Infections of the Central Nervous System
MLAB 2434 – Microbiology Keri Brophy-Martinez
General Concepts
Infections of the CNS are of critical concern and positive laboratory findings are “critical values”
Infections may be caused by bacteria, fungi, viruses or parasites
Anatomy of CNS
Brain, Spinal Cord and Cranial Nerves Meninges
Dura mater Pia arachnoid Pia mater
Subarachnoid space between pia arachnoid and pia mater; CSF
CSF
Unique body fluid produced by filtration and secretion from specialized capillary tufts of the four ventricles of the brain
Circulates around brain and spinal cord under pressure
Serves as cushion for brain and spinal cord
CSF Characteristics
CSF is a clear and colorless sterile fluid Adults
• Protein 15-45 mg/dL• Glucose 40-80 mg/dL• WBC 0-5 mm3
Newborns• Higher levels of protein & glucose
Infections indicated by increased cell counts and alterations in protein and glucose
Portals of entry for bacteriaRespiratory (most common route
of community-acquired infections)
AuditoryBloodstreamNeural routesContiguous sites
CNS Infections
Types of CNS Infections
Meningitis Also called “leptomeningitis” (pia
mater & arachnoid) Inflammation around blood vessels
within the subarachnoid space Classifications
• Acute• Chronic• Aseptic
Bacterial CNS Infections
Acute Bacterial MeningitisPeaks in winter & early springSymptoms include photophobia,
headache, nausea, vomiting, and stiff neck or Brundzinski sign
In infants, only symptoms may be irritability, poor feeding, and restlessness
Bacterial CNS Infections (cont’d) Acute Bacterial Meningitis (cont’d)
Infants• Group B Strep• L. monocytogenes• H. influenzae• S. pneumoniae• E. coli
Children• N. meningitidis• S. pneumoniae• H. influenzae
Bacterial CNS Infections (cont’d) Acute Bacterial Meningitis (cont’d)
Adolescents• N. meningitidis• S. pneumoniae
Adults• N. meningitidis• S. pneumoniae
Elderly• Gram-negative bacilli• N. meningitidis• S. pneumoniae
Bacterial CNS Infections (cont’d) Acute Bacterial Meningitis
(cont’d)Lab findings
• Leukocytosis with left shift and toxic changes, such as toxic granulation & Dohle bodies
• Increased CSF protein• Decreased CSF glucose
Increased intracranial pressure
Types of CNS Infections (cont’d)
“Aseptic Meningitis”CSF shows predominance of
lymphocytes with no organisms on direct examination
Commonly caused by viruses• Enterovirus• Arbovirus• Mumps virus• Herpes Simplex Virus
Types of CNS Infections (cont’d) Encephalitis/ Meningoencephalitis
Diffuse inflammation of the cerebral cortex
Observe mental changes or neurologic signs
Common in viral infections
Types of CNS Infections (cont’d)
Brain Abscesses Areas of tissue destruction containing
organisms and inflammatory cells Occur as spread from other body sites
or trauma to skull; infecting organism depends on predisposing conditions
Non-traumatic abscesses mostly are aerobic and anaerobic streptococci, Staphylococcus aureus, anaerobic GNR
Bacterial CNS Infections (cont’d) Mycobacterial Infection
Most commonly M. tuberculosis Enters by respiratory route and spread
via bloodstream CSF cells are mostly lymphocytes and
monocytes Mycobacteria are few and CSF should
be centrifuged to concentrate before culturing/gram staining
Bacterial CNS Infections (cont’d) Spirochetal Infections
Lyme Disease Neurosyphilis
Viral Infections Enterovirus Arboviruses Herpes Virus
Parasitic Infections Toxoplasma gondii Naegleria fowleri
Fungal Infections Crytococcus neoformans Candida species
Laboratory Diagnosis of CNS Infections CSF Collection
Lumbar puncture in lower back 3 to 4 tubes collected
• 1st = chemistries• 2nd = microbiology• 3rd = cell counts• 4th = special procedures (if collected)
Process ASAP
Laboratory Diagnosis of CNS Infections (cont’d) CSF Analysis
Acute Bacterial Meningitis• CSF turbid or cloudy• WBC increased with predominance of neutrophils• Increased CSF protein• Decreased CSF glucose
Centrifugation concentrates organisms Should be plated on at least BAP, CA and
broth; MAC if Gram-negative bacilli are expected
Bacterial Antigen Testing • Current literature indicates routine bacterial
antigen testing is of limited value
Laboratory Diagnosis of CNS Infections (cont’d) CSF Analysis
Viral Meningitis• Number of lymphocytes increased• Diagnosis based on PCR or EIA
Fungal Meningitis• Gram stain or India ink• Culture held for 4-6 weeks
Laboratory Diagnosis of CNS Infections (cont’d) CSF Analysis
Tuberculous meningitis• Increase in lymphocytes• Increase in protein• Decrease in glucose• AFB stain & culture • PCR can be used; however, false
positives common
Characteristic Findings in Meningitis
Bacterial Fungal Tuberculous Viral Syphilitic Parasitic
Organisms seen in CSF
See notes See notes See notes None None See notes
Cell count
(leukocytes/mL)
100-100,000
Neutrophils predominate
Normal- 500
Lymphocytes
predominate
50-500
Lymphocytes
predominate
Normal-200
Lymphocytes
predominate
100-750
Lymphocytes
predominate
Normal-200
Lymphocytes/
Eosinophils
predominate
Protein (mg/dL) 100-500 Normal- 250 Normal- 150 Frequently normal
50- 250 Usually increased
Glucose (mg/dL) <30 Normal- decreased
<45 Normal Normal Normal- decreased
References
Kiser, K. M., Payne, W. C., & Taff, T. A. (2011). Clinical Laboratory Microbiology: A Practical Approach . Upper Saddle River, NJ: Pearson Education.
Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook of Diagnostic Microbiology (4th ed.). Maryland Heights, MO: Saunders.
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