Alzheimer’s disease is a progressive neurological disorder that gradually destroys memory and thinking skills. While memory loss is a widely recognized symptom, detecting the disease early is gaining importance for effective management and potential future treatments. Recent research increasingly points to changes in the sense of smell as a subtle, early indicator of this condition.
The Link Between Smell and Alzheimer’s
Individuals might find it difficult to identify particular odors, such as coffee, lemon, or even smoke, or struggle to distinguish between different scents. This reduced ability to smell, known as hyposmia, or a complete loss of smell, called anosmia, can appear several years before more commonly recognized cognitive symptoms, like memory difficulties, become apparent. Research, including a study tracking participants from the Baltimore Longitudinal Study of Aging, has shown that lower performance on odor identification tests is associated with a higher chance of developing mild cognitive impairment (MCI), often linked to Alzheimer’s. This connection has been observed in both human and animal studies.
A rapid decline in the sense of smell over time can also predict a loss of cognitive function and structural changes in brain regions relevant to Alzheimer’s disease. One study found that a quick decline in smell among older adults with normal cognition predicted smaller gray matter volume in brain areas associated with smell and memory, worse cognitive function, and an increased risk of dementia. The risk of smell loss in these cases was comparable to carrying the APOE-e4 gene, a known genetic risk factor for Alzheimer’s disease. This suggests olfactory changes are closely linked to neuropathological damage progression.
Why Smell Changes Occur
Olfactory changes in Alzheimer’s disease involve the condition’s characteristic pathologies. Amyloid plaques and tau tangles, the hallmark protein aggregates of Alzheimer’s, begin to accumulate in specific brain regions that are important for processing smell. These regions include the olfactory bulb, which receives direct input from the nasal cavity, and the entorhinal cortex, a significant input area to the hippocampus, a brain area deeply involved in memory. This accumulation can occur in these areas even before spreading to other parts of the brain associated with memory and other cognitive functions.
Damage to these neural pathways disrupts the brain’s ability to interpret smell signals. Neurofibrillary tangles, composed of tau protein, have been detected in the olfactory bulb at very early stages of Alzheimer’s pathology, sometimes even preceding damage to the entorhinal cortex. While amyloid-beta also accumulates in the olfactory bulb, tau pathology shows a stronger association with disease progression and cognitive decline. The presence of these protein aggregates in olfactory regions is directly linked to a higher incidence of incorrectly identified odors, suggesting that neuronal alterations related to tau pathology might be a direct cause of olfactory dysfunction.
Recognizing and Addressing Smell Changes
Individuals and families should be aware of subtle smell changes. These changes might manifest as difficulty detecting once-recognizable odors, such as coffee, lemons, or the dangerous smell of a gas leak or spoiled food. These alterations are often subtle and might be dismissed as minor or attributed to other factors. Someone might also notice they no longer enjoy favorite perfumes or flowers as intensely as before.
If persistent or concerning smell changes are noticed, consult a healthcare professional for a comprehensive evaluation. This consultation is for a professional assessment to determine the cause of smell loss, not self-diagnosis. A doctor can conduct specific tests to assess olfactory function and consider other symptoms or medical history to arrive at an accurate diagnosis. Early detection of any underlying condition allows for timely intervention and management.
Distinguishing Smell Changes from Other Conditions
While smell changes can indicate Alzheimer’s, many other common conditions also cause diminished smell. These causes are often benign and do not indicate a serious neurological disorder.
Temporary smell loss is common with upper respiratory infections like colds, flu, or sinus infections, due to nasal congestion and inflammation. Allergies, like hay fever, can also lead to similar congestion and reduced smell. Other physical obstructions in the nasal passages, such as nasal polyps or tumors, can block airflow and prevent odor molecules from reaching the olfactory receptors. Head injuries can sometimes damage the nerves involved in smell, leading to a loss of olfactory function. Certain medications, including some antibiotics, antihistamines, and drugs for high blood pressure, can also affect the sense of smell as a side effect. Aging itself, known as presbyosmia, is a common cause of gradual smell decline, typically after age 60, as nasal membranes thin and nerves become less efficient.
Beyond these common causes, other neurological conditions can also lead to smell disturbances. Parkinson’s disease, for example, frequently involves early smell loss, often preceding motor symptoms. Conditions like multiple sclerosis, Huntington’s disease, and even certain vitamin deficiencies (such as vitamin B-12 or zinc) can impact the olfactory system. Given the wide range of potential causes for smell changes, a professional diagnosis is necessary to differentiate the underlying reason and determine if it is related to Alzheimer’s or another condition.