Limbic encephalitis occurring in association with Alzheimer's disease

Memory lost, memory regained: neuropsychological findings and neuroimaging in two cases of paraneoplastic limbic encephalitis with radically different outcomes

OBJECTIVE

To report two cases of paraneoplastic limbic encephalitis (PNLE) with similar clinical presentation, but dramatically different outcome and to highlight the role of neuropsychological and radiological evaluation in PNLE.

METHODS

Both patients underwent an extensive battery of neuropsychological tests designed to document general intellectual function, anterograde verbal and visual memory, naming, knowledge and executive ability. In addition, structural (CT and MRI) and functional (HMPAO-SPECT) brain scans were performed.

RESULTS

Both patients presented with fairly sudden onset of profound and persistent memory loss in the absence of other neurological symptoms. Their subsequently diagnosed small cell lung cancer was treated with a combination of radiotherapy and chemotherapy, leading to remission of the tumour. The memory of patient 1 recovered fully and he died from an unrelated cause 1 year later; neuropsychological testing showed a severe, but isolated, anterograde amnesia, brain MRI was normal and HMPAO-SPECT showed left medial temporal hypoperfusion. Patient 2 remained densely amnesic despite regression of her lung tumour; neuropsychological testing disclosed both anterograde and extensive retrograde amnesia together with more generalised cognitive deficits including anomia and executive impairments, MRI showed gross atrophy of the hippocampus and amygdala bilaterally, and HMPAO-SPECT showed pronounced frontal and temporal hypoperfusion.

CONCLUSION

Complete remission from PNLE may occur and seems to be associated with pure anterograde amnesia without evidence of structural hippocampal damage in MRI. By contrast, cognitive deficits beyond severe anterograde amnesia and evidence of destructive medial temporal lobe pathology on MRI seem to be poor prognostic features.

Limbic encephalitis investigated by 18FDG-PET and 3D MRI

Two patients with clinically probable or possible limbic encephalitis (LE) are reported, both cases with typical findings in clinical symptoms (severe neuropsychological deficits and complex partial seizures) and in routine magnetic resonance imaging (MRI) (hyperintense mesiotemporal lesions). Underlying malignancy was identified (rectal carcinoma) in one case but could not be detected in the other patient. The 2 patients were investigated by cerebral 18F-fluoro-2-deoxy-D-glucose-positron emission tomography (FDG-PET) and 3-dimensional (3D) MRI, and abnormalities in metabolic activity were mapped using coregistration of spatially normalized PET and MRI. Highly significant focal hypermetabolism in bilateral hippocampal areas was found in both cases. The authors' findings support FDG-PET coregistered to 3D MRI as a potentially valuable additional tool in the imaging diagnostics of LE. Results are discussed with respect to the clinical symptoms and previously reported imaging findings in the disease.

Neurologic paraneoplastic syndromes

Several neurologic paraneoplastic disorders are believed to be caused by an autoimmune reaction against antigen(s) co-expressed by tumour cells and neurons. Of the paraneoplastic syndromes, the evidence for an autoimmune etiology is strongest for the Lambert-Eaton myasthenic syndrome, in which autoantibodies downregulate voltage-gated calcium channels at the presynaptic nerve terminal. For other syndromes, including cerebellar degeneration, multifocal encephalomyelitis, sensory neuronopathy, limbic encephalitis, opsoclonus-myoclonus, stiff person syndrome, and retinal degeneration, the autoimmune theory is supported by the presence of specific antineuronal antibodies. These antibodies serve as a useful diagnostic tool, but their actual role in causing neuronal injury and clinical disease remains unclear. Further understanding of immunopathogenesis awaits successful experimental models. Among different syndromes, a varied proportion of patients shows neurologic improvement with immunosuppressive treatments; it is likely that many patients have already suffered irreversible neuronal injury at the time of diagnosis.

Rapidly progressive dementia: a clinicopathologic correlation

We evaluated a 66-year-old man with a rapidly progressive, akinetic-rigid dementia syndrome. Despite extensive testing, which included magnetic resonance imaging (MRI), we were unable to make the correct antemortem diagnosis. Autopsy demonstrated spontaneous progressive multifocal leukoencephalopathy. This report illustrates that even in the absence of characteristic MRI findings, this uncommon cause of dementia should be considered in the differential diagnosis of rapidly progressive, akinetic-rigid syndromes with dementia.

Anatomy of the medial temporal lobe

The medial temporal lobe concept is an example of neurojargon rich in clinical and behavioral meaning, but sparse in neuroanatomical meaning except for topography. Like the concept of anterior speech area, many know roughly where it is located and what its functional correlates are, but not a lot else. At least three anatomical entities qualify as components of the medial temporal lobe. These include the amygdaloid body, the hippocampal formation, and the parahippocampal cortices that cover them superficially and are visible on the external surface of the hemisphere. For the greater part of this century, topographical observations, dissection, and descriptive data from passive staining methods have formed the principal source of information about the anatomy of the medial temporal lobe. However, in the past two decades much new information has emerged from experimental neuroanatomical studies in nonhuman primates and from neuropathological studies in humans. With magnetic resonance imaging (MRI), previous neuroanatomical detail, which earlier may have seemed like descriptive minutia, has now come alive and assumed substantial relevance in neurological and psychiatric diagnosis. Some of the emerging concepts as they relate to the neuroanatomy of the primate brain are highlighted and summarized here.