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Smell Disorders 101

Author Julia Casazza covers Smell Disorders 101 on BackTable ENT

Julia Casazza • Sep 10, 2023 • 38 hits

Though olfactory dysfunction greatly influences patients’ quality of life, it remains an under-studied topic within otolaryngology. Dr. Zara Patel, rhinologist and internationally-renowned expert on olfaction, recently discussed this topic on the BackTable ENT podcast. In this article, we define important terms, explain how to test olfaction, and review the potential causes of olfactory dysfunction.

This article features excerpts from the BackTable ENT Podcast. We’ve provided the highlight reel in this article, and you can listen to the full podcast below.

The BackTable ENT Brief

• Olfactory dysfunction is a common rhinology and general otolaryngology chief complaint.

• COVID-19-associated smell loss brought new attention to the importance of smell in overall health.

• Anosmia is complete absence of smell, whereas hyposmia is reduction in ability to smell

• Olfactory tests include “scratch-and-sniff” booklets to assess odor identification and more sophisticated kits such as “Sniffin Sticks” that can assess identification, threshold, and odor discrimination.

• Post-viral syndromes, congenital illnesses, chemical exposures, metabolic disorders, and neurodegenerative disease can all present with smell loss.

• In an older patient presenting with smell loss as an isolated complaint, baseline cognitive testing should be ordered.

Causes for olfactory dysfunction include viral infection, trauma, and toxic medications.

Table of Contents

(1) Olfactory Dysfunction Definitions

(2) Assessment of Olfactory Dysfunction

(3) Etiologies of Olfactory Dysfunction

Olfactory Dysfunction Definitions

Due in part to COVID-19 smell loss, both clinicians and patients now better appreciate the importance of olfaction. On a broad level, olfactory dysfunction can be categorized as anosmia, hyposmia, parosmia, or phantosmia. Anosmia is a complete inability to smell, whereas hyposmia is a diminished ability to smell. Parosmia is a situation-dependent distortion of smell; for example, patients with cacosmia smell feces upon perception of a known trigger. Hundreds of molecules must be detected to perceive a complex smell, so neuronal death (due to infection, trauma, or other etiologies) will leave a patient able to perceive only a fraction of these molecules and thus with an incomplete perception of the smell. Phantosmia is a consistent perception of a smell that isn’t present. Phantosmia results from damage at any level of the olfactory pathway, and can be the presenting symptom of a tumor, or even a migraine aura.

[Dr. Gopi Shah]
Let's get into it. I wanted to first start with terminology, because I know anosmia, but then we have hyposmia, parosmia, phantosmia. I don't know if there's other “-osmias,” but can we just go into some of the terminology that we should know, and the differences?

[Dr. Zara Patel]
Anosmia is no smell. They have no smelling ability. Hyposmia would just be a decrease in smelling ability. Parosmia is the distortion or alteration of smell. Phantosmia is when someone is smelling something that just isn't there actually in the environment, a phantom smell. Then there's other asmias that more specifically characterize parosmias. For example, a very unfortunately common parosmia is cacosmia, where someone smells feces anytime they smell something or eat something that smells like feces. There's like very specific “-osmias” characterizing exactly the way that smell is altered also.

[Dr. Gopi Shah]
I did peds and what I enjoy is pediatric sinus. I'd see some of the adolescents with small distortion. I recall one patient specifically. He's there with mom and every time they would sit down for dinner, it would smell like garbage. Any dinner that they would have. Is that a parosmia or a phantosmia?

[Dr. Zara Patel]
It sounds like it's a parosmia because he wasn't smelling garbage all day long no matter where he was or what he was doing. It sounds like specifically when he was trying to eat, that is what smelled and probably tasted like garbage. That's a parosmia. Unfortunately, it is never a good smell. It's always some terrible smell. People will say feces or rotting flesh, or garbage, or a sweet chemical scent.

There's very specific categories of parosmias. We don't know exactly why, but I suspect that just from an evolutionary standpoint, when our brain is receiving a signal that it is not familiar with, it wants us to avoid whatever that unknown thing is and not put it in our body, and so it automatically distorts into this bad odor and taste that we are repulsed by it.

[Dr. Gopi Shah]
Leading into how patients present to you, if the chief complaint is something like parosmia, or when they're eating dinner but it smells like trash or something like that, is that workup differential or question set your history different than if a patient comes in with just anosmia?

[Dr. Zara Patel]
It really is not that different because when you think about why we have parosmia, again, there's a lot of research going on about specifically what's wrong, and it likely may be on a multi-level process of what's going wrong with parosmias, but what we do know is that we see parosmia quite often when we have hyposmia or anosmia, especially in a post-viral situation, and especially with COVID-19. The percentage of people with parosmia after COVID-19 is much higher than other post-viral illnesses, honestly. It's almost a mark of the smell loss that occurs with COVID-19.

Likely, one of the major reasons why we develop parosmia is that we have not been able to have our full complement of olfactory neurons bounce back and regenerate after the initial insult. It is intuitive that if you lose some neurons, they're not able to regenerate correctly, then you're not going to be able to smell as well. You'll have a decrease in smell. Also, if you think about the way in which we smell, take a complex odor like coffee, for example. That's dozens or hundreds of different molecules that are filtering into your nose, dissolving into the mucus layer, and then attaching to many, many different olfactory receptor neurons.

What happens with parosmia is that some of those neurons are still there. They're able to pick up on particular molecules in that entire aroma, but they're not able to pick up on all of them. Instead of smelling that full complement of both the slightly bitter but also the warm and toasty aspects of coffee, some people will only be able to pick up on that acrid, bitter component and miss the pleasant component because they don't have those neurons to pick up on those particular molecules.

It makes sense that when you're missing some neurons, that you may not be able to pick up on the entire odor, and that tends to end up being a bad odor or smell. There may be, on the higher level in the olfactory bulb or olfactory cortex, other things also going on that are causing this aberrant signaling, but I suspect that the majority of it is just that you're not able to send the entire signal correctly.

Listen to the Full Podcast

Evaluation & Management of Patients with Olfactory Dysfunction with Dr. Zara Patel on the BackTable ENT Podcast)
Ep 122 Evaluation & Management of Patients with Olfactory Dysfunction with Dr. Zara Patel
00:00 / 01:04

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Assessment of Olfactory Dysfunction

Olfactory tests assist clinicians in defining an olfactory disturbance. Unlike vision or hearing, there is no standard assessment for smell, and patient perception of odors can be affected by previous experience. Whenever olfactory testing is used, keep in mind that a patient’s socio-cultural background influences what is detected. For example, a patient who recently immigrated from a culture that doesn’t drink coffee might be able to smell – but not name – the odor of American coffee. On a basic level, “scratch-and-sniff” booklets such as the University of Pennsylvania Smell Identification Test (UPSIT) can tell clinicians whether patients are able to identify specific odors. More sophisticated tests such as Sniffin’ Sticks can assess identification, threshold, and discrimination between different odors.

[Dr. Zara Patel]
I will tell you that the way that we define [hyposmia] and the way that we test smell in general is fairly limited. We have these scratch and sniff booklets like the UPSIT, which is what many of our listeners will be familiar with, the University of Pennsylvania Smell Identification Test. Then there's more granular type of tests like Sniffin’ Sticks, which don't just test identification, but also test threshold and discrimination. That's also just smelling pens and introducing them to someone and asking them whether they can pick up on it and what they smell.

Can they discriminate? Things like that. All of those types of tests. Honestly, if you look at that review, that ICARs team, you'll see there are dozens of different smell tests out there on the world. There's a 23-page table outlining all of the different smell tests that we have. That shows you that when we have so many different options, there is no one really amazing option to test smell. That's actually something that I'm currently working on right now, an objective measure of smell function. Because if you think about those ways that we test smell, the main problems are the subjectivity.

It matters whether someone was born in the US versus a different country, the smells that they're going to be able to identify and that they're familiar with. It even matters whether you're testing someone who has lived their entire life in an inner city, urban environment, or if they were raised in the countryside within the US. It matters whether someone can speak English to participate in a test. It matters whether someone is literate, whether they can read and circle the correct answer on something like the UPSIT. Of course, cognition plays a huge role in whether someone understands how to do a test like that.

Because of all these barriers and biases, it really prompted me to start developing this device that I'm currently working on, an endoscopic device to actually directly record the neuronal signal from the olfactory epithelium, which I think will be just a huge change in the way that we think about smell. We think about the way that we test vision and the way that we test hearing, it's all very objective at this point.

We really need to bring our field of olfaction into the realm of objectivity. To get back to your question the reason that probably a lot of the people that come to see me are hyposmic but they still feel, they say that they can't smell anything, and that's because we're just really testing for the one facet of smell in that UPSIT. There's so much more nuance of whether people can pick up on smells in their environments. We're missing probably a lot of information when we test smell.

Etiologies of Olfactory Dysfunction

Much like other sensory disorders, olfactory dysfunction often results from systemic processes. Common etiologies of smell loss include post-viral infection, chemical exposures, metabolic disorders, psychiatric illness, congenital illnesses, and neurodegenerative disease. Given greater public awareness of smell loss as the initial symptom of neurodegenerative diseases (including Alzheimer’s and Parkinson’s), clinicians must broach this topic with sensitivity. Dr. Patel suggests addressing neurodegenerative disease as one of many possible explanations for olfactory dysfunction in older patients. When she sees patients over the age of 65 presenting with isolated smell loss, she reassures them regarding their current cognition and suggests getting a baseline cognitive assessment in case issues arise later.

[Dr. Gopi Shah]
I love that you brought up all of those factors regarding language, where people have lived most of their lives, where they are by the time they get tiers at a new country, city, place, because that has a lot of different aspects of equity to it. I wanted to ask you about more equitable care, but we're going to get to that. We'll get to that because all that-- When a patient comes in to clinic, what is always part of your list of questions? What are you always, in your history, checking for? Are you looking at the medication list? What are the key things?

[Dr. Zara Patel]

I probably will not list everything here that you could potentially think about. I will just mention that in that ICAR document and that statement, at the very end of that document, because it's a huge document, it's like 600 pages or something, but at the very end, there is an algorithm that takes people all the way from diagnosis through treatment. At the beginning of that algorithm, we really list out all the different factors that can affect it. The highlights of what I ask about are certainly any history of sinus or nasal inflammation, that's still really the number one reason why people have smell issues.

Also, of course, any preceding post-viral type of event. COVID-19 is a very common reason why people come into our clinics now for smell loss, but even before the pandemic, I was treating and researching post-viral smell loss for the last decade before the pandemic even hit. That has always been one of the reasons. COVID-19 is one, but other coronaviruses, influenza viruses, rhinoviruses. It's not just whether they had COVID or not, but any viral events is important to ask about. Then any trauma. Certainly, you go through their medication list.

You ask them about their environment, so their job. Sometimes you'll be surprised what people are exposed to in their jobs. Then you certainly want to just take a really full medical history. You want to know if people have had cancer, whether they've had radiation or chemotherapy. It doesn't matter if it's a head and neck cancer or a cancer of their leg, if they had chemotherapy, then that certainly will still affect neuronal ability and function. You want to ask about metabolic issues, endocrine issues. We know that things like diabetes and hypothyroidism affect smell. You want to ask about other neurotransmitter or cognitive issues.

Any depression, any schizophrenia, any autism, all of those types of issues will have an impact on smell. Of course, especially in our older patient population, you want to ask them about any other neurodegenerative-type symptoms. You want to ask about any family history of that to know if they have any risk factors, because as we're learning more and more in the last five years or so, smell loss really is probably the very earliest sign of neurodegenerative disease. I suspect that a lot of the smell loss that in the past was categorized in the idiopathic category, we didn't know why, will eventually end up being proven to be early signs of neurodegenerative disease.

[Dr. Gopi Shah]
Wow. Do you talk to your patients about that as a potential? How does that conversation go?

[Dr. Zara Patel]
Yes, I do. It is definitely a delicate conversation. You certainly don't want to scare people or make them worry unnecessarily. The way that I frame it, and I really tend to bring this up in people who are already 65 and older, or if they have a direct family member like their mother or father, a neurodegenerative disease, then I frame it and I go through all the different possible things that are contributing to their smell loss or smell dysfunction. Often when I see patients, there's more than just one factor that could potentially be contributing, either causing the smell loss or preventing people from being able to bounce back from a smell loss from something else like COVID-19.

I talk about all the different factors that are in their history that could be contributing. Then I do mention at the end, just to give you the information that we do know now that smell loss can be one of the earliest signs of neurodegenerative disease. Most of the time I say this, after having this conversation with you, I feel like you are currently mentally completely intact and have high cognitive function, and so I wouldn't worry about that currently, but if you start noticing any difference in your memory, any difference in your motor movement ability, I want you to remember what I've told you about smell loss being an early sign.

I do think it would be a good idea for you to get a baseline cognitive test now, so that if you ever do develop those other symptoms, you have something to compare against. That's how I frame it. Every so often, I will see a patient who I actually do think has cognitive dysfunction during my interview with them. I gently make note of that to them, and I ask them to see one of my neurology colleagues to be further evaluated.

Podcast Contributors

Dr. Zara Patel discusses Evaluation & Management of Patients with Olfactory Dysfunction on the BackTable 122 Podcast

Dr. Zara Patel

Dr. Zara Patel is director of endoscopic skull base surgery and a professor of otolaryngology and neurosurgery at Stanford in California.

Dr. Gopi Shah discusses Evaluation & Management of Patients with Olfactory Dysfunction on the BackTable 122 Podcast

Dr. Gopi Shah

Dr. Gopi Shah is a practicing ENT at UT Southwestern Medical Center in Dallas, TX.

Cite This Podcast

BackTable, LLC (Producer). (2023, August 1). Ep. 122 – Evaluation & Management of Patients with Olfactory Dysfunction [Audio podcast]. Retrieved from https://www.backtable.com

Disclaimer: The Materials available on BackTable.com are for informational and educational purposes only and are not a substitute for the professional judgment of a healthcare professional in diagnosing and treating patients. The opinions expressed by participants of the BackTable Podcast belong solely to the participants, and do not necessarily reflect the views of BackTable.

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