Neurodiagnostic Studies CT and MRI scans in patients with OPCA typically show pancerebellar and brain stem atrophy, enlarged 4th ventricle and cerebellopontine angle cisterns, and demyelination of transverse pontine fibers on T2-weighted MRl images.{4} PET scans show reduced metabolic rate in the brain stem and cerebellum.{36} In one study, 73% of brain stem auditory evoked responses were abnormal.{104} Neuropathology and Neurochemistry Pathologic studies show a wide variation of neuronal damage. The fundamental pathologic change in OPCA is loss of Purkinje cells in the cerebellar cortex, particularly in the vermis.{56} In addition to cerebellar atrophy, substantia nigra degeneration and depigmentation, neuronal loss in other brain stem nuclei, and demyelination of corticospinal tracts and posterior columns are seen.{62} Recently clear, argyrophilic, introcytoplasmic structures that react with antitubulin antibodies were demonstrated in nearly all patients with OPCA.{58,82,87} These glial cytoplasmic inclusions are located prominently in the white matter of the putamen, internal capsule, cerebral peduncles, and substantia nigra. They were found in the brains of all 11 patients diagnosed with MSA but not in any of the other 284 brains from patients with different neurologic diseases.{87} All of Nakazato et al's {82} six patients with sporadic OPCA were found to have 'oligodendroglial cytoplasmic inclusions', whereas one of two brains with the hereditary forms showed similar lesions. The concurrent cellular and myelin destruction was suggested to be caused by generation of oligodendroglial cytoplasmic inclusions in affected cells triggering a complement activated secondary demyelinating response.{4} The observation of oligodendroglial cytoplasmic inclusions in nearly all cases of MSA suggest the possibility that the three forms of MSA are related and that the disease process originates in the myelin or axons. Detailed morphometric and biochemical studies correlated reductions in aspartic and glutamic acid with Purkinje cell loss in the cerebellar cortex and with neuronal cell loss in the inferior olives (aspartic acid).{1} In addition, quisqualate receptors appear to be decreased, whereas quinolinic acid metabolism is increased.{61,73} The increased quinolinic acid phosphoribosyl-transferase activity in OPCA was interpreted as a compensatory mechanism designed to protect quinolinic acid-sensitive granule cells.{61} In addition, low glutamate dehydrogenase activity was found in most but not all studies; however, this defect probably is not disease-specific.{4,65} Recently mitochondrial DNA abnormalities were postulated to be important in the pathogenesis of OPCA in some patients.{112} CORTICOBASAL GANGLIONIC DEGENERATION Clinical Aspects ln 1968, Rebeiz et al {96} reported three patients of Irish decent with parkinsonism, myoclonus, supranuclear palsy, and apraxia who were found at autopsy to have 'corticodentatonigral degeneration with neuronal achromasia', Recently this complex neurobehavioral disorder has become increasingly recognized. Its most striking features include marked asymmetry of involvement. apraxia. and parkinsonism. In their review of 15 patients, Riley et al {97} categorized the typical features of what is now termed 'corticobasal ganglionic degeneration' into movement disorders (akinesia, rigidity, p&~stural instability, limb dystonia, cortical myoclonus, and postural/intention tremor), cortical sensory loss, apraxias (ideational and ideomotor), and the 'alien limb' phenomenon. Dementia is a late feature of CBGD. In our experience, limb contractures, often preceded by the alien hand phenomenon, are more common in this condition than in any other parkinsonism plus syndrome.{17} Neurologic examination often reveals hyperreflexia, Babinski signs, ocular motility disturbance, and bulbar impairment but no ataxia. Of the 15 patients reported by Riley et al,{97} the age at onset ranged from 51 to 71 years, and men were more commonly affected than women (3:2). Two patients died 7 and 10 years after disease onset. Neurodiagnostic Studies CT scans were abnormal in 14 of the 15 patients in one series; eight had asymmetrical parietal lobe atrophy corresponding to the most affected side, and six had bilateral parielal atrophy.{97} PET scanning reveals reduced fluorodopa uptake in the caudate and putamen and cortical hypometabolism especially in the superior temporal and inferior parietal lobe.{100} Neuropathology and Neurochemistry Pathologic features in this disease include neuronal degeneration in the precentral and postcentral cortical areas with achromatic neural inclusions seen not only in the cortex, but also in the thalamus, subthalamic nucleus, red nucleus, and substantia nigra.{31,69} Dopamine concentration throughout the striatum and substantia nigra was reduced when compared with age-matched controls.{97} To date, no effective treatment has been found, although the dystonia may improve with botulinun' toxin. ALZHEIMER'S DISEASE WITH PARKINSONISM Clinical Aspects Dementia occurs in approximately 20% of patients with Parkinson's disease, but this usually develops later in the course of the illness.{91} Cognitive decline seems to be more prominent in patients with the PIGD form of Parkinson's disease as compared with the more typical tremor variety.{25,54} Alternatively extrapyramidal signs were reported in about one third of patients with Alzheimer's disease.{77} Diffuse Lewy body disease (DLBD), considered by some a variant of Alzheimer's disease or at' overlap between Alzheimer's disease and l2arkinson's disease, is now well recognized, but its clinical and pathologic criteria have not yet been fully defined.{11,13} Differentiation of Alzheimer's disease and DLBD on clinical findings alone may he difficult. Crystal et al {13} reviewed the course of six DLBD, three Alzheimer's disease and one Parkinson's disease patient with autopsy- confirmed diagnosis and found that patients with DLBD were more likely to have gait impairment, rigidity, and resting tremor early in the course of the illness. In another study of 30 patients with DLBD, psychosis and dementia were often found to precede parkinsonism.{11} Agitation, hallucinations, delusions, and abnormal electroencephalograms were more common in patients with DLBD than in the patients with Alzheimer's disease.{11} No differences were found with respect to age, gender, or disease progression between autopsy-proved cases of Alzheimer's disease and DLBD.{30} On neurologic examination, rigidity, bradykinesia, and action tremor were more frequent in the patients with DLBD, whereas impairment of up-gaze was surprisingly more common in the Alzheimer's disease group. The differentiation between DLBD and the other parkinsonism plus syndromes, particularly PSP, can be particularly difficult when a patient with parkinsonism and dementia is also found to have oculomotor disturbance, as noted in some patients with DLBD.{68} Neurodiagnostlc Studies There are no diagnostic studies that call reliably differentiate between Parkinson's dementia, Alzheimer's disease, and DLBD. PET scanning showed no differences between Alzheimer's disease and DLBD.{113} In one series of 37 patients with Alzheimer's disease, however, concentrations of cerebrospinal fluid homovanillic acid and biopterin were noted to be significantly lower in the patients with Alzheimer's disease with extrapyramidal signs as compared with the group without extrapyramidal signs matched for age and dementia severity.{59} Neuropathology and Neurochemistry About one third of patients with Alzheimer's disease have Lewy bodies at autopsy, but patients with DLBD, seem to have a greater neuronal loss in the substantia nigra, substantia inominata, and locus ceruleus and have lower cortical choline acetyltransferase levels than the patients with Alzheimer's disease.{30} Besides diffuse distribution of Lewy bodies throughout the basal forebrain, brain stem and hypothalamus, a paucity of neurofibrillary tangles in DLBD helps to diffrentiate DLBD from Alzheimer's disease.{11,30} Although Gibb et al {30} found no Lewy bodies in the hippocampus or cortex, Burkhardt et al {11}reported 'Lewy-like' bodies in the limbic system and neocortex. Others report different antigenic coniponents of the Lewy body in DLBD and Parkinson's disease, with tau protein being present only in the dementing disorder.{22} Pathologic changes in Alzheimer's disease have been well characterized: Neuritic plaques containing beta-amyloid protein were demonstrated in both sporadic and familial Alzheimer's disease, and neurofibrillary tangles (NFTs) consist of the paired helical filament tau. The abnormal phosphate substitution is thought to interfere with normal neurotubule formation.{64} In patients with Alzheimer's disease, NFTs are found predominantly in the hippocampus, but extrapyramidal signs are more likely associated with increased NFTs in the substantia nigra.{32} Tabaton et at{110} report antigenic similarities between the NFTs of PSP and Alzheimer's disease when derived from neurons of similar populations and postulated that anatomic location rather than disease specificity is the deterinining factor for antigenicity. In a study contrasting Parkinson's dementia and Alzheimer's disease, de Ia Monte et al{80} found similar reductions in the cross sectional areas of the globus pallidus-putamen; greater cell loss was noted in the amygdala of Parkinson's dementia brains, however, whereas Alzheimer's disease was associated with prominent cortical atrophy. The relative frequencies of Lewy bodies, neuritic plaques, and NFTs, however, were not discussed. PARKINSONISM-DEMENTIA COMPLEX OF GUAM (PDCG) Clinical Aspects The combination of parkinsonisn, dementia, and motor neuron disease was first noted in a population of Guam.{44} In a review of 363 Chammoro and three Filipino immigrants with this disease, men were affected twice as frequently as women, but no differences in age of onset (57 years) or death (62 years) were seen between the genders.{115} Besides parkinsonism. supranuclear ocular motility disorder was reported in all 37 patients in one series.{67} Patients with parkinsonism usually present later than those with amytrophic lateral sclerosis, possibly because patients presenting with motor neuron disease do not survive long enough to develop extrapyramidal symptoms.{114} Furthermore basal ganglia signs may be masked by the motor neuron disease. Neurodiagnostic Studies PET scanning in patients with the parkinsonian form of this disease reveals decreased presynaptic l8F-6-fluorodopa uptake similar to patients with Parkinson's disease, whereas those with amyotrophic lateral sclerosis have an intermediate picture between Parkinson's disease and control populations, suggesting a preclinical lesion.{105} Neuropathology and Neurochemistry The finding of increased frequencies of this neurodegenerative disorder on the island of Guam suggest a possible environmental etiology.{44} The neurotoxin beta-N-methylamino-L-alanine (BMAA), a compound found in the cycad plant and believed to be in high concentrations in flour made from this plant, produces a similar spectrum of neurologic decline in monkeys.{106} An analysis of the BMAA content of cycad flour, however, suggests that the quantities of BMAA normally consumed by the inhabitants of the endemic areas were not sufficient to produce neurologic toxicity.{20} Other hypotheses concerning abnormalities in mineral metabolism and hypomagnesemia or hypocalcemia also have been suggested, but supporting evidence is lacking.{27} Pathologically this condition resembles Alzheimer's disease more than Parkinson's disease; tau-containing NFTs are present particularly in the hippocampus{35,101} and other brain and spinal cord areas.{75} Immunohistochemical studies of autopsied brains of patients with PDCG showed marked reduction in the number of dopaminergic neurons in both the lateral and the medial substantia nigra.{39} Despite marked reduction of nigrostriatal dopamine concentration, the striatal output system was well preserved and glutamate, gamma-aminobutyric acid (GABA), choline acetyltransferase, and serotonin were spared. 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