The cerebrospinal fluid (CSF) is the optimal source for biomarkers to establish ante-mortem a link between clinical features and underlying pathologic features (see also Section 1 : Altered proteins in brain neurodegenerative diseases).

Brain neurodegenerative diseases (NDDs) are commonly classified by distinct clinical presentations, e.g. impairment in cognitive functioning involving anatomical region showing neuronal dysfunction and loss. Most of these diseases follow a deteriorating course to dementia. 50-75% of dementia are due to Alzheimer’s pathology.

Most cases of disease are sporadic, but some are inherited in a dominant manner. In these cases, the overexpression of mutant proteins rises to disease-associated phenotypes with early occurrence of disease often.

Most frequent Neurodegenerative diseases

For some diseases, only the clinical phenotype criteria are used for subtyping, whereas for others biochemical modifications or gene polymorphism could also to be considered. The classification gains in confidence based on positive evidence of the presence of pathological protein deposits in brain.

The accumulation of brain pathologies seems to be a nearly inevitable consequence of aging; there is frequently an overlap of concomitant pathologies.

How well do the CSF biomarkers perform diagnostically?

Although a multitude of CSF biomarkers for specific pathologic changes and nonspecific markers of oxidative damage or inflammation involved in neurodegenerative diseases were studied, only three core biomarkers are validated for a differential diagnosis of AD, i.e. Aβ1-42 peptide (Aβ42), total tau (T-tau), and its phosphorylated form (P-tau) measured in vitro using CSF specimen. 

CSF α-Syn is currently studied for its possible value as a Parkinson Disease biomarker and in the differential diagnosis of NDDs, but not validated. Current commercial assays detecting total α-syn levels seem not able to distinguish Lewy body disorders from other neurodegenerative disorders.

The detection of CSF PrPSc is performed only in reference laboratories for Prion diseases with the nonspecific 14.3.3 protein.

For now, only results of CSF Aβ42, T-tau and P-tau can be used in routine clinical setting.

Overview of clinical effectiveness of established Alzheimer disease biomarkers

The concomitant pathologies in elderly individuals lead to an artificial clinical subgrouping drawn by the dominant clinical phenotype. The common age-associated brain pathologies are amyloid plaques, tangles, ischemic cerebrovascular disease but also microinfarctions, hippocampal sclerosis, alpha synuclein deposits (Lewy bodies), TDP43 inclusions, and argyrophilic grains.

A clinical phenotype (typical, atypical or unclear) and the CSF core biomarkers reflecting the dynamic changes of protein metabolism in brain help for differentiating pathological neurodegenerative decline from normal aging.

Change of specific biomarkers for Alzheimer’s disease in other dementias

Source: K.Blennow, Clinical Neurochemistry Laboratory, Sahlgrenska University Hospital, Sweden

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