Synapse and primary cilia dysfunctions in Autism Spectrum Disorders. Avenues to normalize these functions.

AIM: An update on the plasticity of the brain networks involved in autism (autism spectrum disorders [ASD]), and the increasing role of their synapses and primary non-motile cilia. METHODS: Data from PubMed and Google on this subject, published until February 2024, were analyzed. RESULTS: Structural and functional brain characteristics and genetic particularities involving synapses and cilia that modify neuronal circuits are observed in ASD, such as reduced pruning of dendrites, minicolumnar pathology, or persistence of connections usually doomed to disappear. Proteins involved in synapse functions (such as neuroligins and neurexins), in the postsynaptic architectural scaffolding (such as Shank proteins) or in cilia functions (such as IFT-independent kinesins) are often abnormal. There is an increase in glutaminergic transmission and a decrease in GABA inhibition. ASD may occur in genetic ciliopathies. The means of modulating these specificities, when deemed useful, are described. INTERPRETATION: The wide range of clinical manifestations of ASD is strongly associated with abnormalities in the morphology, functions, and plasticity of brain networks, involving their synapses and non-motile cilia. Their modulation offers important research perspectives on treatments when needed, especially since brain plasticity persists much later than previously thought. Improved early detection of ASD and additional studies on synapses and primary cilia are needed.

Measles Sclerosing Subacute PanEncephalitis (SSPE), an intriguing and ever-present disease: Data, assumptions and new perspectives.

BACKGROUND: Subacute sclerosing panencephalitis (SSPE) is a rare, non-treatable and fatal neurological complication of measles, still present due to the return of the epidemic linked to the loosening of vaccination policies. Its mechanism remains unexplained. OBJECTIVE: The main objective was to investigate explanatory variables relating to the risk of developing SSPE and its pathophysiology. METHODS: Literature analysis was focused on different varieties of SSPE: perinatal forms, short-incubation forms similar to acute measles inclusion body encephalitis (MIBE), rapidly evolving forms, forms occurring in the immunosuppressed, adult forms, and family forms. In addition, several studies on the parameters of innate immunity and interferon responses of patients were analyzed. RESULTS: Two main data were highlighted: a relationship between the so-called fulminant forms and the prescription of corticosteroids was established. In familial SSPE, two groups were individualized according to the duration of the latency period, prompting an analysis of patient exomes. CONCLUSION: Treatment with corticosteroids should be banned. Knowledge of the genes involved and epigenetics should be useful for understanding the pathophysiology of SSPE and other late-onset neurological infections with RNA viruses.

Plasma total prion protein as a potential biomarker for neurodegenerative dementia: diagnostic accuracy in the spectrum of prion diseases.

AIMS: In the search for blood-based biomarkers of neurodegenerative diseases, we characterized the concentration of total prion protein (t-PrP) in the plasma of neurodegenerative dementias. We aimed to assess its accuracy in this differential diagnostic context. METHODS: Plasma t-PrP was measured in 520 individuals including healthy controls (HC) and patients diagnosed with neurological disease control (ND), Alzheimer's disease (AD), sporadic Creutzfeldt-Jakob disease (sCJD), frontotemporal dementia (FTD), Lewy body dementia (LBD) and vascular dementia (VaD). Additionally, t-PrP was quantified in genetic prion diseases and iatrogenic CJD. The accuracy of t-PrP discriminating the diagnostic groups was evaluated and correlated with demographic, genetic and clinical data in prion diseases. Markers of blood-brain barrier impairment were investigated in sCJD brains. RESULTS: Compared to HC and ND, elevated plasma t-PrP concentrations were detected in sCJD, followed by FTD, AD, VaD and LBD. In sCJD, t-PrP was associated neither with age nor sex, but with codon 129 PRNP genotype. Plasma t-PrP concentrations correlated with cerebrospinal fluid (CSF) markers of neuro-axonal damage, but not with CSF t-PrP. In genetic prion diseases, plasma t-PrP was elevated in all type of mutations investigated. In sCJD brain tissue, extravasation of immunoglobulin G and the presence of swollen astrocytic end-feet around the vessels suggested leakage of blood-brain barrier as a potential source of increased plasma t-PrP. CONCLUSIONS: Plasma t-PrP is elevated in prion diseases regardless of aetiology. This pilot study opens the possibility to consider plasma t-PrP as a promising blood-based biomarker in the diagnostic of prion disease.

[Doctor Francoise Cathala and history of prions diseases]. / Le docteur Francoise Cathala Pagesy et l'histoire des maladies à prions.

Doctor Françoise Cathala Pagesy, MD, MS, born on July 7, 1921 in Paris, passed away peacefully at home on November 5, 2012. Unconventional, passionate and enthusiastic neurologist and virologist, she devoted her life to research on latent and slow viral infections, specializing mainly on unconventional transmissible agents or prions. As a research member of Inserm (French Institute for Medical Research), she soon joined the team of Carlton Gajdusek (the NINCDS - National Institute of Nervous Central System and Stroke - of NIH), who first demonstrated the transmissibility of kuru and Creutzfeldt-Jakob disease to monkeys. When she came back to Paris, where she was followed by one of NIH members, Paul Brown, she joined the Centre de Recherches du Service de Santé des Armées (Army Health Research Center), in Percy-Clamart, where she found the experimental design and the attentive help needed for her research, which appeared heretical to many French virologists, including some authorities. A large number of research programs were set up with numerous collaborations involving CEA (Center for Atomic Energy) and other institutions in Paris and Marseilles on epidemiology, results of tissue inoculation, electrophysiology and neuropathology of human and animal prions diseases, and resistance of the infectious agent. International symposia were set up, where met, in the Val-de-Grâce hospital in Paris, the research community on "slow viral diseases". Stanley Prusiner introduced the concept - then badly accepted and still in evolution - of prion, a protein only infectious agent. Before retiring from Inserm, Françoise Cathala predicted and was involved in some of the huge sanitary crises in France. These were, first, Creutzfeldt-Jakob disease from contaminated growth hormone extracted from cadavers, which led parents to instigate legal procedure - a quite unusual practice in France. The second was Mad cow disease in the United Kingdom then in France, followed by new variant of Creutzfeldt-Jakob human epidemics, paradigmatic food safety crisis bringing together the poles of production (beef and meat-and-bone meal) and consumption, and leading to an unexpected social bang. Through Françoise Cathala exemplary life, the history of French, and more generally of worldwide prions diseases is dealt with.

[Spreading of protein misfolding: A new paradigm in neurology]. / La transconformation protéique, nouveau paradigme en neurologie.

Protein misfolding and spreading ("transconformation") are being better understood. Described in Prions diseases, this new paradigm in the field of neurodegenerative disorders and brain aging also implies sporadic inclusion myositis, type 2 diabetes, some cancers, sickle cell disease... Misfolding is transmitted from a protein or peptide to a normally folded one. Often associated with a stress of the endoplasmic reticulum, it may spread along the neurites, following anterograde or retrograde axonal transport. In the central nervous system, it occurs in a few cells and there is invasion of adjacent cells by cell-to-cell spread. Three varieties of protein misfolding occur along neuroanatomical pathways. It can be a 'centripetal' process. The synucleinopathy of Parkinson disease has been carefully studied: the changes first occur in cardiac or enteric plexuses... and reach later on the mesencephalon and neocortex. Thus, skin biopsy might prove a diagnostic tool. Protein misfolding may also occur along 'centrifugal' pathways, from motor cortex to peripheral motor neurons. Examples are provided by SOD and pTDP-43 in Amyotrophic Lateral Sclerosis. Amyloid ß peptide in cerebral aging and Alzheimer's disease also spread from occipital cortex to the brainstem. Lastly, the propagation may remain 'central' for TDP-43 in behavioral variant frontotemporal dementia, following only pathways of the encephalic neural network. This has to be confirmed, however, since the spreading of some proteins (such as tau or Aß peptides) has been considered central for a long time and has proved today to involve extracerebral tissues. The complex mechanisms of protein misfolding, still in analysis, include the involvement of chaperone proteins, the formation of very toxic labile proteins molecules (oligomers?), and provide a number of new therapeutic perspectives.

Increased expression of BIN1 mediates Alzheimer genetic risk by modulating tau pathology.

Genome-wide association studies (GWAS) have identified a region upstream the BIN1 gene as the most important genetic susceptibility locus in Alzheimer's disease (AD) after APOE. We report that BIN1 transcript levels were increased in AD brains and identified a novel 3 bp insertion allele ∼28 kb upstream of BIN1, which increased (i) transcriptional activity in vitro, (ii) BIN1 expression levels in human brain and (iii) AD risk in three independent case-control cohorts (Meta-analysed Odds ratio of 1.20 (1.14-1.26) (P=3.8 × 10(-11))). Interestingly, decreased expression of the Drosophila BIN1 ortholog Amph suppressed Tau-mediated neurotoxicity in three different assays. Accordingly, Tau and BIN1 colocalized and interacted in human neuroblastoma cells and in mouse brain. Finally, the 3 bp insertion was associated with Tau but not Amyloid loads in AD brains. We propose that BIN1 mediates AD risk by modulating Tau pathology.

Quantification of surviving cerebellar granule neurones and abnormal prion protein (PrPSc) deposition in sporadic Creutzfeldt-Jakob disease supports a pathogenic role for small PrPSc deposits common to the various molecular subtypes.

AIMS: Neuronal death is a major neuropathological hallmark in prion diseases. The association between the accumulation of the disease-related prion protein (PrP(Sc) ) and neuronal loss varies within the wide spectrum of prion diseases and their experimental models. In this study, we investigated the relationships between neuronal loss and PrP(Sc) deposition in the cerebellum from cases of the six subtypes of sporadic Creutzfeldt-Jakob disease (sCJD; n=100) that can be determined according to the M129V polymorphism of the human prion protein gene (PRNP) and PrP(Sc) molecular types. METHODS: The numerical density of neurones was estimated with a computer-assisted image analysis system and the accumulation of PrP(Sc) deposits was scored. RESULTS: The scores of PrP(Sc) immunoreactive deposits of the punctate type (synaptic type) were correlated with neurone counts - the higher the score the higher the neuronal loss - in all sCJD subtypes. Large 5- to 50-µm-wide deposits (focal type) were found in sCJD-MV2 and sCJD-VV2 subtypes, and occasionally in a few cases of the other studied groups. By contrast, the highest scores for 5- to 50-µm-wide deposits observed in sCJD-MV2 subtype were not associated with higher neuronal loss. In addition, these scores were inversely correlated with neuronal counts in the sCJD-VV2 subtype. CONCLUSIONS: These results support a putative pathogenic role for small PrP(Sc) deposits common to the various sCJD subtypes. Furthermore, the observation of a lower loss of neurones associated with PrP(Sc) type-2 large deposits is consistent with a possible 'protective' role of aggregated deposits in both sCJD-MV2 and sCJD-VV2 subtypes.

Evidence for induction of the ornithine transcarbamylase expression in Alzheimer's disease.

To more rapidly identify candidate genes located within chromosomal regions of interest defined by genome scan studies in Alzheimer's disease (AD), we have developed a customized microarray containing all the ORFs (n=2741) located within nine of these regions. Levels of gene expression were assessed in total RNA from brain tissue of 12 controls and 12 AD patients. Of all genes showing differential expression, we focused on the ornithine transcarbamylase (OTC) gene on Xp21.1., a key enzyme of the urea cycle which we found to be expressed in AD brains but not in controls, as confirmed by RT-PCR. We also detected mRNA expression of all the other urea cycle enzymes in AD brains. Immunochemistry experiments revealed that the OTC expression was strictly restricted to vascular endothelial cells in brain. Furthermore, OTC activity was 880% increased in the CSF of probable AD cases compared with controls. We analysed the association of the OTC -389 G/A and -241 A/G promoter polymorphisms with the risk of developing AD. We observed that rare haplotypes may be associated with the risk of AD through a possible modulation of the methylation of the OTC promoter. In conclusion, our results suggest the involvement of a new pathway in AD brains involving the urea cycle.

Transcriptomic and genetic studies identify IL-33 as a candidate gene for Alzheimer's disease.

The only recognized genetic determinant of the common forms of Alzheimer's disease (AD) is the epsilon 4 allele of the apolipoprotein E gene (APOE). To identify new candidate genes, we recently performed transcriptomic analysis of 2741 genes in chromosomal regions of interest using brain tissue of AD cases and controls. From 82 differentially expressed genes, 1156 polymorphisms were genotyped in two independent discovery subsamples (n=945). Seventeen genes exhibited at least one polymorphism associated with AD risk, and following correction for multiple testing, we retained the interleukin (IL)-33 gene. We first confirmed that the IL-33 expression was decreased in the brain of AD cases compared with that of controls. Further genetic analysis led us to select three polymorphisms within this gene, which we analyzed in three independent case-control studies. These polymorphisms and a resulting protective haplotype were systematically associated with AD risk in non-APOE epsilon 4 carriers. Using a large prospective study, these associations were also detected when analyzing the prevalent and incident AD cases together or the incident AD cases alone. These polymorphisms were also associated with less cerebral amyloid angiopathy (CAA) in the brain of non-APOE epsilon 4 AD cases. Immunohistochemistry experiments finally indicated that the IL-33 expression was consistently restricted to vascular capillaries in the brain. Moreover, IL-33 overexpression in cellular models led to a specific decrease in secretion of the A beta(40) peptides, the main CAA component. In conclusion, our data suggest that genetic variants in IL-33 gene may be associated with a decrease in AD risk potentially in modulating CAA formation.

Golgi apparatus, GERL, and lysosomes of neurons in rat dorsal root ganglia, studied by thick section and thin section cytochemistry.

New insights into the ultrastructure and phosphatase localizations of Golgi apparatus and GERL, and into the probable origin of lysosomes in the neurons of fetal dorsal root ganglia and the small neurons of adult ganglia have come from studying thick (0.5-1.0 micro) as well as thin (up to 500 A) sections by conventional electron microscopy. Tilting the thick specimens, by a goniometer stage, has helped to increase our understanding of the three-dimensional aspects of the Golgi apparatus and GERL. One Golgi element, situated at the inner aspect of the Golgi stack, displays thiamine pyrophosphatase and nucleoside diphosphatase activities. This element exhibits regular geometric arrays (hexagons) of interconnected tubules without evidence of a flattened portion (saccule or cisterna). In contrast, GERL shows acid phosphatase activity and possesses small cisternal portions and anastomosing tubules. Lysosomes appear to bud from GERL. Osmium deposits, following prolonged osmication, are found in the outer Golgi element. Serial 0.5-micro and thin sections of thiamine pyrophosphatase-incubated material demonstrate that, in the neurons studied, the Golgi apparatus is a continuous network coursing through the cytoplasm. Serial thick sections of acid phosphatase-incubated tissue suggest that GERL is also a continuous structure throughout the cytoplasm. Tubules of smooth endoplasmic reticulum, possibly part of GERL, extend into the polygonal compartments of the inner Golgi element. The possible physiological significance of a polygonal arrangement of a phosphatase-rich Golgi element in proximity to smooth ER is considered. A tentative diagram of the Golgi stack and associated endoplasmic reticulum in these neurons has been drawn.

Transmission of the BSE agent to mice in the absence of detectable abnormal prion protein.

The agent responsible for transmissible spongiform encephalopathies (TSEs) is thought to be a malfolded, protease-resistant version (PrPres) of the normal cellular prion protein (PrP). The interspecies transmission of bovine spongiform encephalopathy (BSE) to mice was studied. Although all of the mice injected with homogenate from BSE-infected cattle brain exhibited neurological symptoms and neuronal death, more than 55 percent had no detectable PrPres. During serial passage, PrPres appeared after the agent became adapted to the new host. Thus, PrPres may be involved in species adaptation, but a further unidentified agent may actually transmit BSE.

[The neuropathology of sleep in human neurodegenerative diseases]. / Neuropathologie du sommeil des maladies neurodégénératives humaines.

The neuropathology of human sleep remains an ill-defined issue. The data concerning the main structures of human brain areas involved, or supposed to be implicated, in sleep organisation are reviewed. Five levels of organisation can be schematically recognized: (i) the ascending arousal system, (ii) the non REM and REM systems (iii) regulated by hypothalamic areas, (iv) and the biological clock, (v) modulated by a number of "allostatic" influences. These are briefly described, with emphasis on the location of structures involved in humans, and on the recently revised concepts. Current knowledge on the topography of lesions associated with the main sleep disorders in degenerative diseases is recalled, including REM sleep behavior disorders, restless legs syndrome and periodic leg movements, sleep apneas, insomnia, excessive daily sleepiness, secondary narcolepsy and disturbed sleep-wake rhythms. The lesions of sleep related structures observed in early and late stages of four degenerative diseases are then reviewed. Two synucleinopathies (Lewy lesions associated disorders, including Parkinson's disease and Dementia with Lewy bodies, and Multiple System Atrophy) and two tauopathies (Progressive Supranuclear Palsy and Alzheimer's disease) are dealt with. The distribution of lesions usually found in affected patients fit with that expected from the prevalence of different sleep disorders in these diseases. This confirms the current opinion that these disorders depend on the distribution of lesions rather than on their biochemical nature. Further studies might throw insight on the mechanism of normal and pathological sleep in humans, counterpart of the increasing knowledge provided by animal models. Specially designed prospective clinicopathological studies including peculiar attention to sleep are urgently needed.

Creutzfeldt-Jakob disease with slow progression. A mimickry of progressive supranuclear palsy.

Sporadic Creutzfeldt-Jakob disease (sCJD) does not always present with typical clinical signs, such as myoclonus in association with periodic sharp-wave complexes. We present a 67-year old female patient with initial falls and vertical gaze palsy, suggesting the diagnosis of Progressive Supranuclear Palsy (PSP). EEG and MRI were not contributory. Typical clinical and paraclinical CJD signs were only seen after 17 months. The diagnosis was confirmed by autopsy. - CJD can be a neurodegenerative chameleon. The present case adds to the scare literature of slowly evolving CJD mimicking Parkinsonism related to tauopathies.

Outcome of patients with HIV-1-related cognitive impairment on highly active antiretroviral therapy.

OBJECTIVE: To examine the impact of highly active antiretroviral therapy (HAART) on the outcome of HIV-1-related cognitive impairments using a neuropsychological (NP) battery to assess separately the psychomotor, executive function and memory fields. DESIGN: A longitudinal study of HIV-1-infected patients based on serial NP tests in a Paris University Hospital. METHODS: A group of 91 HIV-1-infected patients, of whom 47 were already taking HAART at their first NP examination, were initially categorized as cognitively impaired (n = 53) or non-impaired (n = 38) and underwent one to six serial NP batteries (mean follow-up 12.3+/-8.3 months). Generalized estimating equations (GEE) were used to evaluate performance in a given NP test according to the number of days on HAART. RESULTS: Despite a 25% mortality rate among patients who had cognitive impairment at their first NP examination, GEE showed a positive relationship between the duration of HAART and cognitive performance. Performance in psychomotor tests (e.g. Purdue Pegboard dominant hand) improved continuously during the study period, while memory test performance (e.g. Grober and Buschke free recall) tended to reach a plateau. CONCLUSIONS: HAART improves subcortical cognitive functions during the first year of treatment. Distinct neuropathological mechanisms appear to underlie psychomotor and memory dysfunctions in AIDS.

Regressive or lethal lead encephalopathy in the suckling rat. Correlation of lead levels and morphological findings.

Lead encephalopathy was produced in immature Sprague-Dawley rats with an intraperitoneal (IP) injection of 60 micrograms/g body weight of lead acetate administered daily from the fifth day after birth. Macroscopic and light microscopic study of the nervous system, estimations of the blood-brain barrier permeability to proteins and brain water content were performed every two days thereafter. Lead levels in total blood, plasma, and several brain areas were measured at the same intervals by flameless atomic absorption spectrometry. Electron microscopic study of the cerebellum was done 2, 6, and 12 days after beginning lead administration. After two days of lead administration and before any pathological change occurred the increase in lead level was greater in the cerebellum than in other brain areas. After four to six days, hemorrhagic lead encephalopathy developed and was most prominent in regions with higher lead levels. From day 11 to 14, there were two possible courses: a) improvement of the clinical status and morphological findings in 25% of the animals, or b) progression of abnormal clinical signs and death. Cerebral edema, both intra- and extracellular, may have contributed to the fatal evolution. The mechanism of this edema appeared complex and may have involved resorption failure. Good correlations were observed among progression of the clinical signs, high water content in the brain, morphological evidence of cerebral edema, and a high cerebellar lead level. In contrast, high blood lead levels could be associated with clinical improvement, normal brain water content, and regression of the pathological findings. These data suggest that differences in evolution are more likely related to differences in the development of resistance of the cerebral capillary to lead, or in the efflux of lead, rather than to the blood lead concentrations.

Autometallographic tracing of bismuth in human brain autopsies.

For decades, drugs containing bismuth have been used to treat gastrointestinal disorders. Although a variety of adverse effects, including neurological syndromes, have been recorded, the biological/toxicological effects of bismuth ions are far from disclosed. Until recently, only quantitative assessments were possible, but resent research has made histochemical tracing of bismuth possible. The technique involves silver enhancement of bismuth crystallites by autometallography (AMG). In the present study, the localization of bismuth was traced by AMG in sections of paraffin-embedded brain tissue obtained by autopsy from 6 patients suffering from bismuth intoxication in a period ranging from 1975 through 1977. Tissue was analyzed at light and electron microscopical levels, and the presence of bismuth further confirmed by proton-induced x-ray emission (PIXE). Clinical data and bismuth concentrations in blood, cerebellum, and thalamus were measured by atomic absorption spectrophotometry (AAS) and are reported here. Histochemical analyses demonstrate that bismuth accumulated in neurons and glia cells in the brain regions examined (neocortex, cerebellum, thalamus, hippocampus). Cerebellar blood vessels stained most intensely. The PIXE and AAS data correlated with the histochemical staining patterns and intensities. At the ultrastructural level, bismuth was found to accumulate intracellularly in lysosomes and extracellularly in the basement membranes of some vessels.

Human immunodeficiency virus (HIV) leukoencephalopathy and the microcirculation.

We studied the brains of three patients with acquired immune deficiency syndrome (AIDS), all of whom developed subacutely progressive dementia unassociated with opportunistic infection or neoplasm in the central nervous system. Computed tomographic (CT) scans of the head revealed cortical atrophy, ventricular dilation, and diffuse hypodensity of the centrum semiovale. On microscopic examination, the cerebral and cerebellar white matter in all cases showed diffuse and focal, angiocentric regions of myelin pallor, focal vacuolization, and extensive gliosis. Variable axonal loss and axonal spheroids were evident. The microvasculature showed striking changes, including mural thickening, increased cellularity, and enlargement and pleomorphism of endothelial cells with variable numbers of macrophages and multinucleated giant cells (MNGC), which often contained hemosiderin pigment. Human immunodeficiency virus type 1 (HIV-1) antigens were identified immunocytochemically within perivascular macrophages and MNGC and in some microglial cells. We suggest that the morphologic abnormalities of the microcirculation may be associated with an alteration of the blood-brain barrier. The increased vascular permeability could contribute to damage and loss of the white matter including both myelin and axons, and result in subcortical cerebral atrophy. The HIV-1 infected cells present in relation to the microvasculature may play a role in mediating the vascular injury.

Validity and reliability of the preliminary NINDS neuropathologic criteria for progressive supranuclear palsy and related disorders.

We investigated the validity and reliability of diagnoses made by eight neuropathologists who used the preliminary NINDS neuropathologic diagnostic criteria for progressive supranuclear palsy (PSP) and related disorders. The specific disorders were typical, atypical, and combined PSP, postencephalitic parkinsonism, corticobasal ganglionic degeneration, and Pick's disease. These disorders were chosen because of the difficulties in their neuropathologic differentiation. We assessed validity by measuring sensitivity and positive predictive value. Reliability was evaluated by measuring pairwise and group agreement. From a total of 62 histologic cases, each neuropathologist independently classified 16 to 19 cases for the pairwise analysis and 5 to 6 cases for the group analysis. The neuropathologists were unaware of the study design, unfamiliar with the assigned cases, and initially had no clinical information about the cases. Our results showed that with routine sampling and staining methods, neuropathologic examination alone was not fully adequate for differentiating the disorders. The main difficulties were discriminating the subtypes of PSP and separating postencephalitic parkinsonism from PSP. Corticobasal ganglionic degeneration and Pick's disease were less difficult to distinguish from PSP. The addition of minimal clinical information contributed to the accuracy of the diagnosis. On the basis of results obtained, we propose clinicopathologic diagnostic criteria to improve on the NINDS criteria.

Neuropathological diagnostic criteria for Creutzfeldt-Jakob disease (CJD) and other human spongiform encephalopathies (prion diseases).

Neuropathological diagnostic criteria for Creutzfeldt-Jakob disease (CJD) and other human transmissible spongiform encephalopathies (prion diseases) are proposed for the following disease entities: CJD--sporadic, iatrogenic (recognised risk) or familial (same disease in 1st degree relative): spongiform encephalopathy in cerebral and/or cerebellar cortex and/or subcortical grey matter; or encephalopathy with prion protein (PrP) immunoreactivity (plaque and/or diffuse synaptic and/or patchy/perivacuolar types). Gerstmann-Sträussler-Scheinker disease (GSS) (in family with dominantly inherited progressive ataxia and/or dementia): encephalo(myelo)pathy with multicentric PrP plaques. Familial fatal insomnia (FFI) (in member of a family with PRNP178 mutation): thalamic degeneration, variable spongiform change in cerebrum. Kuru (in the Fore population). Without PrP data, the crucial feature is the spongiform change accompanied by neuronal loss and gliosis. This spongiform change is characterised by diffuse or focally clustered small round or oval vacuoles in the neuropil of the deep cortical layers, cerebellar cortex or subcortical grey matter, which might become confluent. Spongiform change should not be confused with non-specific spongiosis. This includes status spongiosus ("spongiform state"), comprising irregular cavities in gliotic neuropil following extensive neuronal loss (including also lesions of "burnt-out" CJD), "spongy" changes in brain oedema and metabolic encephalopathies, and artefacts such as superficial cortical, perineuronal, or perivascular vacuolation; focal changes indistinguishable from spongiform change may occur in some cases of Alzheimer's and diffuse Lewy body diseases. Very rare cases might not be diagnosed by these criteria. Then confirmation must be sought by additional techniques such as PrP immunoblotting, preparations for electron microscopic examination of scrapie associated fibrils (SAF), molecular biologic studies, or experimental transmission.