Microbiological Diagnosis of Central Nervous System Infections
The central nervous system (CNS) may be infected by various organisms, including bacteria, viruses, fungi, and parasites. Numerous noninfectious etiologies may mimic CNS infections. The causative organisms in postoperative infections are quite different from those of nonsurgical infections. The patient′s endogenous skin flora, primarily gram-positive bacteria, are a frequent cause of postoperative infection. Hospital-acquired pathogens, including Staphylococcus aureus, gram-negative bacilli, and fungi, are the other group of pathogens commonly encountered; the latter group are often resistant to multiple antimicrobial agents. CNS infections often necessitate emergent interventions because of high mortality and complications. This chapter reviews the microbiological diagnosis of CNS infections.
Specimen Selection and Collection
Selection of the appropriate specimens and tests for the detection of causative organisms is crucial in treating infectious diseases. Whenever possible, antimicrobial therapy should be held until appropriate specimens are collected unless delay in treatment poses an unacceptable risk to the patient.
Cerebrospinal Fluid
Lumbar puncture with cerebrospinal fluid (CSF) analysis is usually essential to diagnose CNS infections. Typical CSF findings in meningitis are shown in Table 2.1 .
Opening Pressure and Appearance
The CSF opening pressure is measured with an air–water manometer. In adults, the normal CSF opening pressure ranges from 50 to 190 mm H2O. Variation of pressure with deep respiration provides assurance that fluid flow into the manometer is unobstructed. Opening pressure in bacterial meningitis ranges from 200 to 500 mm H2O.
The CSF is normally clear and colorless but may appear cloudy or turbid in patients with increased concentrations of white blood cells (> 200/mm3), red blood cells (> 400/mm3), bacteria (> 105 colony-forming units/mL), or protein. In patients with a traumatic tap, an initially bloody CSF should clear as the flow of CSF continues.
Cell Count
The normal CSF white blood cell (WBC) count in children and adults is 0 to 5/mm3. The CSF WBCs may be slightly higher in neotates. In a study of uninfected neonates aged 0–28 days, the median count was 3 WBCs/mm3, but it was up to 19/mm3 (95th percentile). It decreased to a median of 2 WBCs/mm3. The 95th percentile was 9 WBCs/mm3.1 The CSF WBC count may be increased after neurosurgical procedures. Elevations of CSF WBC counts can occur in patients with traumatic lumbar puncture or in patients with intracerebral or subarachnoid hemorrhage. In these situations, the following formula should be used as a correction factor for the true WBC count in the presence of CSF red blood cells (RBCs):
Corrected WBC in CSF = Total WBC in CSF – (WBC in Blood × RBC in CSF/RBC in Blood)
Glucose and Protein
The actual CSF glucose concentration may be falsely low in the presence of hypoglycemia; therefore, the CSF glucose should always be compared with a simultaneous serum glucose; the normal ratio of CSF glucose to serum glucose is approximately 0.6, and ratios of less than 0.5 should be considered abnormal. Lumbar CSF protein concentrations greater than 50 mg/dL and ventricular CSF concentrations greater than 15 mg/dL are considered abnormal.
A CSF glucose concentration of less than 34 mg/dL, a ratio of CSF glucose to blood glucose of less than 0.23, a CSF protein concentration greater than 220 mg/dL, a CSF leukocyte count of more than 2,000/mm3, or a CSF neutrophil count of more than 1,180/mm3 is highly predictive of bacterial meningitis.2
CSF Culture
From 1 to 2 mL of CSF specimen should be sent for bacterial culture, and optimally from 5 to 10 mL for mycobacterial and fungal culture.
Other Diagnostic Methods
Many immunologic tests and nucleic acid amplification tests are useful for the evaluation of CNS infections. Please refer to each section for details.
Acute Meningitis/Ventriculitis
Microbiology
Common organisms and useful diagnostic tests are shown in Table 2.2 .
Viral Meningitis
Enteroviruses (primarily echoviruses and coxsackieviruses) are the most common cause of viral meningitis. HSV can also cause a viral meningitis. HSV-2 meningitis is generally seen with a primary genital outbreak. HSV-1 may also cause viral meningitis. When this is recurrent it is thought to be a cause of Mollaret′s meningitis, which is defined as a benign, recurrent lymphocytic meningitis. Other herpesviruses, including HSV-1, varicella-zoster virus (VZV), cytomegalovirus (CMV), Epstein-Barr virus (EBV), and human herpesviruses types 6, 7, and 8 (HHV-6, -7, -8), cause aseptic meningitis and encephalitis. Aseptic meningitis and encephalitis due to mumps are commonly seen in unimmunized patients. From 5 to 10% of patients infected with human immunodeficiency virus (HIV) develop acute viral meningitis either when the virus is acquired or during the seroconversion phase.4 Arthropod-borne viruses, such as West Nile virus, cause meningoencephalitis in areas of endemicity.
Bacterial Meningitis
Streptococcus pneumoniae is the most common organism. S. pneumoniae, Neisseria meningitidis, S. aureus, other streptococci, Listeria monocytogenes, and Haemophilus species account for most of the community-acquired meningeal pathogens.5 Hospital-acquired meningitis typically occurs after neurosurgical procedures. Postoperative CSF leak and traumatic CSF leak are known risk factors for meningitis. About half of cases are caused by gram-negative bacilli (primarily Pseudomonas aeruginosa, Klebsiella species, and Escherichia coli). S. aureus and coagulase-negative staphylococci are also common.
Spirochetal Meningitis
Treponema pallidum invades the CNS during early infection. Early infections may result in cranial nerve palsies and particularly a facial nerve palsy. Clinical neurosyphilis is divided into four distinct syndromes: syphilitic meningitis and meningovascular syphilis usually occur within 5 years of infection, whereas parenchymatous neurosyphilis and gummatous neurosyphilis occur 10 to 20 years after infection.
Borrelia burgdorferi usually disseminates to the CNS early in infection. From 10 to 15% of patients with Lyme disease have nervous system involvement, including lymphocytic meningitis, in the third stage, several months to years after initial infection.6
Amebic Meningitis
Naegleria fowleri and Acanthamoeba species are the common organisms. N. fowleri usually lives in freshwater. It causes a rapidly fatal meningoencephalitis in healthy persons with recreational freshwater exposure. Granulomatous amebic encephalitis with Acanthamoeba species occurs in immunocompromised patients subacutely.
Helminthic Meningitis
Angiostrongylus cantonensis and Gnathostoma spingerum are the most common organisms of eosinophilic meningitis outside Europe and North America.7 They are acquired by eating infected hosts (i.e., snails or freshwater prawns for A. cantonensis, undercooked freshwater fish or chicken for G. spingerum).
Diagnosis
Tests that should be sent routinely when acute meningitis is suspected are summarized in Table 2.3 . Additional tests should be considered based on suspected organisms.
Viral Meningitis
Cerebrospinal Fluid
Viral meningitis usually causes a lymphocytic pleocytosis with mildly elevated protein and decreased glucose concentrations; however, one study reported that about half of patients with viral meningitis had a neutrophil predominance in the CSF for more than 24 hours after the onset of symptoms.8 A CSF neutrophil predominance is not useful as a sole criterion in distinguishing between aseptic and bacterial meningitis. Viral culture is not clinically useful because it takes too long for identification and the sensitivity is low.9
White blood cell count with differential |
Red blood cell count |
Glucose concentration |
Protein concentration |
Gram stain |
Bacterial culture |
Immunology
Paired serology is diagnostic for herpes simplex encephalitis and enterovirus infections but is not useful clinically as serocon-version typically takes weeks, emphasizing the importance of polymerase chain reaction (PCR) in the detection of viruses in CSF specimens. Complement fixation and hemagglutination inhibition in serum specimens are the most reliable for mumps meningitis because the viral culture is not practically useful. Testing of paired acute and convalescent sera, collected 2–3 weeks apart, should demonstrate a diagnostic fourfold rise in mumps antibody titer.
Nucleic Acid Amplification Tests
PCR is the standard test for the detection of enteroviruses and HSV, VZV, EBV, CMV, or HHV-6 meningoencephalitis. The sensitivity of PCR for HSV encephalitis were 98% and the specificity were 94%.10 PCR may become positive only a few days after the onset of symptoms.
Bacterial Meningitis
Cerebrospinal Fluid
The CSF WBC count is usually elevated to 1,000 to 5,000/mm3; however, it can be normal in neonates and in patients with meningococcal or L. monocytogenes meningitis. Also, neutrophils usually predominate, although approximately 10% of patients present with a predominance of lymphocytes in the CSF. This is more common in neonatal gram-negative bacillary meningitis and meningitis caused by L. monocytogenes.11 Patients with very low CSF WBC counts (0–20/mm3) despite high CSF bacterial concentrations tend to have a poor prognosis.12
Elevated CSF protein and decreased CSF glucose concentrations are seen in bacterial meningitis; however, normal CSF WBC and protein concentrations may be seen in patients with neonatal meningitis and severely immunocompromised patients and in specimens obtained at the onset of symptoms.
Cerebrospinal Fluid Culture
Gram stain of the CSF shows bacteria in 90% of S. pneumoniae and Haemophilus influenzae cases, 75% of N. meningitidis cases, 50% of gram-negative bacilli cases, and 24 to 50% of L. monocytogenes cases.13,14 Prior antimicrobial therapy decreases the probability of identifying organisms by 50%.
CSF culture is considered a gold standard. The probability of identifying the organism may be decreased to less than 50% in patients with prior antimicrobial therapy. Fungal culture and acid-fast bacillus (AFB) culture need to be considered in immunocompromised patients or patients with subacute or chronic symptoms.
Blood Culture
Blood culture is also crucial to diagnose bacterial meningitis. The causative organism was recovered from 86% of pediatric patients.15 Blood samples should be drawn at first, even if antimicrobial therapy needs to be started before lumbar puncture because of clinical deterioration.
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