The organization of the Dominantly Inherited Alzheimer Network [5] has been a major accomplishment in creating the logistic basis of such clinical trials, although owing to the small sample size, it is not likely that else all drugs can be tested in this specific population. On clinical grounds, EOAD and LOAD are distinguished on the basis of age of onset (AOO) alone. Several studies attempted to delineate the clinical, neuropsycho-logical, imaging, pathological, and biomarker differences between EOAD and LOAD based on the 65-year arbitrary cutoff proposed by Amaducci and colleagues [1] in 1986. The age of disease onset of patients with AD ranged from 50 to 99 in most studies but included subjects with AOO as low as 41 years in a few.

As AOO is an estimate, the attempt to dichotomize the AOO distribution introduces both misclassification of subjects around the cutoff and noise into the datasets. Furthermore, diverse onset ages within genetically defined families demonstrated that, even with the same upstream trigger, AOO can vary, suggesting that other genetic and environmental factors contribute to the AOO phenotype [6]. In addition, in vivo diagnosis of AD is estimated at 95% accuracy, and therefore introduces noise due to some misclassification bias [7]. After consideration of these limitations, there have been few replicable clinical differences between the EOAD and LOAD groups. Differences in the neuropsychological profiles are controversial and inconsistent between studies.

While there is a consensus that LOAD appears to have a more predominant impairment of memory (with verbal memory affected more severely than nonverbal memory in general [8]), it remains unclear whether language, visuospatial abilities, and praxis are more affected or preserved in EOAD compared with LOAD [9]. The literature suggested that AV-951 language is more affected in EOAD with preservation of visuospatial function [10,11], whereas more recently, praxis and visuospatial function appeared to be more affected when compared with LOAD [12]. Most research data support the hypothesis that there is greater involvement of the frontal-parietal structures in EOAD and more predominant deficits in temporal lobe function with a propensity for the left hemisphere in LOAD [10,13,14] Studies investigating the rate of disease progression by measuring cognitive and functional abilities over time yielded variable results.

Some reports demonstrated that EOAD shows a more rapid progression [15-17], and others found that AOO is not a major predictor of the rate of progression [18,19]. Most voxel-based volumetric magnetic resonance imaging studies found that, in LOAD, hippocampal atrophy is prominent [20] whereas the pattern in EOAD is more variable. In EOAD, instances of atrophy of the temporal-parietal [21], then parietal-occipital [20], temporal and posterior cingulate [22], and precuneus [23] areas have been reported.