Overview

What is Anosmia?

Anosmia is a condition where you lose your sense of smell. It can be short-term, often resulting from upper respiratory infections or non-communicable diseases such as diabetes. In some instances, anosmia can be permanent, stemming from underlying illnesses or congenital factors (from birth).¹ It is also important to note that anosmia can occur as a prodromal symptom, meaning that it could also be an early sign of a more significant or serious underlying health issue. As people get older, they are more likely to experience a decline in their sense of smell. Research indicates that the percentage of people with smell-related issues ranges from 3-25%.²

The Importance of Prodromal Symptoms 

Prodromal symptoms refer to early warning signs that appear before a disease starts to develop entirely. They are especially crucial when it comes to detecting neurodegenerative diseases, particularly non-motor (movement) and non-cognitive (memory) signs. These early warning signs can emerge during what is known as the ‘prodromal phase’, which means the period before the more recognisable motor or cognitive signs show up.

Connection to Neurodegenerative Diseases

One example of a prodromal symptom is anosmia. It has been linked to early stages of neurodegenerative diseases like Parkinson’s disease (PD) and Alzheimer’s disease (AD). Since the sense of smell, also referred to as olfaction, relies on specific brain regions, research has found that both PD and AD can affect areas that help smell function, leading to impaired olfaction. Identifying anosmia as a potential early sign of these diseases is especially important for those who may be genetically predisposed (hereditary) to developing them. Being aware of such subtle changes in smell could support earlier detection and, potentially, early interventions.

Anosmia in Parkinson’s Disease

Parkinson’s disease (PD) is a progressive brain disorder that gradually worsens a person's ability to move and their cognitive function. Over time, it leads to issues such as tremors (constant shaking), decreased mobility, muscle stiffness, and problems with balance and coordination, which are the most recognisable symptoms of PD.

Link to Anosmia

Interestingly, research suggests that problems related to sense of smell, known as olfactory impairment, can be one of the earliest signs of PD.³ In fact, anosmia is considered one of the five most common symptoms in the early stages of the disease, affecting both motor and non-motor functions. What makes this fundamental is that anosmia often appears several years or decades before the more visible symptoms, such as tremors. This early onset is because the brain regions responsible for smell are among the first to be affected by the disease. Both clinical signs (like anosmia or hyposmia, which is a reduced sense of smell) and autopsy studies support this view, revealing early accumulation of abnormal proteins in the brain areas linked to smell. This means that testing someone’s sense of smell and taste could be a valuable tool for identifying PD earlier. Ageusia (loss of taste) is another early, though less recognised, symptom that may also point to the disease. Both could potentially act as early warning signs or biomarkers for PD. There is also evidence to suggest a connection between loss of smell and cognitive decline in people with PD. However, researchers are still working to fully understand how this link fits into the broader picture of the disease.⁴ 

Prevalence

• Recent research shows that over 95% of people with PD experience a noticeable loss of smell even before the more well-known symptoms of PD appear³
• Studies on this date back to the 1970s and consistently show that smell loss is ubiquitous in PD patients, with reported rates ranging from 45% to 90%³
• In one more recent study, over 96% of people with PD were found to have some kind of problem with their sense of smell, and around 75% of them had a clear diagnosis of smell loss (also known as olfactory dysfunction)³

Supporting Evidence

In short, research supports the idea that anosmia is a prodromal symptom of PD. One major piece of evidence comes from the Parkinson’s Associated Risk Syndrome (PARS) study, which explored whether combining smell tests with brain scans could help identify people at risk of developing PD. While smell loss on its own is a useful early signal, it is not exclusive to PD; it can occur in other conditions. Despite this, the study found that when someone has both hyposmia and reduced dopamine activity on brain scans, their chances of eventually developing the classic motor symptoms of PD are significantly higher.⁵ Another study also showed that having problems with the sense of smell greatly increases the risk of developing PD, especially in groups that included more women, older adults, or where the follow-up period was relatively short.⁶ 

Anosmia in Alzheimer’s Disease

Understanding Alzheimer’s Disease

Alzheimer's Disease is the most common cause of dementia, and the two terms are often confused, so let’s clear that up. Dementia is not a specific disease; it is a broad term used to describe a set of symptoms that affect memory, thinking, and social abilities enormously enough to impact their everyday life. So, in simple terms, dementia refers to the symptoms, while Alzheimer’s is a disease that causes those symptoms.

Alzheimer’s usually starts with mild memory problems but gradually progresses into more severe and life-altering symptoms. It mostly affects older adults and people with a family history of the condition, though age is the biggest risk factor.

Anosmia and Alzheimer’s

Losing your sense of smell has been recognised as a potential prodromal symptom of Alzheimer’s disease. Research from the Rush Memory and Aging project has shown that older adults who struggle to identify familiar smells were more likely to later develop mild cognitive impairment, which is often the first noticeable stage before Alzheimer’s sets in.⁷ 

One study focused precisely on whether a decline in the ability to recognise smells was connected to how quickly episodic memory declined, a core symptom of Alzheimer’s. The findings were clear: people who scored lower on smell identification tests experienced a faster decline in memory, even before receiving any formal diagnosis. On top of that, the same researchers studied whether smell loss was linked to actual brain changes due to Alzheimer’s, even in individuals who never showed signs of memory loss while alive. Their autopsy results showed that those with poorer smell identification scores had more Alzheimer’s related brain damage, despite appearing symptom-free during their life.⁷ 

This all points to one major insight: a poor sense of smell may act as a valuable early warning sign for Alzheimer’s, not just in terms of memory decline, but also in detecting hidden brain changes before typical symptoms emerge. In the same one large study of 500 older adults with no signs of memory issues at the start, those who had difficulty identifying everyday smells were:

• More likely to show early brain changes linked to Alzheimer’s
• More likely to develop mild cognitive impairment
• More likely to experience memory decline over time

We already know that smell loss is common in people who have Alzheimer’s, and also in those with mild cognitive impairment or genetic predisposition. But this study reinforced that even people with no symptoms can still be at increased risk if their sense of smell is impaired. Long story short, testing someone’s ability to identify smells could be a simple way to spot Alzheimer’s risk early, possibly years before symptoms like memory loss and confusion appear. Additionally, when this testing is combined with other medical and genetic assessments, it may offer an even clearer picture of the actual risk.⁷  

Lastly, research estimated that 85% to 90% of people in the early stages of Alzheimer’s show some level of olfactory dysfunction. Due to this strong and consistent link in research, smell testing is gaining attention as a potential diagnostic tool. There is growing interest in employing  it to help both Parkinson’s and Alzheimer’s diseases in their earliest stage before cognitive decline and physical disability begin to take place.⁸ 

Summary

Anosmia, the loss of smell, has been identified as a fundamental prodromal symptom of both Parkinson’s and Alzheimer’s disease. A growing body of research consistently supports its role as a potential warning sign years before these neurodegenerative conditions are diagnosed. It is especially valuable for early detection in older adults or those with a family history of Alzheimer’s, as olfactory dysfunction could serve as an early indicator that further medical evaluation might be needed.

Since anosmia has only recently been recognised as a prodromal symptom, it has opened up new opportunities in research and clinical practice, encouraging scientists and healthcare professionals to explore it as a non-invasive, early diagnostic tool for identifying those at risk before the obvious symptoms develop.