Laboratory Studies
For practical purposes, N fowleri meningoencephalitis must be rapidly diagnosed. Patients who present with a clinical picture of meningitis (ie, fever, headache, meningismus, nausea and vomiting) should undergo a spinal tap as soon as they present.
In patients with primary amebic meningoencephalitis (PAM), the cerebrospinal fluid (CSF) pressure is often elevated, and the CSF is hemorrhagic. The while blood cell (WBC) count can be within the reference range in early infections but rapidly increases to range from 400-26,000 cells/µL with a neutrophilic predominance. The CSF glucose level may be low or within the reference range, but the CSF protein is usually elevated. Results on a Gram stain of the CSF sediment are negative for bacteria. A wet mount must be made because the trophozoites of N fowleri lyse during the heat fixation that precedes the Gram stain. On the wet preparation, motile trophozoites are evident. Care must be taken to avoid confusing N fowleri trophozoites with WBCs and vice versa. In examining CSF for N fowleri, a regular glass slide for a wet mount is preferred to a WBC counting chamber. The regular glass slide allows for better definition of internal structures.
The CSF is centrifuged at 150g for 5 minutes. The supernatant is carefully aspirated, and the sediment is gently suspended in the remaining fluid. A drop of this suspension is placed on a slide and covered with a No. 1 coverslip. The slide is observed under a compound microscope using 10 and 40 objectives. Phase contrast optics is preferable. The slide may be warmed to 35°C (to promote amebic movement). The amebae are detected based on their active directional movements. CSF indices in N fowleri include the following:
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CSF protein levels are elevated.
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CSF glucose levels are within the reference range or reduced.
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CSF WBC count is elevated (400-26,000 cells/µL).
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CSF RBC count is high, and the CSF is often hemorrhagic.
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CSF Gram stain results are negative for bacteria.
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CSF wet mount is positive for motile trophozoites and is of paramount importance for the diagnosis.
Additional methods of diagnosing N fowleri infection include polymerase chain reaction (PCR), monoclonal antibodies, DNA probes, and isoenzyme profile analysis. However, these methods are more time consuming and labor intensive than routine CSF studies. They are useful in postmortem diagnoses and for research purposes.
The CDC can be helpful in working up a possible case of Naegleria infection: http://www.dpd.cdc.gov/dpdx/HTML/Contactus.htm
Other nonspecific laboratory findings in peripheral blood include the following:
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The WBC count is elevated with a neutrophilic predominance.
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Complete metabolic panel (CMP) may show abnormalities, including hyponatremia associated with acquired diabetes insipidus, hyperglycemia, or both.
Other Diagnostic Studies and Procedures
Imaging studies
Limited data are available on imaging studies. One patient has been reported who had a CT scan of the head that demonstrated diffuse enhancement of the gray matter and obliteration of the interpeduncular and quadrigeminal cisterns.
Other tests
Electroencephalography may show signs of reduced cerebral blood flow, including slow and disorganized fundamental rates.
Procedures
The main diagnostic procedure for PAM is to obtain CSF for wet-mount examination for N fowleri, along with standard laboratory examination of the CSF (eg, WBC, red blood cell [RBC], glucose, protein, bacterial and fungal cultures). N fowleri can be cultured on nonnutrient agar plates, which have a lawn of gram-negative bacteria, such as Escherichia coli, covering its surface. "Trails" in the lawn of bacteria that are left by migrating amebae can be visualized following incubation.
Histologic findings
Histologic findings are as follows:
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Amebic trophozoites in perivascular spaces and paraventricular areas
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Fibrinoid necrosis in some blood vessels
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Hemorrhage and necrosis
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Meningeal exudate composed of neutrophils, chronic inflammatory cells, and degenerating amebae
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Focal demyelination in the white matter of the brain and spinal cord
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Acute inflammatory reaction in nasal mucous membranes
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Trophozoites demonstrated on hematoxylin- and eosin-stained slides in involved tissues
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Number of identified cases of primary amoebic meningoencephalitis (PAM), 1937-2007.
