The critical laboratory test in ABM is analysis of the CSF. In ABM, there is usually a pleocytosis
of the CSF. In ABM, the cells in the CSF are nearly all PMNs. As the meningeal infection is
treated, the number of PMNs decreases and there is a parallel rise in the number of CSF
lymphocytes. Bacterial meningitis begins with a PMN predominance and ends with a
lymphocytic predominance. Other CNS infections, e.g., tuberculosis, viral infections, fungal
infections, and syphilis, may all present initially with a PMN-predominant pleocytosis. These
disorders are characterized by a lymphocytic CSF pleocytosis, but initially may present with a
PMN predominance. Importantly, with the exception of HSV-1,90% PMNs in the CSF
initially always indicates ABM. A PMN predominance of<90% is compatible with a wide
variety of CNS pathogens and does not, of itself, indicate a bacterial etiology. In patients with
fever and nuchal rigidity, a lumbar puncture should always be performed before a head CT/
MRI scan is obtained. Patients with bacterial meningitis are acutely ill and have a potentially
rapidly fatal disorder. To waste valuable time obtaining a head CT/MRI can result in a fatal
outcome. Fear of supratentorial herniation is the main reason why head imaging studies are
done before lumbar puncture, which is appropriate if a mass lesion is suspected, but not if the
diagnosis includes ABM. Far more people will die from a delay in therapy than have died from
supratentorial herniation (1–5,18,25,26) (Table 6 to 9).
Table 5 Complications of Meningitis
Complications Associated Organisms
.Deafness/hearing loss Haemophilus influenzae
Neisseria meningitidis
TB
RMSF
Mumps
.Seizures Streptococcus pneumoniae(early)
H. influenzae
Group B streptococci
HSV-1
Septic arthritis
Histoplasmosis
TB
Brain abscess
H. influenzae
.Subdural effusions S. pneumoniae
N. meningitidis
.Septic arthritis S. aureus
.Hemiplegia S. pneumoniae
.Cerebral-vein thrombosis H. influenzae(associated Jacksonian seizures)
.Hydrocephalus H. influenzae
TB
Neurosarcoidosis
Neurocysticercosis
.Cranial nerve abnormalities N. meningitidis(CN VI, VII, VIII)
Tuberculosis (CN VI)
Neurosarcoidosis (CN VII)
Meningeal carcinomatosis (multiple CNs)
.Herpes labialis N. meningitidis
S. pneumoniae
.Panophthalmitis N. meningitidis
S. pneumoniae
H. influenzae
.Purpura/petechiae or shock N. meningitidis
S. pneumoniae
Listeria monocytogenes
Staphylococcus aureus
Abbreviations: RMSF, Rocky Mountain spotted fever; HSV, herpes simplex virus.
142 Cunha and Smith