Clinical Features

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Primary Amebic Meningoencephalitis due to Naegleria fowleri

Clinical Signs and Symptoms

PAM is an acute, rapidly progressing illness. It is characterized by bifrontal or bitemporal headaches, fever, nausea, vomiting and stiff neck. The symptoms progress rapidly, leading to lethargy, confusion, coma and, in most cases, to death in a few hours. Seizures, and sometimes abnormalities in taste or smell and ataxia, may be seen. Nuchal rigidity with positive Kernig's and Brudzinski's signs may be present. Photophobia may be present late in the clinical course. Palsies involving the third, fourth and sixth cranial nerves may also be present in some patients and indicate brain edema and herniations. Raised intracranial pressure has been reported in the majority of patients. Cardiac rhythm abnormalities detected by ECG have been found in some cases. The peripheral white blood cell count is generally elevated, with a marked increase in polymorphonuclear leukocytes and with some lymphocytosis. The majority of cases end fatally within 1 week from the beginning of the symptoms; however, a few cases have been reported to have survived without neurological sequelae. The cause of death is usually increased intracranial pressure with brain herniation leading to cardiorespiratory arrest (Butt, 1996; Fowler and Carter, 1965; Seidel et al., 1982).

Pathological Findings

Gross CNS findings. The cerebral hemispheres are usually swollen and edematous. Hemorrhagic necrosis of the cerebral cortex is characteristic (Figure 12.5A). Uncal and cerebellar tonsillar herniations may be seen. The leptomeninges are congested, with scant purulent exudate that may be seen along the sulci and around blood vessels. The olfactory bulbs and the orbitofrontal cortices are usually necrotic and hemorrhagic.

Microscopic findings. Histopathologically, PAM is characterized by modest amounts of purulent exudate, necrosis and edema, with diffuse hemorrhages of the cortical areas and CNS parenchyma (Figure 12.5B). Amebic troph-ozoites are present within the perivascular spaces, with minimal or no inflammatory reaction (Figure 12.5B). Cysts are not present within the CNS lesions. Necrotizing angiitis is occasionally seen. The leptomeninges show a fibrinopurulent exudate composed of polymorphonuclear leukocytes and eosinophils with fewer macrophages and lymphocytes. These changes are usually found at the base of the cerebral hemispheres, the brain-stem, the cerebellum and the upper portions of the spinal cord. Amebic trophozoites can be seen within the purulent exudate (Butt, 1996; Fowler and Carter, 1965; Martinez and Visvesvara, 1997; Visvesvara and Stehr-Green, 1990).

Granulomatous Amebic Encephalitis (CAE) produced by Acanthamoeba spp. and B. mandrillaris

Clinical Signs and Symptoms

Granulomatous amebic encephalitis (GAE) is characterized by a protracted, insidious clinical

Fig. 12.5 (A) Primary amebic meningoencephalitis. Coronal section at the level of the optic chiasm showing focal necrosis with hemorrhage on the orbitofrontal cortices and cingulate gyri. From the Medical College of Virginia, A338-67, with permission. (B) The cerebral cortex contains multiple clusters of N. fowleri trophozoites with negligible acute inflammatory reaction. Hematoxylin and eosin, x250. From the Medical College of Virginia, A338-67, with permission

Fig. 12.5 (A) Primary amebic meningoencephalitis. Coronal section at the level of the optic chiasm showing focal necrosis with hemorrhage on the orbitofrontal cortices and cingulate gyri. From the Medical College of Virginia, A338-67, with permission. (B) The cerebral cortex contains multiple clusters of N. fowleri trophozoites with negligible acute inflammatory reaction. Hematoxylin and eosin, x250. From the Medical College of Virginia, A338-67, with permission course (Anzil et al., 1991; Martinez and Visves-vara, 1997). GAE has a clinical picture that mimics a single or multiple space-occupying lesion. Localizing neurologic signs and symptoms, such as hemiparesis and seizures, appear early in the clinical course. Mental status abnormalities, headache and stiff neck may be present. Palsies involving the third and the sixth cranial nerves may be seen. Nausea, vomiting, low-grade fever, lethargy, cerebellar ataxia and diplopia are also part of the clinical features. Chest X-rays of the lungs may demonstrate focal consolidated areas and pneumonitis. The direct cause of death in GAE is usually acute broncho-pneumonia, liver or renal failure septicemia (Carter et al., 1981; Martinez and Visvesvara, 1997; Visvesvara and Stehr-Green, 1990).

Pathological Features

The route of invasion and penetration into the brain in cases of GAE is hematogenous, probably from a primary focus in either the lower respiratory tract or the skin. Amebic trophozoites and cysts may be found within the pulmonary parenchyma or skin lesions. The CNS is the target of clinical illness, but other organs including lungs, kidneys, uterus, prostate and testes may be involved, secondary to terminal hematogenous spread (Martinez and Visvesvara, 1997).

Gross CNS, Dermatologie and Other Findings

In the cerebral hemispheres there are multifocal areas of cortical and basal ganglia softening, with necrosis of CNS tissue and hemorrhages (Figure 12.6A). The brainstem, cerebral hemispheres and cerebellum may show areas of 'hemorrhagic infarcts' (Figure 12.6B). Ulcerations of the skin may be seen mainly in patients with AIDS, with acute and chronic inflammation (see numerous references in endlist). A skin biopsy may demonstrate amebic trophozoites and cysts. Ulcerated skin lesions may serve as the portal of entry for amebas or they may represent 'terminal' dissemination of the infection. Several cases of skin involvement without dissemination to the CNS have also been reported.

Neuropathological Findings of CAE

The histopathologic changes consist of multifocal, subacute or chronic necrotizing granulomatous encephalitis with multinucleated giant cells in the

Fig. 12.6 (A) Granulomatous amebic encephalitis due to B. mandrillaris, showing areas of disruption of cerebral cortex and encephalomalacias. Courtesy of E. Yunis MD, Children's Hospital of Pittsburgh, CA-76-79. (B) Coronal section of the cerebral hemisphere, showing multifocal areas of encephalomalacia involving cerebral cortex and subcortical white matter. Courtesy of E. Yunis MD, Children's Hospital of Pittsburgh, CA-76-79. (C) Microscopic appearance of cerebral cortex and subarachnoid space with profuse inflammatory reaction. Courtesy of Presbyterian University Hospital, PA-80-28. (D) Arteriole showing amebic trophozoites and cysts of Acanthamoeba castellanii within the vascular walls. Hematoxylin and eosin, x 350. Courtesy of Presbyterian University Hospital, PA-80-28

Fig. 12.6 (A) Granulomatous amebic encephalitis due to B. mandrillaris, showing areas of disruption of cerebral cortex and encephalomalacias. Courtesy of E. Yunis MD, Children's Hospital of Pittsburgh, CA-76-79. (B) Coronal section of the cerebral hemisphere, showing multifocal areas of encephalomalacia involving cerebral cortex and subcortical white matter. Courtesy of E. Yunis MD, Children's Hospital of Pittsburgh, CA-76-79. (C) Microscopic appearance of cerebral cortex and subarachnoid space with profuse inflammatory reaction. Courtesy of Presbyterian University Hospital, PA-80-28. (D) Arteriole showing amebic trophozoites and cysts of Acanthamoeba castellanii within the vascular walls. Hematoxylin and eosin, x 350. Courtesy of Presbyterian University Hospital, PA-80-28

cerebral hemispheres, midbrain, basal ganglia and cerebellum (Figure 12.6C). Trophozoites and cysts are found within the CNS lesions, particularly around and within blood vessel walls (Figure 12.6D). CNS tissues show inflammatory changes that vary depending on the immunologic status of the host. Focal chronic leptomeningitis may be seen. In patients with AIDS, the lesions may be mostly necrotic, with minimal or negligible inflammation. This probably is due to an impairment of cell-mediated immunity or a defect in histiocytic response, with failure to produce multinucleated giant cells (Anzil et al., 1991; DiGregorio et al., 1992; Denney et al., 1997; Jaramillo-Rodriguez et al., 1989; Lowichik et al., 1995; Martinez and Visvesvara, 1997). Arteritis with the presence of trophozoites and cysts may also be seen (Jaramillo-Rodriquez et al., 1989; May et al., 1992; Murakaura et al., 1995).

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