What is the role of PCR of Cerebrospinal Fluid (CSF) in diagnosing Primary Amoebic Encephalitis (PAE) and Granulomatous Amoebic Encephalitis (GAE)?

PCR of CSF in Primary Amoebic Encephalitis (PAE) and Granulomatous Amoebic Encephalitis (GAE)

PCR testing of CSF has limited utility for diagnosing amoebic encephalitis and is only available through specialized laboratories like the CDC, while direct microscopic examination of CSF remains the most practical diagnostic approach for PAE, particularly when performed on fresh specimens.

Diagnostic Role of PCR in Amoebic Encephalitis

Primary Amoebic Encephalitis (PAE - Naegleria fowleri)

• Direct microscopic examination of CSF is the primary diagnostic method for PAE, with motile trophozoites visible on wet mount preparation of fresh CSF 1
• PCR testing for Naegleria fowleri is available only at specialized laboratories such as the CDC and includes PCR testing on brain tissue, other tissue specimens, and CSF 1
• The diagnosis of PAM was made before death in only 27% of cases in the United States, highlighting the diagnostic challenges even with available testing 2
• CSF wet mount is specifically recommended for Naegleria fowleri testing and should be performed immediately on fresh specimens 1

Granulomatous Amoebic Encephalitis (GAE - Balamuthia mandrillaris and Acanthamoeba spp.)

• PCR testing for Balamuthia mandrillaris and Acanthamoeba species is only available at specialized laboratories and includes serum immunofluorescence assay, immunohistochemistry on brain or other tissue, and PCR testing on brain tissue, other tissue, and CSF 1
• Next-generation sequencing (NGS) of CSF has successfully identified Balamuthia mandrillaris in GAE cases, with most mapped reads belonging to the organism 3
• PCR on Chelex-treated extracts from CSF has confirmed Balamuthia mandrillaris identification post-mortem, suggesting that PCR studies on CSF may be valuable diagnostic tools 4
• Brain tissue from the affected region offers optimal sensitivity and specificity, but other specimens including CSF can be tested 1

Practical Diagnostic Approach

When to Consider Amoebic Encephalitis

• PAE should be suspected in children and young adults with acute meningoencephalitis following freshwater exposure (swimming, diving, water sports) in warm bodies of water 2
• GAE should be considered in patients with subacute to chronic encephalitis, particularly those who are immunocompromised, with white matter lesions or basilar meningeal enhancement on imaging 1, 5

CSF Characteristics

For PAE (Naegleria fowleri):

• CSF resembles bacterial meningitis with high opening pressures, elevated WBC counts with predominantly neutrophils (median 2400 cells/μL), low glucose (median 23 mg/dL), and elevated protein (median 365 mg/dL) 2
• Immediate examination of fresh CSF for motile trophozoites is critical 1, 2

For GAE:

• CSF typically shows lymphocytic pleocytosis, but amoebic trophozoites may be detected during routine cytological examination 6
• Culture using human brain microvascular endothelial cells (HBMECs) can isolate Balamuthia mandrillaris from CSF, though cytopathic effects may take 20-28 days 4

Critical Limitations and Pitfalls

• PCR testing is not routinely available and requires coordination with specialized laboratories or the CDC 1
• The delay in obtaining PCR results makes it impractical for acute management decisions in PAE, which progresses rapidly and is nearly always fatal 2
• For suspected PAE cases, clinicians should immediately contact the CDC at 770-488-7100 (available 24/7) to discuss diagnostic testing and treatment options 2
• Imaging results are abnormal in approximately three-fourths of PAE patients but are not diagnostic for amoebic infection 2
• Early identification of amoebae in CSF is mandatory to prevent fatal outcomes, making immediate microscopic examination more valuable than waiting for PCR results 6, 5