Hippocampal Atrophy Subtypes of Alzheimer’s Disease Using Automatic MRI in a Memory Clinic Cohort: Clinical Implications

Introduction: One pathological hallmark of Alzheimer’s disease (AD) is atrophy of medial temporal brain regions that can be visualized on magnetic resonance imaging (MRI), but not all patients will have atrophy. The aim was to use MRI to categorize patients according to their hippocampal atrophy status and to present prevalence of the subtypes, difference in clinical symptomatology and progression, and factors associated with hippocampal subtypes. Methods: We included 215 patients with AD who had been assessed with the clinically available MRI software NeuroQuant (NQ; CorTechs labs/University of California, San Diego, CA, USA). NQ measures the hippocampus volume and calculates a normative percentile. Atrophy was regarded to be present if the percentile was ≤5. Demographics, cognitive measurements, AD phenotypes, apolipoprotein E status, and results from cerebrospinal fluid and amyloid positron emission tomography analyses were included as explanatory variables of the hippocampal subtypes. Results: Of all, 60% had no hippocampal atrophy. These patients were younger and less cognitively impaired concerning global measures, memory function, and abstraction but impaired concerning executive, visuospatial, and semantic fluency, and more of them had nonamnestic AD, compared to those with hippocampal atrophy. No difference in progression rate was observed between the two groups. In mild cognitive impairment patients, amyloid pathology was associated with the no hippocampal atrophy group. Conclusion: The results have clinical implications. Clinicians should be aware of the large proportion of AD patients presenting without atrophy of the hippocampus as measured with this clinical MRI method in the diagnostic set up and that nonamnestic phenotypes are more common in this group as compared to those with atrophy. Furthermore, the findings are relevant in clinical trials.