Impaired sense of smell (including after coronavirus)

Functions of smell

The sense of smell is a person’s ability to perceive odors. It plays an important role in human life and performs the following functions:

1. Informational – obtain information about a substance that may pose a threat to humans;
2. Aesthetic – pleasant aromas promote the production of endorphins, or joy hormones, which improve your mood;
3. Gustatory - olfactory receptors are connected with taste receptors, they are involved in the formation of various tastes. Therefore, to a person with nasal congestion, food seems tasteless;
4. Communicative (this function is more developed in animals) - the brain determines the state of another organism due to the release of subtle odorous substances, recognizes states of joy, fear, aggression, sexual desire.

Olfactory brain

The olfactory brain (rhinencephalon) is located on the lower and medial surfaces of the cerebral hemispheres and is conventionally divided into peripheral and central sections.

The peripheral part of the olfactory brain includes the olfactory bulb (bulbus olfactorius) and tract (tractus olfactorius), located on the lower surface of the frontal lobe in the olfactory sulcus (sulcus olfactorius). The olfactory tract ends with the olfactory triangle (trigonum olfactorium), which diverges in front of the anterior perforated substance (substantia perforata anterior) into two olfactory stripes (striae olfactoriae laterales) (see Fig. 473). The lateral stripe goes around the bottom of the lateral sulcus (sulcus lateralis) and ends in the cortex of the temporal lobe (uncus). The medial strip is directed to the medial longitudinal fissure into the subcallosal gyrus (gyrus subcallosus) and the paraolfactory field (area paraolfactoria), which are located under the beak of the corpus callosum (rostrum corporis callosi) (Fig. 474).

474. Diagram of the structure of the olfactory brain. 1 - bulbus olfactorius; 2 - stria olfactoria medialis; 3 - stria olfactoria lateralis; 4 - uncus hippocampi; 5-corpus mamillare; 6 - fissura hippocampi; 7 - lamina terminalis; 8 - area olfactoria; 9 - trigonum olfactorium; 10 - tr. olfactorius

The central part of the olfactory brain includes: the vaulted gyrus, the hippocampus, the dentate gyrus, the uncinus, the intramarginal gyrus, the fasciculate gyrus and the gray layer above the corpus callosum.

The vaulted gyrus (gyrus fornicatus) (see Fig. 472) has a ring shape, goes around the corpus callosum and is located on the medial surface of the hemispheres. The vaulted gyrus consists of three parts: the cingulate gyrus (gyrus cinguli) and the parahippocampal gyrus (gyrus parahippocampalis), connected by an isthmus (isthmus gyri cinguli).

The cingulate gyrus lies above the corpus callosum on the medial surface of the cerebral hemisphere and is not only the center of smell, but also the regulation of the function of internal organs (primarily the cardiovascular system). It is bounded above by the cingulate groove (sulcus cinguli), below by the groove of the corpus callosum (sulcus corporis callosi). In front, the cingulate gyrus connects with the parietal gyrus (sulcus paraterminalis), and behind, at the level of the parieto-occipital sulcus (sulcus parietooccipitalis), it passes into the isthmus of the fornix (isthmus fornicatus), which below the posterior edge of the corpus callosum connects with the hippocampal gyrus (gyrus parahippocampalis).

The hippocampus is an invagination of the gray matter due to the sulcus hyppocampi from the medial wall of the lower horn of the lateral ventricle (Fig. 475). The hippocampus is clearly visible in the cavity of the inferior horn in the form of a club-shaped body. It is bounded on the lateral side and in the back by the roundabout groove (sulcus collateralis), in front by the nasal groove (sulcus rhinalis). The hippocampus at the anterior perforated substance is bent in the form of a hook (uncus), being the center of smell.

475. The structure of the central part of the olfactory brain. 1 - uncus; 2 - fimbria hyppocampi; 3 - gyrus dentatus, 4 - gyrus parahippocampalis

The dentate gyrus (gyrus dentatus) represents the twisted part of the cortex of the medial edge of the sulcus hippocampi. The gray matter of the dentate gyrus extends to the inner edge of the hippocampus, as well as to the dorsal surface of the corpus callosum, forming the so-called gray vestment (indusium griseum), which ends in the supracallosal gyrus.

The hook (uncus) represents the anterior end of the hippocampal fissure (fissura hippocampi), which is divided by a cord into two parts: anterior and posterior. The anterior part belongs to the hook, and the posterior part forms the intramarginal gyrus (gyrus intralimbicus), which passes between the dentate gyrus and the white fimbria, ending in the ligamentous gyrus (gyrus fasciolaris).

Types of smell impairment

There are several types:

• Hyposmia is a decrease in the function of smell.
• Anosmia is a complete loss of the sense of smell (typical of COVID-19).
• Hyperosmia is an increased reaction (usually negative) to faint odors.
• Parosmia is a distortion of perception (a person’s sensations do not coincide with how the substance actually smells).
• Cacosmia is the sensation of unpleasant odors in the absence of their sources.
• Olfactory hallucinations are the sensation of unusual odors in the absence of a real stimulus.

Causes of impaired sense of smell

The area of ​​smell perception is located in the upper parts of the nasal cavity. This area is called the olfactory cleft; there are sensitive receptors (olfactory bulbs) that can become irritated even when one molecule of an odorous substance enters. Next, an impulse arises that penetrates the fibers of the olfactory nerve into the brain, where the received signal is analyzed.

Thus, disruption of this function can occur at 3 levels:

• Level of the olfactory fissure. This is the most common cause of hyposmia or anosmia. At this level there is a mechanical barrier on the path of the air stream to the sensitive receptors. This situation can be caused by swelling of the nasal mucosa, deviated septum in the upper sections, hypertrophy of the nasal turbinates, proliferation of polyps and other neoplasms.
• Level of receptors and olfactory nerve. This level is achieved as a result of the action of toxic substances, viral infections, injuries, as well as diseases leading to damage to nerve fibers and sensitive receptors. At this level, the sense of smell is impaired during coronavirus.
• Brain level. Such disorders are caused by: brain injuries, tumors of the anterior cranial fossa, changes in hormonal levels, mental illness, and the effects of narcotic substances. Damage at this level, as a rule, does not lead to a decrease or absence of the ability to perceive and distinguish odors, but to disorders of a different nature: the appearance of olfactory hallucinations, increased sensitivity, and distortion of perception.

Receptor department

It is concentrated in the depths of the nasal cavity and is equipped with olfactory receptors - neurons. In their work they are similar to the receptors that perceive taste. When you inhale air, molecules penetrate deep into the nose and settle on the mucous membrane. And they are perceived by these receptors. Just the epithelium in the nasal cavity is the receptor section. Its main tasks: encoding and transmitting information reflecting the intensity, quality and duration of the aroma in the olfactory bulb. The inhaled air irritates sensitive neurons, and on their surface there are receptors that perceive a certain substance. Today, 200-400 receptors are known that perceive highly specific substances. These cells are renewed every 30-60 days, and their source is the anterior surfaces of the brain, where areas containing poorly differentiated cells capable of dividing and maturing are concentrated.

Conductor and brain department

There are receptors on the surface of epithelial cells. And an impulse in them occurs when irritated by certain aromatic molecules. A person can perceive aroma only when exposed to 8-10 molecules of an odorous substance and irritated at least 40% of all neurons. Simply put, for a person to smell, the action of 320 odorous molecules is needed, but a dog needs only 8. And all because it has 200-220 million olfactory cells, and a person has 6 million. This irritation is transmitted through special nerve cells directly to the hypothalamus. This miniature organ is important because it controls numerous processes in the body: it maintains normal temperature, thirst, hunger, blood sugar levels, sleep, emotions, sexual arousal, and some emotions. Along with the hypothalamus, signals are also sent to the hippocampus, the part of the brain responsible for memory and attention. This is where pleasant memories are formed that are associated with aromas.

Mechanisms of smell

There are still about 30 theories devoted to the mechanism of smell. The most popular one says that all smells can be divided into 6 main ones:

• camphor;
• ethereal;
• floral;
• spicy;
• putrefactive;
• mint.

Each molecule has its own formula and contacts only a specific receptor. This selectivity is explained by the shape of the molecule, for example, the camphor molecule is round, while flowers resemble a kite. And the receptors are shaped accordingly: a spherical molecule cannot contact receptors designed for a kite shape. If the molecule “fits” correctly into its hole, then we feel the aroma.

Taste and smell

In the course of research conducted in the USA, it was possible to find out that olfactory receptors are located not only in the nasal cavity, but also on the tongue. After all, smell gives an idea of ​​​​the taste of food, but until recently these two processes were considered independent and did not interact with each other, but recent research has shown that this is not so. This interaction begins at the very first stage - perception. In the course of research, it was possible to establish that the sense of smell is responsible for 65% of the perception of taste: if we do not feel the aroma of food, then its taste is worse, and in difficult cases, it is impossible to distinguish one type from another. It’s easy to check: just remember how difficult it is to perceive the taste of food during nasal congestion and during a cold, when the mucous membrane is swollen and the functioning of the olfactory epithelium suffers.

Smell as a sign of health or pathology

A decrease or exacerbation of the sense of smell can be regarded as a diagnostic sign of certain diseases.
It is known that children suffering from autism spectrum disorders do not distinguish between pleasant and unpleasant aromas; they inhale them equally deeply. This data was obtained through clinical research: such a “breath” test showed that children who “failed” the smell test were diagnosed with autism spectrum disorders. A deterioration in the ability to perceive aromas can be a symptom of Alzheimer's disease, because the brain structures responsible for feelings are affected. In addition to reducing the ability to smell, the opposite picture can be observed. The reasons for an exacerbation of the sense of smell are very diverse, for example, it can accompany a migraine attack, in which everything, even the lightest aromas, is perceived more strongly and can cause increased pain. Hormonal disruptions, disturbances in the functioning of the central nervous system, certain types of brain injuries or tumor processes are also important. Text: Yulia Lapushkina

Methods for diagnosing smell disorders

When a problem is identified, a comprehensive examination is necessary.

• Olfactometry - allows you to determine the degree of decrease in perception.
• Endoscopic examination of the nasal cavity - examination is necessary to identify pathologies.
• Computed tomography of the nasal cavity and paranasal sinuses is necessary to identify neoplasms and inflammation.
• Laboratory examination - allows you to identify concomitant diseases that lead to impaired perception (diabetes mellitus, hyperthyroidism).
• MRI of the brain - allows you to identify brain diseases that lead to a disorder of smell (demyelinating diseases, tumors of the anterior cranial fossa).

Causes of loss of smell

The condition in question can be provoked by:

1. Irregular structure of the nose and sinuses can be congenital or acquired. In the first case, we are talking about the fact that a person is already born with a similar problem and from birth does not distinguish between aromas. Acquired disorders of the structure of the nose are the consequences of injuries and in this case the sense of smell is lost immediately after the onset of the pathology.
2. Diseases of the nasal cavity - chronic and acute. In this case, they talk about a temporary problem - for example, loss of smell during a cold does not require any specific treatment and everything recovers on its own.
3. Head injuries. If there was a serious traumatic brain injury, then the nerve fibers are damaged by the bones of the skull (their fragments). Most often, the problem occurs when the occipital, temporal and ethmoid bones are damaged.
4. Tumors of the brain, nose and sinuses are benign or malignant. A distinctive feature of such anosmia is its slow development; the patient may not pay attention to an already progressing problem for a long time.
5. Smoking. Tobacco smoke, as well as the toxins and tars in it, are irritating to the mucous membranes of the nose and sinuses. With prolonged smoking, combined anosmia can develop - loss of taste and smell at the same time.

Intoxication can also affect the intensity and level of smell. If the body is constantly influenced by benzene and cadmium, then over time the development of anosmia is inevitable. This can happen against the backdrop of specific working conditions.

Treatment methods

Conservative treatment tactics depend on the cause. It is imperative to take into account whether the patient has suffered from Covid.

Conservative treatment

Impaired sense of smell caused by inflammation of the nasal mucosa.

Conservative therapy is carried out aimed at reducing swelling of the mucous membrane and eliminating the cause of inflammation.

For this, nasal rinsing using antiseptics is used.

Drugs are used that reduce inflammation and swelling of the nasal mucosa.

Physiotherapy procedures are carried out: magnetic laser therapy, halotherapy, phonophoresis.

Antiviral therapy, antibacterial therapy, and oral antiallergic therapy may be prescribed.

Toxic damage to the olfactory nerve and receptors.

Anti-inflammatory and detoxification therapy is carried out.

Drugs are prescribed that act to improve the transmission of nerve impulses (prozerin, B vitamins).

Physiotherapy is used to improve microcirculation, as well as restore the function of the olfactory nerve (magnetic laser therapy, phonophoresis).

Surgery

Surgical treatment is most effective for smell disorders caused by:

• polyps or chronic sinusitis
• deviated septum
• hypertrophy of the nasal turbinates

Surgical methods for treating pathology:

• polypotomy or polysinsotomy
• septoplasty
• conchoplasty

These types of operations are performed under general anesthesia using endoscopic techniques, radio wave or laser equipment. This makes it possible to reduce the risk of postoperative bleeding, reduces the time spent in the hospital, and reduces the rehabilitation period.

Our clinic deals with various cases of deterioration in the ability to perceive odors, including impaired sense of smell after coronavirus.

A little about smell

The sense of smell is a special sense that allows us to respond to aromas. Odorous substances act on the upper part of the nasal mucosa, where the olfactory nerve is located. Simply put, the sense of smell is the ability to sense smells. Each person perceives them differently, which is why experts distinguish three groups of people:

• Macromatics - have a keen sense of smell and are sensitive to aromas. They can distinguish all existing shades of odors.
• Micromatics - it will take them a little time to determine the intensity of the aroma. According to statistics, there are more such people.
• Anosmatics are people who cannot smell at all. Their number is small.