Infectious Diseases in Critical Care Medicine

The various causes of viral/aseptic meningitis are uniformly associated with a normal
CSF glucose with a few important exceptions. The presence of a normal CSF glucose in a
patient with suspected meningitis argues strongly against a bacterial tuberculous or fungal
etiology and suggests a viral or noninfectious mimic of meningitis, i.e., carcinomatous
meningitis. The viruses that are capable of decreasing the CSF glucose include HSV,
lymphocytic choriomeningitis (LCM), mumps, and occasionally enteroviruses. With these
exceptions aside, a normal CSF glucose virtually excludes a bacterial etiology of ABM
(1,5,8,14,18,28,29).
RBCs are not a feature of ABM, and the physician should suggest an alternate
explanation for the patient’s symptoms. Excluding a traumatic tap, CNS leaking aneurism, etc.,
RBCs in the CSF limit diagnostic possibilities toL. monocytogenes, amebic meningoencephalitis,
leptospirosis, tuberculous meningitis, HSV, and anthrax. RBCs in the CSF can also decrease the
CSF glucose and increase the CSF lactic acid. The abnormalities in CSF glucose and lactic acid
are proportional to the number of RBCs present in the CSF, and can account for mild to
moderate abnormalities in these two CSF parameters (1,5,8,18,27).
The white blood cell response in the CSF typically is early and brisk with bacterial
meningitis. Many CNS infections characteristically associated with a lymphocytic predominance
often present acutely with a PMN predominance, e.g., tuberculosis (TB), fungi, syphilis, and
viruses. With the exception of HSV-1, only ABM presents with a CSF PMNs90%. Patients with
partially treated meningitis have a mixed picture with both PMNs and lymphocytes as well as a
moderately decreased glucose versus the profoundly decreased glucose and untreated ABM,
and will have CSF lactic acid levels that are intermediate between aseptic/viral meningitis and
ABM. The clinician uses not only the CSF Gram stain but analyzes the patient’s clinical
information integrating the CSF findings of the number of WBCs in relationship to the PMN
predominance glucose levels, CSF lactic acid levels, and the presence or absence of RBCs in the
absence of trauma to correctly analyze CSF findings (1,5,8,30) (Tables 10 and 11).

Table 10 CSF Gram Stain Clues in Meningitis

.Purulent CSF/no organisms seen
Neisseria meningitidis
Streptococcus pneumoniae
.Cloudy CSF/without WBCs
S. pneumoniae
.Gram-positive bacilli
Listeria monocytogenes
Pseudomeningitis(Bacillus,Corynebacterium,etc)
.Gram-negative bacilli
Haemophilus influenzae(small encapsulated,
pleomorphic)
Enteric aerobic GNBs (larger, unencapsulated)
.Gram-positive cocci
Group A, B streptococci (pairs and chains)
S. pneumoniae(pairs)
Staphylococcus aureus(clusters)
Staphylococcus epidermidis
VA/VP shunt infections only (clusters)
.Gram-negative cocci
N. meningitidis
.Mixed organisms/polymicrobial
Pseudomeningitis
Brain abscess with meningeal leak
VP shunt infection
DisseminatedStrongyloides stercoralis
Meningitis (2 8 to penetrating head trauma)

.Clear CSF/no organisms seen

Viral meningitis

TB/fungal meningitis

Neurosarcoidosis meningitis

Early bacterial meningitis

Partially treated bacterial meningitis

Meningitis in leukopenic hosts

Meningeal carcinomatosis

Brain abscess

Parameningeal infection

Bland emboli (2 8 to SBE)

Cerebritis

Neuroborreliosis

LCM

L. monocytogenes

HIV

Syphilis

Leptospirosis

Abbreviations: RMSF, Rocky Mountain spotted fever; SBE, subacute bacterial endocarditis; HIV, human
immunodeficiency virus; VA, ventriculo-atrial; VP, ventriculo-peritoneal; LCM, lymphocytic choriomeningitis.

Meningitis and Its Mimics in Critical Care 145