Volumetry and cortical thickness of olfactory‐related structures in Alzheimer's disease: A systematic review and meta‐analysis
Bibliographic record
Abstract
Abstract Background A lesser known early symptom of Alzheimer’s disease is the olfactory decline. The ability to identify an odour may even be a predictor of conversion from mild cognitive impairment (MCI) to AD. This sensory decline could reflect AD‐related brain damage, including cerebral atrophy of structures related to the sense of smell. Method The aim of this systematic review is to summarize current knowledge about the volume and cortical thickness of olfactory brain structures in the course of AD. Articles were collected from the PubMed database using the keywords olfac*, smell, MRI, volume, VBM, thickness, Alzheimer, MCI, POC, olfactory bulb, hippocamp*, amygdala, entorhinal cortex, piriform cortex. Articles were included in the review according to the following criteria: (1) participants had a diagnosis of AD or Mild Cognitive Impairment (MCI); (2) the presence of a healthy control group of the same age range; (3) cortical volume or thickness was calculated from MRI scans; and (4) there was an assessment of olfactory capacities. A meta‐analysis is in preparation. Result Twelve studies met the inclusion criteria. All of them report lower olfactory capacities in patients (AD or MCI). Eight studies report a reduced hippocampal volume and 9 studies relate olfactory capacities to hippocampal volume in these patients. Analogous results are found with the primary olfactory cortex. Amygdala and entorhinal cortex volumes are also affected, but their relationship to olfactory capacity is inconsistent. Only one study measured the volume of the olfactory bulb and reported no significant results. Links were also found between olfactory measurements and the volume of the pre‐central gyrus, ventricles and neocortex. Conclusion The olfactory decline observed in the early stages of AD may reflect brain damage already present in these stages, mainly to the hippocampus and the primary olfactory cortex. These findings support the proposition that an olfactory decline could be an early marker of AD.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.008 | 0.021 |
| Meta-epidemiology (narrow) | 0.002 | 0.001 |
| Meta-epidemiology (broad) | 0.015 | 0.023 |
| Bibliometrics | 0.005 | 0.007 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.003 | 0.001 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.002 | 0.001 |
| Insufficient payload (model declined to judge) | 0.004 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".