Dysosmia

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Dysosmia
Specialty Neurology

Dysosmia is a disorder described as any qualitative alteration or distortion of the perception of smell. [1] Qualitative alterations differ from quantitative alterations, which include anosmia and hyposmia. [2] Dysosmia can be classified as either parosmia (also called troposmia) or phantosmia. Parosmia is a distortion in the perception of an odorant. Odorants smell different from what one remembers. Phantosmia is the perception of an odor when no odorant is present. The cause of dysosmia still remains a theory. It is typically considered a neurological disorder and clinical associations with the disorder have been made. [3] Most cases are described as idiopathic and the main antecedents related to parosmia are URTIs, head trauma, and nasal and paranasal sinus disease. [4] Dysosmia tends to go away on its own but there are options for treatment for patients that want immediate relief. [1]

Contents

Signs and symptoms

Smell disorders can result in the inability to detect environmental dangers such as gas leaks, toxins, or smoke. In addition to safety, nutritional and eating habits can also be affected. There is a loss of appetite because of unpleasant flavor and fear of failing to recognize and consuming spoiled food. A decreased or distorted sense of smell therefore results in a decreased quality of life. [5] Distortions are believed to have a greater negative impact on people than the complete loss of smell because they are constantly reminded of the disorder and the distortions have a greater effect on eating habits. [1] [5]

Classification and terminology

Olfactory dysfunction can be quantitative and/or qualitative. Quantitative smell disorders are disorders in which there is complete or partial loss of olfaction. Anosmia, the complete loss of olfaction, and hyposmia, the partial loss of olfaction are the two disorders classified as quantitative because they can be measured. Qualitative smell disorders cannot be measured and are disorders in which there is alternation or distortion in the perception of smell. Qualitative disorders include parosmia (also called troposmia) and phantosmia. [2] Dysosmia is a qualitative olfaction disorder and includes both parosmia and phantosmia. Olfactory dysfunction including anosmia, hyposmia, and dysosmia can be either bilateral or unilateral on either nostril. Anosmia only on the left nostril would be termed unilateral left anosmia while bilateral anosmia would be termed total anosmia. [3] If the distortion is unpleasant, the disorder can be referred to as cacosmia. [6] Under an alternative definition, cacosmia is used for an unpleasant perception of an odorant due specifically to nasosinusal or pharyngeal infection. [4] The rare term torquosmia can be used when the perceived smell is chemical, burning or metallic. [7]

Parosmia

Parosmia is a distortion in the perception of an odorant. Odorants smell different from what one remembers. [3]

Phantosmia

Phantosmia is the perception of an odor when there is no odorant present. When a phantom smell lasts less than a few seconds, the term olfactory hallucination can be used.[ citation needed ]

Cause

Even though the causes of dysosmia are not yet clear, there are two general theories that describe the etiology: the peripheral and central theories. In parosmia, the peripheral theory refers to the inability to form a complete picture of an odorant due to the loss of functioning olfactory receptor neurons. The central theory refers to integrative centers in the brain forming a distorted odor. In phantosmia, the peripheral theory refers to neurons emitting abnormal signals to the brain or the loss of inhibitory cells that are normally present in normal functioning. The central theory for phantosmia is described as an area of hyper-functioning brain cells that generate the order perception. Evidence to support these theories include findings that for the majority of individuals with distortions, there is a loss of sensitivity to smell that accompanies it and the distortions are worse at the time of the decreased sensitivity. [1] It has been reported in parosmia cases that patients can identify triggering stimuli. Common triggers include gasoline, tobacco, coffee, perfume, fruits and chocolate. [4]

The cause of dysosmia has not been determined but there have been clinical associations with the neurological disorder: [2] [8]

Most of cases are described as idiopathic and the main antecedents related to parosmia are URTIs, head trauma, and nasal and paranasal sinus disease. [4] Psychiatric causes for smell distortion can exist in schizophrenia, alcoholic psychosis, depression, and olfactory reference syndrome. [1]

Pathophysiology

Anatomy and physiology

Human olfactory system. 1: Olfactory bulb 2: Mitral cells 3: Bone 4: Nasal epithelium 5: Glomerulus (olfaction) 6: Olfactory receptor cells Olfactory system.svg
Human olfactory system. 1: Olfactory bulb 2: Mitral cells 3: Bone 4: Nasal epithelium 5: Glomerulus (olfaction) 6: Olfactory receptor cells

Olfactory receptors

The olfactory neuroepithelium, located in the roof of the nasal chambers, comprises bipolar receptor cells, supporting cells, basal cells, and brush cells. [5] There are approximately 6 million bipolar sensory receptor neurons whose cell bodies and dendrites are in the epithelium. The axons of these cells aggregate into 30-40 fascicles, called the olfactory fila, which project through the cribriform plate and pia matter. These axons collectively make up the olfactory nerve (CN I) and serve the purpose of mediating the sense of smell. [9]

Characteristics of the bipolar receptor neurons include cilia on the dendritic ends which project into the mucus, regeneration from basal cells after damage, and each receptor neuron is also a first order neuron. The first order neurons project axons directly from the nasal chamber to the brain. Its first neuron characteristic allows direct exposure to the environment, which makes the brain vulnerable to infection and invasion of xenobiotic agents. Supporting cells, called sustentacular cells, provide metabolic and physical support to the receptors by insulating the cell and regulating the composition of the mucus. Basal stem cells give rise to both neurons and non-neuronal cells and allow for constant regeneration of receptor cells and its surrounding cell. [3]

Olfactory transduction

Olfactory transduction begins with the movement of odorants from the air phase to the aqueous phase in the olfactory mucus. Odorants are transported by odorant binding proteins or diffuse through the mucus and reach the cilia on the dendritic ends of bipolar receptor neurons. Stimulation causes action potentials to be initiated and the signals are sent to the brain via the olfactory fila. [3] [5]

Olfactory bulb

Olfactory receptor neuron axons project through the cribriform plate to the olfactory bulb. The olfactory bulb is a structure at the base of the frontal lobe. It comprises neurons, nerve fibers, interneurons, microglia, astrocytes, and blood vessels. It is made up of 6 layers: olfactory nerve layer, glomerular layer, external plexiform layer, mitral cell layer, internal plexiform layer, and granule layer. The terminals of the receptor axons synapse with the dendrites of mitral and tufts cells within the glomeruli of the olfactory bulb. The axons of the mitral and tufts cells send signals to the olfactory cortex. [3] [5]

Olfactory cortex

Signals from odor sensation are sent from the olfactory bulb through mitral and tufts cell axons via the lateral olfactory tract and synapse at the primary olfactory cortex. The primary olfactory cortex includes the anterior olfactory nucleus, the piriform cortex, the anterior cortical nucleus of the amygdala, the periamygdaloid complex, and the rostral entorhinal cortex. A unique characteristic of olfaction is its independence from the thalamus. The odor signals are sent directly from the sensory receptor neuron to the primary cortex. However, communication between the primary and secondary olfactory cortex requires connections with the thalamus. [3] [5]

Odor perception

Odor identity, quality, and familiarity are mainly deciphered by the piriform cortex. Consciousness of smell is achieved by projections from the piriform cortex to the medial dorsal nucleus of the thalamus and to the orbitofrontal cortex, which the secondary olfactory cortex is part of. [5]

There are approximately 1,000 olfactory receptors coded for in the human genome. [10] Less than 500 receptors are functional in the nasal epithelium. Each receptor neuron is a single type of olfactory receptor and is not specific to any one odorant. [3] An odorant is recognized by more than one type of receptor and thus odorants are recognized by a combination of receptors. The olfactory system relies on different excitation patterns to obtain different codes for different odorants. Nobel prize recipient Linda B. Buck compared this system to combining different letters of the alphabet to produce different words. In this case, each word represents an odor. This coding explains why we can detect more odors than there are receptors in the nasal epithelium. [5]

Olfaction and flavor

Flavor is perceived by the combination of the sense of taste, sense of smell, and the trigeminal nerve (CN V). The gustatory system is responsible for differentiation between sweet, sour, salty, bitter, and umami. [10] The olfactory system recognizes the odorants as they pass to the olfactory epithelium via a retronasal pathway. [5] This explains why we can identify a variety of flavors in spite of only having five types of taste receptors. The trigeminal nerve senses texture, pain, and temperature of food, and related qualities such as the cooling effect of menthol or the burning sensation of spicy food. [10]

Diagnosis

Diagnosing dysosmia precisely can be difficult due to the variety of causes and symptoms. Often, patients may be unsure whether or not they are having issues with smell or taste specifically. It is important to identify whether the distortion applies to an inhaled odorant or if an odor exists without the stimulus. Distortion of an odorant is presented in two types: when the stimuli are different from what one remembers, and when everything has a similar smell. A clinical history can also help determine what kind of dysosmia one has, as events such as respiratory infection and head trauma are usually indications of parosmia. Phantosmia often occurs spontaneously. Unfortunately, there are no wholly accurate diagnostic tests or methods for dysosmia.; [1] evaluation must be done through questionnaires and medical history. [8]

Treatment

Even though dysosmia often goes away on its own over time, there are both medical and surgical treatments for dysosmia for patients who want immediate relief. Medical treatments include the use of topical nasal drops and oxymetazoline HCL, which give an upper nasal block so that the air flow can't reach the olfactory cleft. Other medications suggested include sedatives, anti-depressants, and anti-epileptic drugs. The medications may or may not work and for some patients, and side effects may not be tolerable. Most patients benefit from medical treatment, but for some, surgical treatment is required. Options include a bifrontal craniotomy and excision of the olfactory epithelium, which cuts all of the fila olfactoria. [1] According to some studies, transnasal endoscopic excision of the olfactory epithelium has been described as a safe and effective phantosmia treatment. [11] The Bifrontal craniotomy results in permanent anosmia, and both surgeries are accompanied with the risks associated with general surgery. [1]

Epidemiology

The frequency of phantosmia is rare in comparison with the frequency of parosmia. Parosmia has been estimated to be in 10-60% of patients with olfactory dysfunction and from studies, it has been shown that it can last anywhere from 3 months to 22 years. [2] [4] Smell and taste problems result in over 200,000 visits to physicians annually in the US. [4] Lately, it has been thought that phantosmia might co-occur with Parkinson's disease. However, its potential to be a premotor biomarker for Parkinson's is still up for debate as not all patients with Parkinson's disease have olfactory disorders [12]

Related Research Articles

<span class="mw-page-title-main">Anosmia</span> Inability to smell

Anosmia, also known as smell blindness, is the loss of the ability to detect one or more smells. Anosmia may be temporary or permanent. It differs from hyposmia, which is a decreased sensitivity to some or all smells.

<span class="mw-page-title-main">Olfactory nerve</span> Cranial nerve I, for smelling

The olfactory nerve, also known as the first cranial nerve, cranial nerve I, or simply CN I, is a cranial nerve that contains sensory nerve fibers relating to the sense of smell.

<span class="mw-page-title-main">Olfactory bulb</span> Neural structure

The olfactory bulb is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus where it plays a role in emotion, memory and learning. The bulb is divided into two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb connects to the amygdala via the piriform cortex of the primary olfactory cortex and directly projects from the main olfactory bulb to specific amygdala areas. The accessory olfactory bulb resides on the dorsal-posterior region of the main olfactory bulb and forms a parallel pathway. Destruction of the olfactory bulb results in ipsilateral anosmia, while irritative lesions of the uncus can result in olfactory and gustatory hallucinations.

<span class="mw-page-title-main">Olfactory system</span> Sensory system used for smelling

The olfactory system, or sense of smell, is the sensory system used for smelling (olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main olfactory system and an accessory olfactory system. The main olfactory system detects airborne substances, while the accessory system senses fluid-phase stimuli.

<span class="mw-page-title-main">Olfactory receptor neuron</span> Transduction nerve cell within the olfactory system

An olfactory receptor neuron (ORN), also called an olfactory sensory neuron (OSN), is a sensory neuron within the olfactory system.

<span class="mw-page-title-main">Olfactory epithelium</span> Specialised epithelial tissue in the nasal cavity that detects odours

The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures 9 cm2 and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils. The olfactory epithelium is the part of the olfactory system directly responsible for detecting odors.

<span class="mw-page-title-main">Glomerulus (olfaction)</span>

The glomerulus is a spherical structure located in the olfactory bulb of the brain where synapses form between the terminals of the olfactory nerve and the dendrites of mitral, periglomerular and tufted cells. Each glomerulus is surrounded by a heterogeneous population of juxtaglomerular neurons and glial cells.

Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants which give rise to the sense of smell. Activated olfactory receptors trigger nerve impulses which transmit information about odor to the brain. These receptors are members of the class A rhodopsin-like family of G protein-coupled receptors (GPCRs). The olfactory receptors form a multigene family consisting of around 800 genes in humans and 1400 genes in mice.

Parosmia is a dysfunctional smell detection characterized by the inability of the brain to correctly identify an odor's "natural" smell. Instead, the natural odor is usually transformed into an unpleasant aroma, typically a "burned", "rotting", "fecal", or "chemical" smell. There can also be rare instances of a pleasant odor called euosmia. The condition was rare and little-researched until it became relatively more widespread since 2020 as a side effect of COVID-19.

In medicine and anatomy, the special senses are the senses that have specialized organs devoted to them:

Phantosmia, also called an olfactory hallucination or a phantom odor, is smelling an odor that is not actually there. It can occur in one nostril or both. Unpleasant phantosmia, cacosmia, is more common and is often described as smelling something that is burned, foul, spoiled, or rotten. Experiencing occasional phantom smells is normal and usually goes away on its own in time. When hallucinations of this type do not seem to go away or when they keep coming back, it can be very upsetting and can disrupt an individual's quality of life.

Hyposmia, or microsmia, is a reduced ability to smell and to detect odors. A related condition is anosmia, in which no odors can be detected. Some of the causes of olfaction problems are allergies, nasal polyps, viral infections and head trauma. In 2012 an estimated 9.8 million people aged 40 and older in the United States had hyposmia and an additional 3.4 million had anosmia/severe hyposmia.

A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system. Topographic maps can be found in all sensory systems and in many motor systems.

Olfactory fatigue, also known as odor fatigue, olfactory adaptation, and noseblindness, is the temporary, normal inability to distinguish a particular odor after a prolonged exposure to that airborne compound. For example, when entering a restaurant initially the odor of food is often perceived as being very strong, but after time the awareness of the odor normally fades to the point where the smell is not perceptible or is much weaker. After leaving the area of high odor, the sensitivity is restored with time. Anosmia is the permanent loss of the sense of smell, and is different from olfactory fatigue.

Hyperosmia is an increased olfactory acuity, usually caused by a lower threshold for odor. This perceptual disorder arises when there is an abnormally increased signal at any point between the olfactory receptors and the olfactory cortex. The causes of hyperosmia may be genetic, hormonal, environmental or the result of benzodiazepine withdrawal syndrome.

<span class="mw-page-title-main">Sense of smell</span> Sense that detects smells

The sense of smell, or olfaction, is the special sense through which smells are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.

Olfactory memory refers to the recollection of odors. Studies have found various characteristics of common memories of odor memory including persistence and high resistance to interference. Explicit memory is typically the form focused on in the studies of olfactory memory, though implicit forms of memory certainly supply distinct contributions to the understanding of odors and memories of them. Research has demonstrated that the changes to the olfactory bulb and main olfactory system following birth are extremely important and influential for maternal behavior. Mammalian olfactory cues play an important role in the coordination of the mother infant bond, and the following normal development of the offspring. Maternal breast odors are individually distinctive, and provide a basis for recognition of the mother by her offspring.

The University of Pennsylvania Smell Identification Test (UPSIT) is a test that is commercially available for smell identification to test the function of an individual's olfactory system.

<span class="mw-page-title-main">Insect olfaction</span> Function of chemical receptors

Insect olfaction refers to the function of chemical receptors that enable insects to detect and identify volatile compounds for foraging, predator avoidance, finding mating partners and locating oviposition habitats. Thus, it is the most important sensation for insects. Most important insect behaviors must be timed perfectly which is dependent on what they smell and when they smell it. For example, olfaction is essential for locating host plants and hunting prey in many species of insects, such as the moth Deilephila elpenor and the wasp Polybia sericea, respectively.

Retronasal smell, retronasal olfaction, is the ability to perceive flavor dimensions of foods and drinks. Retronasal smell is a sensory modality that produces flavor. It is best described as a combination of traditional smell and taste modalities. Retronasal smell creates flavor from smell molecules in foods or drinks shunting up through the nasal passages as one is chewing. When people use the term "smell", they are usually referring to "orthonasal smell", or the perception of smell molecules that enter directly through the nose and up the nasal passages. Retronasal smell is critical for experiencing the flavor of foods and drinks. Flavor should be contrasted with taste, which refers to five specific dimensions: (1) sweet, (2) salty, (3) bitter, (4) sour, and (5) umami. Perceiving anything beyond these five dimensions, such as distinguishing the flavor of an apple from a pear for example, requires the sense of retronasal smell.

References

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  2. 1 2 3 4 Frasnelli, J (2004). "Clinical presentation of qualitative olfactory dysfunction". Eur Arch Otorhinolaryngol. 261 (7): 411–415. doi:10.1007/s00405-003-0703-y. PMID   14610680. S2CID   22145674.
  3. 1 2 3 4 5 6 7 8 Doty, R (2009). "The olfactory system and its disorders". Seminars in Neurology. 29 (1): 74–81. doi: 10.1055/s-0028-1124025 . PMID   19214935.
  4. 1 2 3 4 5 6 Bonfils, P; Avan, P; Patrick Faulcon; David Malinvaud (2005). "Distorted odorant perception analysis of a series of 56 patients with parosmia". Arch Otolaryngol Head Neck Surg. 131 (2): 107–112. doi: 10.1001/archotol.131.2.107 . PMID   15723940.
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  6. ↑ Hawkes, Christopher H. (2002). Smell and taste complaints. Boston: Butterworth-Heinemann. pp. 49–50. ISBN   978-0-7506-7287-0.
  7. ↑ Braun, D.; Wagner, W.; Zenner, H.-P.; Schmahl, F. (2002). "Disabling disturbance of olfaction in a dental technician following exposure to methyl methacrylate". International Archives of Occupational and Environmental Health. 75 (1): 73–74. doi:10.1007/s00420-002-0380-y. ISSN   1432-1246. PMID   12397414. S2CID   27483544.
  8. 1 2 Nordin, S; Murphy, C; Davidson, T; Quinonez, C; et al. (1996). "Prevalence and assessment of qualitative olfactory dysfunction in different age groups". Laryngoscope. 106 (6): 739–44. doi:10.1097/00005537-199606000-00014. PMID   8656960. S2CID   29556193.
  9. ↑ Doty, R (2001). "Olfaction". Annu. Rev. Psychol. 52: 423–452. doi:10.1146/annurev.psych.52.1.423. PMID   11148312.
  10. 1 2 3 Hummel, T; Landis, B; Huttenbrink, K-B (2011). "Smell and taste disorders". GMS Current Topics in Otorhinolaryngology, Head and Neck Surgery. 10: Doc04. doi:10.3205/cto000077. PMC   3341581 . PMID   22558054.
  11. ↑ Leopold, D; Loehrl, T; Schwob, J (2002). "Long-term follow up of surgically treated phantosmia". Arch Otolaryngol Head Neck Surg. 128 (6): 642–647. doi: 10.1001/archotol.128.6.642 . PMID   12049557.
  12. ↑ Landis, B; Burkhard, P (2008). "Phantosmias and Parkinson Disease". Arch Neurol. 65 (9): 1237–9. doi: 10.1001/archneur.65.9.1237 . PMID   18779429.
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