Loss of smell may signal dementia years before diagnosis, German study suggests

Losing the sense of smell may be one of the earliest detectable signs of dementia, according to new research from a German team that traced damage to nerve fibres connecting the brain's scent centre with regions that process sensory information.

The investigators found evidence that the immune system attacks fibres linking the olfactory bulb to the locus coeruleus, a small brainstem nucleus involved in sensory processing and arousal. The pattern of damage appeared in experiments with mice, was visible on PET scans of living patients, and was corroborated in post-mortem tissue from people with Alzheimer’s disease.

In the animal studies, researchers observed early degeneration of the nerve projections that carry scent-related information from the olfactory bulb to the locus coeruleus. PET imaging in living subjects showed disruptions along the same pathway, and analyses of brain tissue after death revealed chemical changes that the authors say can provoke immune responses that target those fibres.

Dementia, including Alzheimer’s disease, is characterized by progressive declines in memory, language and mood. Impaired olfaction has long been reported in people who later receive a dementia diagnosis, but the new work aims to explain a biological mechanism that could link early sensory loss to the neurodegenerative process.

The investigators say the findings raise the possibility that changes in smell could serve as an early biomarker of dementia risk, potentially years before clinical diagnosis. They caution, however, that further research is required to confirm the observations across larger and more diverse human populations and to define how such a marker might be used in clinical practice.

Experts not involved in the study have previously noted that olfactory testing is inexpensive and noninvasive, but they also warn that smell loss can result from many causes, including age, nasal disease and environmental exposures. Determining whether olfactory changes specifically predict dementia will require longitudinal studies that track patients over time.

The authors report converging evidence from animal, imaging and post-mortem data pointing to an immune-mediated process that damages olfactory pathways early in the disease course. If validated in future studies, the finding could inform efforts to detect neurodegenerative disease at a stage when interventions might be more effective.