Distinguishing neurocognitive deficits in adult patients with NP-C from early onset Alzheimer’s dementia

Andreas Johnen, Matthias Pawlowski, Thomas Duning

Niemann-Pick disease type C (NP-C) is a rare, progressive neurodegenerative disease caused by mutations in the NPC1 or the NPC2 gene. Neurocognitive deficits are common in NP-C, particularly in patients with the adolescent/adult-onset form. As a disease-specific therapy is available, it is important to distinguish clinically between the cognitive profiles in NP-C and primary dementia (e.g., early Alzheimer's disease; eAD). In a prospective observational study, we directly compared the neurocognitive profiles of patients with confirmed NP-C (n = 7) and eAD (n = 15). All patients underwent neurocognitive assessment using dementia screening tests (mini-mental status examination [MMSE] and frontal assessment battery [FAB]) and an extensive battery of tests assessing verbal memory, visuoconstructive abilities, visual memory, executive functions and verbal fluency. Overall cognitive impairment (MMSE) was significantly greater in eAD vs. NP-C (p = 0.010). The frequency of patients classified as cognitively 'impaired' was also significantly greater in eAD vs. NP-C (p = 0.025). Patients with NP-C showed relatively preserved verbal memory, but frequent impairment in visual memory, visuoconstruction, executive functions and in particular, verbal fluency. In the eAD group, a wider profile of more frequent and more severe neurocognitive deficits was seen, primarily featuring severe verbal and visual memory deficits along with major executive impairment. Delayed verbal memory recall was a particularly strong distinguishing factor between the two groups. A combination of detailed yet easy-to-apply neurocognitive tests assessing verbal memory, executive functions and verbal fluency may help distinguish NP-C cases from those with primary dementia due to eAD.