Odor Tests Could Help Detect Alzheimer’s Disease Before Onset of Symptoms, New Study Suggests

Odor identification tests may help track Alzheimer’s disease (AD) in persons at risk even before the onset of symptoms, a new study shows.

The study, “Odor identification as a biomarker of preclinical AD in older adults at risk,” appeared in the journal Neurology.

Gradual memory loss is observed in several types of dementia, including Alzheimer’s. Although smaller memory lapses, such as forgetting familiar words or the location of objects, are observed early in the disease progress, pathological alterations in the brain may have been going on for as long as 20 years. As a result, methodologies to detect Alzheimer’s disease prior to memory loss have been the focus of intense research.

“Despite all the research in the area, no effective treatment has yet been found for AD,” John Breitner, MD, the lead author of the study, said in a press release. Breitner is the director of the Centre for Studies on Prevention of Alzheimer’s Disease at the Douglas Mental Health Research Centre of McGill University. “But, if we can delay the onset of symptoms by just five years, we should be able to reduce the prevalence and severity of these symptoms by more than 50%.”

The study involved 274 healthy, aging (average age of 63 years) participants with a parental or multiple-sibling history of the disease. The researchers analyzed the association of odor identification with pre-symptomatic Alzheimer’s disease.

 The subjects undertook scratch-and-sniff tests to identify scents, such as the smell of a lemon, bubble gum, or gasoline. Samples of cerebrospinal fluid (which fills and surrounds the brain and spinal cord) were collected by lumbar puncture from 101 participants to quantify the levels of Alzheimer’s-related proteins.

The results revealed that reduced ability to identify odors was associated with increased levels of Alzheimer’s biomarkers. These included the protein tau and an altered form of tau (containing several phosphate groups), which is the primary component of toxic intracellular tangles, one of the key hallmarks of the disease.

The scientists also observed that decreased odor identification was associated with lower cognitive function and increased age.

Overall, the research shows that “that the loss of the ability to identify smells is correlated with biological markers indicating the advance of the disease,” Marie-Elyse Lafaille-Magnan, the study’s first author and a doctoral student at McGill, said.

The author noted that “for more than 30 years, scientists have been exploring the connection between memory loss and the difficulty that patients may have in identifying different odors. This makes sense because it’s known that the olfactory bulb (involved with the sense of smell) and the entorhinal cortex (involved with memory and naming of odours) are among the first brain structures … to be affected by the disease.”

The possibility that a simple odor test may help track Alzheimer’s disease before the onset of symptoms would be a major advantage over more invasive and expensive tests that require collection of the cerebrospinal fluid. “However, problems identifying smells may be indicative of other medical conditions apart from AD and so should not be substituted for the current tests,” Breitner said.

The scientists cautioned that further research is needed to determine how altered ability to identify smells over time relates to Alzimer’s disease progression.