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The term rapidly progressive dementia (RPD) is used to describe cases with a progression course which usually ranges between weeks and months.1–4 The subacute nature of RPD excludes other conditions with fulminant progression such as infectious or metabolic acute encephalopathies, which progress within hours or days and typically commence as an acute confusional state.
In most cases, the cognitive decline observed in RPD can be attributed to a single underlying disorder. Nevertheless, a rapid course might also represent the aggravation of an undiagnosed disease attributable to a secondary cause, usually an infection or a metabolic dysregulation. Various conditions involving the central nervous system (CNS) can emerge as RPD, including Creutzfeld-Jakob disease (CJD) and other spongiform encephalopathies, vascular disorders, autoimmune and paraneoplastic encephalopathies, subacute infections, metabolic and toxic disorders and systemic diseases (see Table 1). However, it is important to point out that even neurodegenerative disorders such as Alzheimer’s disease (AD), dementia with Lewy bodies and frontotemporal dementia present in rare cases as a subacute dementia instead of a slowly progressive deterioration of higher functions.5 CJD is the prevailing cause of RPD in most related studies. Regarding the relative frequency of other disorders which account for cases of RPD, there is marked variability among scientific groups.1–3 In cases of RPD with an early age of onset, the possibility of an infection, hereditary metabolic disorder or autoimmune encephalopathy should be considered.6,7
The early diagnosis of the undergoing disorder in a patient exhibiting an RPD can be particularly demanding owing to the paucity of clinical signs during the early stages of many disorders and to overlapping laboratory findings. The pattern of cognitive deficits is crucial for clinical assessment. Selective memory, executive function and language deficits might favour one potential diagnosis and exclude others. Thus, a detailed neuropsychological evaluation is crucial.2,3
The evaluation of behavioural alterations and neuropsychiatric symptoms is of high importance, since the initial symptoms can be exclusively a depressive-like or a disinhibited behaviour. Associated signs and symptoms, either neurological or systematic, can unveil an undetermined underlying disease. Ataxia, parkinsonism and specific motor or sensory deficits might herald the clinical expression of an otherwise obscure disorder during the progression of various forms of RPD.