Changes in smell are not always cold. Understand common causes, when to raise the alarm and how to seek diagnosis early to protect memory and quality of life.
Changes in smell are usually seen as a temporary problem and of little clinical relevance. They are generally attributed to flu, allergies or, more recently, Covid-19. However, scientific evidence accumulated over the last few decades shows that, in certain contexts, subtle changes in the perception of smells can be an early sign of neurological diseases, including Alzheimer’s.
Recognizing when the loss of smell is a benign finding and when it deserves medical investigation is essential for early diagnosis, adequate monitoring and preservation of quality of life.
Smell as a neurological marker
The olfactory system has a unique characteristic among the senses: its pathways connect directly to regions of the brain linked to memory, emotions and behavior, such as the olfactory bulb, the limbic system and the entorhinal cortex. Unlike vision or hearing, smell stimuli do not first pass through the thalamus, quickly reaching deep structures in the brain.
These regions are among the first to undergo structural and functional changes in the neurodegenerative process of Alzheimer’s. Therefore, studies show that deficits in identification, discrimination and recognition of odors can appear years before classic cognitive symptoms, functioning as an early functional marker.
Recent research indicates that simple odor identification tests, combined with brief cognitive assessments, can help identify people at higher risk of cognitive decline, expanding screening possibilities in clinical settings and primary care.
Not everything is covid: common causes of loss of smell
Despite the attention given to Covid-19, most changes in smell have non-neurological causes. Among the most common are allergic rhinitis, chronic rhinosinusitis, various viral infections, smoking, use of certain medications, exposure to irritating substances and head injuries.
In these cases, the loss of smell is usually accompanied by nasal or respiratory symptoms and tends to improve with specific treatment. Correctly differentiating local causes from neurological causes is essential to avoid alarmism and delays in diagnosis.
When the symptom should trigger a warning sign
The warning sign appears when the change in smell is progressive, persistent and cannot be explained by obvious nasal diseases. The risk is greater when the symptom appears in people over 60 years of age or is associated with subtle cognitive complaints, such as memory lapses, difficulty concentrating or changes in behavior.
In these scenarios, the olfactory change should be interpreted as part of a broader clinical picture, and not as an isolated finding.
Early diagnosis and follow-up
The initial approach usually includes otorhinolaryngological evaluation, with the aim of excluding inflammatory or nasal structural causes. In the absence of a local explanation, or in the presence of associated neurological signs, the investigation must continue with neurological and neuropsychological evaluation.
The current concept of Alzheimer’s diagnosis has evolved into a biomarker-based model, recognizing that the pathological process begins long before the evident clinical manifestation. In this context, smell can act as an accessible sentinel signal, guiding the need for closer monitoring.
Early identification allows for better care planning, control of modifiable risk factors, adequate guidance for patients and families, and preservation of autonomy and safety.
A small symptom, a big impact
The loss of smell affects not only food and social pleasure, but also everyday safety by reducing the perception of smoke, gas leaks and spoiled food. In certain contexts, it can be a silent sign that the brain is in the process of changing.
Recognizing this symptom with attention, balance and a scientific basis contributes to a more preventive, humane and future-oriented approach to medicine.
José Carlos Rodrigues Junior – CRM 106.636 / EQR 29137 – Neurosurgeon
