Smell disorders

Introduction

Historically, disorders of the sense of smell and taste have been given little consideration, partly due to a lack of knowledge and understanding of their complex functioning and partly because they were considered to have no significant implications for health.

Smell disorders affect approximately 14% of the population, and their incidence is known to increase with age, as there is a deterioration of olfactory sensory neurons around the age of 40. Furthermore, 80% of taste disorders are secondary to olfactory deficits.

Suffering from a disorder of smell and/or taste can have serious consequences on nutrition, lead to social isolation, psychological distress for the affected individual, and can even be life-threatening as one might not detect the smell of gas or consume spoiled food.

 

Common Terminology

Smell Disorders:

  • Anosmia: the inability to detect odors
  • Hyposmia: reduced ability to detect odors
  • Dysosmia: includes any other olfactory alteration

Taste Disorders:

  • Ageusia: the inability to detect flavors
  • Hypogeusia: reduced ability to detect flavors
  • Dysgeusia: a change or alteration in the perception of tastes

 

Anatomy and Physiology of Smell

alteraciones del olfato otorrino madrid

The sense of smell begins when the chemical substances dissolved in the air we breathe reach the top of the nasal passages, where they stimulate receptors or olfactory neur

ons located in

the mucosa. This stimulation generates an electrical signal that travels through the olfactory pathway to the cerebral cortex and the limbic system, which conditions physiological resp

onses to olfactory stimuli, such as hunger, sexual instincts, involuntary memory, or specific emotions.

The intensity of stimulation depends on the quantity of odoriferous substances in the air, and the chemical composition of these substances determines which receptors are stimulated, resulting in different olfactory sensations.

It is estimated that we can detect up to 10,000 different odors, although there are significant variations among individuals.

Odors are categorized into six different groups:

  • Fruity odors (orange, lemon, pineapple, banana, etc.)
  • Floral odors (rose, lavender, jasmine, etc.)
  • Resinous odors (eucalyptus, pine, wood, etc.)
  • Spicy odors (cinnamon, cloves, vanilla, etc.)
  • Burnt or smoky odors (smoke, popcorn, etc.)
  • Decomposed or rotten odors

perdida gusto otorrino madrid

Why do we lose taste when we lose smell?

The perception of taste is a complex sensory experience that involves not only the sense of taste but also the sense of smell, touch, and even sight.

When we eat or drink, we perceive odors through two different pathways: the external environmental pathway (anterior nasal pathway) and through the mouth and nasopharynx (retronasal pathway). Scientific studies have shown that odors are perceived differently depending on which pathway reaches the receptors. It is also known that the retronasal pathway plays a significant role in the ability to identify tastes and likely reflects the evolutionary role of the sense of smell in humans.

The aroma of food is the most important factor contributing to the identification of flavors, and it is common for patients to misidentify olfactory dysfunction as a taste problem.

 

What Are the Causes of Smell Disorders?

Most patients who seek medical attention for a smell disorder present with hyposmia or dysosmia, with a complete loss of smell (anosmia) being less common.

Common causes of these disorders include:

  • Diseases of the nasal passages and paranasal sinuses: This group includes rhinosinusitis with or without polyps and allergic rhinitis. In these cases, olfactory dysfunction results from inflammation of the mucosa. A rarer cause is the presence of a neuroblastoma, a malignant tumor originating in the olfactory epithelium.
  • Infections and post-infection status: In the context of acute upper respiratory infections, temporary hyposmia or anosmia is common due to the effects of the infection on the nasal mucosa. In most patients, olfactory function recovers as the infection resolves.

However, viral upper airway infections can also cause damage to the peripheral olfactory receptor system or even to the central neuronal transmission system, and in 6 to 13% of patients, olfactory dysfunction may persist after the infection has cleared.

In COVID-19, olfactory and taste disorders are the primary neurological symptoms and may even be the only manifestation of the disease in up to 60% of cases. It is known that these symptoms have a sudden onset 2 to 14 days after exposure to the virus.

Research led by a team of neuroscientists at Harvard Medical School (USA) and published in the journal Science Advances has concluded that COVID-19-associated anosmia is due to the virus infecting a type of nerve cell in the nasal cavity that is not a neuron but a support cell (glial cell). This implies that, in the majority of cases, SARS-CoV-2 infection is unlikely to cause permanent damage to the olfactory neural circuits and lead to persistent anosmia.

  • Post-traumatic: Head traumas are a common cause of smell disorders, which can occur even in mild traumas. There is a strong correlation between the severity of the trauma and the extent of olfactory impairment. Olfactory recovery typically occurs during the first few months after the onset of symptoms, and the chances of improvement are low after one year.

Mechanisms by which trauma can cause olfactory impairment include:

  • Nasal fractures
  • Septal deviations
  • Edema and nasal mucosa bleeding Septal or sinus surgeries due to mechanical obstruction or damage to the olfactory neuroepithelium
  • Disruption of the first olfactory axons as they pass through the base of the skull
  • Contusion or destruction of the olfactory bulb
  • Contusion or destruction of the olfactory cortex

 

  • Other Central Nervous System Disorders: Certain conditions, such as neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease) or strokes (ischemic or hemorrhagic), can affect the central olfactory centers.
  • Exposure to Chemicals, Toxins, and Metals: Contact, typically in the workplace, with substances like methacrylate vapor, ammonia, formaldehyde, or sulfuric acid, among others, is known to cause olfactory disorders.
  • Medications and Drugs: Some medications can cause olfactory dysfunction, although to a lesser extent than they affect taste. Common culprits include certain antihypertensive drugs like beta-blockers, calcium channel blockers, and ACE inhibitors. Intranasal cocaine use can damage both the olfactory neuroepithelium in the nasal mucosa and the central transmission system.
  • Tobacco: Smoking tobacco can lead to olfactory disorders, which are generally reversible after quitting smoking.
  • Endocrine System Disorders: Diseases like hypothyroidism or diabetes mellitus can also be associated with olfactory dysfunction.

 

How is the diagnosis of smell disorders made?

The first step in diagnosing any deficit in the sense of smell is to conduct a comprehensive medical history and physical examination. The patient will be asked about respiratory infections, prior head and facial traumas, surgical history, exposure to chemicals, medication or drug use, etc.

At Altiorem, we perform rhinoscopy, pharyngoscopy, and fibroscopy.

Complementary tests such as computed tomography (CT) of the paranasal sinuses or cranial magnetic resonance imaging (MRI) are typically requested as well.

 

How is olfactory capacity measured?

Currently, there are various types of smell tests.

On one hand, there are questionnaires that allow us to assess the olfactory threshold, and on the other hand, there are tests for odor identification, which enable us to assess and quantify olfactory impairment as objectively as possible.

Among the former, at Altiorem, we use the following:

  • International standardized questionnaire or Sv QODNS (Short version of the Questionnaire of Olfactory Disorders-Negative Statements)
  • Questionnaire for the assessment of nasal obstruction symptoms or NOSE (Nasal Obstruction Symptom Evaluation)

The most well-known odor identification tests include:

  • Connecticut Chemosensory Clinical Research Center Olfactory Test or CCCRC
  • University of Pennsylvania Smell Identification Test or UPSIT
  • Burghart Smell Test or Sniffin’ Sticks test, which is the one we use at Altiorem.

Cómo se mide la capacidad olfativa

What is the treatment for smell disorders?

The treatment of smell disorders depends primarily on the underlying cause.

As diseases of the nasal passages and paranasal sinuses are the main causes of olfactory impairment, treating any obstruction and infection, if present, can improve olfactory function.

Systemic corticosteroids are effective in reducing nasal mucosa swelling. However, their prolonged use is associated with a wide range of side effects, so their use is recommended in short cycles. To reduce symptom recurrence, topical corticosteroids may be used for more extended periods.

Currently, there is no evidence of any effective pharmacological treatment for post-viral hyposmia/anosmia or olfactory impairment secondary to head trauma. However, due to the regenerative capacity of olfactory neurons, it is expected that some cases will improve spontaneously over time.

Olfactory rehabilitation can help regenerate olfactory sensory cells and recover olfactory function more rapidly, improving olfactory sensitivity in 30% of cases, as demonstrated by multiple studies. Additionally, rehabilitation has been shown to improve mood and depressive symptoms associated with olfactory disorders, and even cognitive function in patients with Parkinson’s disease.

This has led to the development of various odor training techniques. The most well-known method, and the one we use at Altiorem, was described by Professor Thomas Hummel at the University of Dresden in 2009. It is based on stimulating the sense of smell with four odors, each belonging to one of the basic categories. The most effective odors have been shown to be rose, lemon, clove, and eucalyptus. Patients are recommended to smell these fragrances twice a day, for at least 20 seconds each, over a period of twelve weeks.

 

Can surgery improve smell disorders?

Endoscopic sinonasal surgery (ESS) is particularly effective in improving symptoms such as nasal obstruction, mucus, or facial pain or pressure. However, the results regarding smell are more variable. In this regard, patients who benefit most from surgery are those with chronic rhinosinusitis with polyps. Nonetheless, after surgical intervention, long-term medical treatment is usually necessary to prevent or delay relapse as much as possible.

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