gadolinium enhancement may display the meningeal exudate and cortical reaction, and both types of imaging, with appropriate techniques, will demonstrate venous occlusions and adjacent infarctions. The issues pertaining to an abscess and to brain swelling in meningitis have already been noted and are disucussed further on as well. We find it advisable to obtain some of these imaging procedures.
Recurrent Bacterial Meningitis (See Chap. 32)
This is observed most frequently in patients who have had some type of shunting procedure for the treatment of hydrocephalus or who have an incompletely closed dural opening after cranial or spinal surgery. When the origin of the recurrence is inapparent, one should suspect a congenital neuroectodermal sinus or a fistulous connection between the nasal sinuses and the subarachnoid space. The fistula in these latter cases is more often traumatic than congenital in origin (e.g., a previous basilar skull fracture), although the interval between injury and the initial bout of meningitis may be several years. The site of trauma is in the frontal or ethmoid sinuses or the cribriform plate, and S. pneumoniae is the usual pathogen. Often it reflects the predominance of such strains in nasal carriers. These cases usually have a good prognosis; mortality is much lower than in ordinary cases of pneumococcal meningitis. Neurenteric cysts, although rare, are another source of recurrent meningitis.
CSF rhinorrhea (or otorrhea) is present in some cases of posttraumatic meningitis, but it may be transient and difficult to find. Suspicion of its presence is raised by the recent onset of anosmia or by the occurrence of a watery nasal discharge that is salty to the taste and increases in volume when the head is dependent. One way of confirming the presence of a CSF leak is to measure the glucose concentration of nasal secretions; ordinarily they contain little glucose, but in CSF rhinorrhea the amount of glucose approximates that obtained by lumbar puncture (two-thirds of the serum value). A “dipstick” used for urine testing is sometimes adequate but these are regrettably decresingly available on general hospital wards. Another bedside test for CSF rhinorrhea or otorrhea is to estimate the amount of protein in the fluid. A high protein, sufficient to make a handkerchief stiff on drying, suggests it is of nasal mucosal origin. If the fluid fails to cause a handkerchief to stiffen on drying, a spinal fluid leak is suspected. The most specific and sensitive test for CSF otorrhea and rhinorrhea may be the finding of β2-transferrin (tau), in fluid collected in a polypropylene tube, a substance not found in fluids other than CSF.
The site of a CSF leak can sometimes be demonstrated by injecting a dye, radioactive substance (radionuclide), or water-soluble contrast material into the spinal subarachnoid space and detecting its appearance in nasal secretions or observing its site of exit by CT scanning. This testing is best performed after the acute infection has subsided. Persistence of CSF rhinorrhea or a spinal CSF leak usually requires surgical repair.