Isolated medial temporal lobe amnesia (MTLA): Predictor of cerebral amyloidosis or marker of phenotype-specific vulnerability?

BACKGROUND

Medial temporal lobe amnestic syndrome (MTLA) is classically considered a hallmark of Alzheimer's disease (AD). However, emerging evidence suggests etiological heterogeneity, challenging the assumption that MTLA universally reflects AD pathology.

OBJECTIVE

To determine the prevalence of amyloid pathology in isolated MTLA, identify phenotypic and genetic risk factors, and characterize associated network vulnerabilities in amnestic mild cognitive impairment (aMCI).

METHOD: This retrospective observational study analyzed 55 patients with isolated MTLA at the aMCI stage. Participants underwent neuropsychological testing, cerebrospinal fluid (CSF) biomarker analysis, amyloid PET, and 18FDG-PET. Patients were stratified by amyloid status (positive/negative) and compared for APOE genotype, clinical features, and metabolic patterns. Statistical analyses included the Kruskal-Wallis test for non-parametric group comparisons and chi-square tests for categorical genetic associations.

RESULTS

Amyloid pathology was observed in only 67% (37/55) of MTLA patients, dissociating the syndrome from AD in one-third of cases. Amyloid-positive patients demonstrated a significantly higher APOE ɛ4 carrier rate compared to amyloid-negative peers (χ2=7.02, df=2, P=0.030), while 18FDG-PET revealed inferotemporal hypometabolism in amyloid-positive cases, marking early decontextualized memory impairment.

CONCLUSION

MTLA syndrome is not homogeneous on the biological level and amyloid pathology and APOE ɛ4 genotype stratify patients into distinct subgroups. Amyloid-positive cases demonstrate inferotemporal hypometabolism, suggesting AD-related network vulnerability. By contrast, amyloid-negative MTLA group shows no systemic brain network vulnerabilities, likely due to its heterogeneous etiological origins. These findings advocate for a precision medicine framework integrating biomarkers to guide therapeutic strategies, moving beyond syndromic diagnoses to target underlying mechanisms.