Olfactory Disorders – Pathway, Work up And Causes

WHY IS PROPER NOSE FUNCTION ESSENTIAL?

Material or environmental safety. One way to tell if food is fresh or ruined is to smell it. Additionally, one can detect whether or not there is a gas leak in the surrounding area by using their sense of smell. The aesthetic qualities of commonplace items. The ability to recognize and associate a smell with a certain item, person, or meal.

Foundational components of communication o In order to communicate with one another, animals and other lower species rely on their fermions and odorous stimuli. The sense of smell helps in digestion by inducing gastrointestinal secretions and identifying the flavors of foods and beverages. When a dog is trained to a specific sound, it causes stomach reflux and increases salivation and gastric secretion, just like in Pavlov's experiment.

In a similar vein, gastrointestinal secretions and saliva flow up in response to odorous stimuli, and hunger is experienced. This aids in our comprehension of the food's flavor as well. A retronasal column is activated when we smell, which allows our taste receptors to register the flavor. When someone has poor scent perception, taste is consequently impacted.

AN INDICATOR FOR WHAT IS THE LACK OF OLFACTORY FUNCTION? 

A variety of nasal and tubal conditions can impair the ability to smell. Factors contributing to impaired olfactory perception.-  Parkinson's disease,  Alzheimer's disease,  Nasopharyngeal carcinoma,  Multiple sclerosis, Frontal meningiomas, Sinus infection,  Frontal lobe tumor, Olfactory nerve compression caused by tumor. Neural oligodendroblastoma

OLFACTORY EPITHELIUM AND CLEFT

The olfactory cleft is a specific region of the nose that contains olfactory epithelium, a unique sensory epithelium that is responsible for scent, but lacks pseudostratified ciliated columnar epithelium. The vestibule, which is lined by stratified squamous epithelium, and the olfactory region are the two parts of the nose that are not lined by respiratory epithelium.The septum, superior turbinate, and cribriform plate of the ethmoid circumscribed the olfactory cleft medially, laterally, and superiorly. The olfactory area/cleft is located close to the nose's roof.

OLFACTORY CLEFT AND NASAL ARCHITECTURE  

95% of air, only 10% to 15% reaches the neuroepithelium responsible for smell.  A parabolic curve represents the maximum air volume (inspiratory air current) as it approaches the middle meatus. The olfactory neuroepithelium lacks the maximum air current volume.

Slight modifications to nasal architecture and airflow, such as nasal polyps and a deviated septum, can impact the volume of air that reaches the olfactory cleft, impairing breathing or olfactory perception.

FOUR NEURAL SYSTEMS WITHIN THE NOSE  

The primary olfactory system's cranial nerve 1 [CN 1], commonly known as the olfactory nerve, is the first. The olfactory nerve senses common odors such as chocolate, vanilla, rose, and masala. The second is the vomeronasal system, commonly known as the accessory olfactory system. This can be seen close to the septum's anterior end.

It has no significant role in humans and is primitive. On the other hand, smell sensing serves a specific purpose in reptiles and snakes. Trigeminal somatosensory system (mediated by CN 5): third. It assists in recognizing stimuli that are chemical or non-chemical, such as burning, cooling, itching, and tickling.  Nervous terminalis [CN 0] is the fourth.

It is located close to the vomeronasal nerve and organ and is made up of a loose plexus of ganglionated nerves.

The odorants must enter the nose and either activate by diffusion (passive activation) or sniffing (active stimulation/activation) before neuronal transduction can start. Subsequently, the odorous stimulus enters the aqueous phase of the olfactory mucus after passing through the olfactory cleft.

Can the patient smell if there is no mucus, less music, or dry mucus?

 No. The reason for this is that only the olfactory mucus experiences the aqueous phase, which is where the air phase should transduce to.

As food is eaten or chewed, air movement in the retro nasal direction indicates the flavor of that specific food. The mucus plays a critical role in spreading the smell to olfactory receptors and maintaining a moist, safe environment for the neuroepithelium in the olfactory cleft. Odorant binding proteins are specific proteins that help move odorous compounds from mucus to the olfactory receptor and then to the olfactory tract. They assist in moving the odor stimulus from the olfactory epithelium to the olfactory receptor and beyond.

Olfactory neurons are predominantly excited by the neurotransmitter glutamate; dopamine is required for the proper functioning of olfactory nerves.

OLFACTORY PATHWAY

ENVELOPMENT OF OLFACTORY DISORDERS

 Anosmia, or the lack of smell function, is the inability to perceive any qualitative olfactory sensations. The term "partial anosmia" refers to the inability to detect some smells. The term "hyposmia" or "microsomia" describes a reduction in sensitivity to smells.  A disorder known as hypersomia causes an increase in sensitivity to everyday smells.

Dysosmia is the abnormal sense of smell. Dysnomia includes parosmia and cacosmia. The condition known as cacosmia causes a substance's typical odor to be perceived as offensive or nasty.  Parosmia is characterized by a distorted or altered sense of smell.

Phantosmia, also known as olfactory hallucination, is a dysosmic disorder in which a person perceives an odor even in the absence of an odor stimulus. In certain stroke patients, olfactory processing, language, and general intellectual abilities are generally intact, but they are unable to remember or comprehend the smell sensation. This condition is known as olfactory agnosia.

A condition known as heterosmia occurs when all smells are the same.

Presbyosmia is a disorder in which the sense of smell deteriorates with age. The aversion or dread of specific scents is known as osmophobia.

HISTORY TAKING

Recording history is crucial.  It is nearly impossible to diagnose the illness without a complete medical history. The checklists listed below must be completed before obtaining a patient's medical history if they have olfactory anomalies.

Sudden loss of smell can occur as a result of trauma, ischemia, infections such as COVID-19 or URTIs, or mental health issues including bipolar disorder or anxiety. Progressive and obstructive lesion surrounding the sinonasal region is indicated by gradual loss over time, especially if the loss is unilateral. Patients have a loss of smell from the right side of their nose, but they are able to smell through the left. Periodic loss may indicate sinusitis and inflammatory processes in the nose.

OTHER POINTS TO BE NOTED

A history of drug misuse or other concomitant conditions, such as dementia, hypothyroidism, diabetes, or liver failure, must be taken. Alzheimer’s disease is one type of dementia that can lead to olfactory abnormalities.  Inquire about any history of epistaxis, nasal discharge, face numbness or weakness, nasal blockage, headache, or irritation. • Kallmann's syndrome is suggested if there is anosmia with delayed puberty. These symptoms can assist in determining if the condition is related to the paranasal sinuses or the nose.

If there is a positive family history, it can be determined if the condition has a genetic origin. Headaches in the past may indicate that an intracranial tumor, migraine, or sinusitis needs to be investigated.

Parkinson's and Alzheimer's disorders can impair smell. A space-occupying lesion causing irregularities in smell can clearly be caused by an aura or a seizure. Keep an eye out for any hand or facial stiffness, as well as memory problems.

EXAMINATION  

Neurological exams are crucial, and a comprehensive ENT examination is essential and frequently crucial for individuals with olfactory disorders. Secondary cranial diseases can cause olfactory abnormalities. When intracranial pressure is elevated, an evaluation of the optic and olfactory nerves is necessary since these are the first nerves to be impacted. The functioning of the optic nerve may be compromised if there is a disruption in the olfactory nerves. Elevated intracranial pressure is a sign of abnormal function of the olfactory and optic nerves.

Visual acuity, visual fields, and optic disc examination provide information on the possibility of an intracranial mass-related increase in intracranial pressure, which can lead to optic atrophy. Testing of the trigeminal nerve is crucial because it distinguishes between chemical and non-chemical stimuli. 

To determine whether or not a trigeminal nerve is functional, trigeminal nerves in the ophthalmic, maxillary, and mandibular regions can be tested using the cotton wool test, trigeminal reflex, or by determining the sensory and motor functions. The way that facial nerves work is also crucial. Whenever the chorda tympani nerve is damaged, taste perception and, to a lesser extent, smell perception might be affected.

WORK UP OF PATIENTS WITH SMELL DISORDERS  

The physician has to actively search for anything, such as a polyp, tumor, DNS, or mass, that is obstructing airflow to the olfactory cleft. Endoscopic inspection is one method for doing this.

UNIVERSITY OF PENNSYLVANIA SMELL IDENTIFICATION TEST [UPSIT]

UPSIT, also known as the Smell Identification Test in the commercial world, is the most often used olfactory test and is the initial test performed on patients with olfactory disorders. The self-administered test takes ten to fifteen minutes to complete.  In this, the patient is given a card containing several olfactory stimuli in the form of microcapsules. The patient is instructed to sniff each one and mark the one that most closely matches the stimulus.

The test can be completed in under a minute, and the score obtained will indicate whether or not the patient has an olfactory anomaly.

Extremely low score indicates dishonesty. Four booklets total, each containing ten microencapsulated odorants, are included in UPSIT.

OLFACTORY THRESHOLD TESTING

Phenyl ethyl alcohol is administered to patients as an odorant during olfactory testing.  It is administered like a staircase, going from a modest concentration to a huge concentration. Initially 5 ml, then 10 ml, and so forth, up to 25 ml.  The threshold is the lowest volume at which the subject detects the smell, regardless of the amount of olfactory stimuli applied. Either one or both parties may carry out this action.

OLFACTORY EVENT-RELATED POTENTIALS [OERPS]

In this, the patient is hooked up to a device that resembles an electrocardiogram. An odorous stimulus is administered to the patient, resulting in electrical activity in the brain (EEG activity), which is graphed and recorded.  This graph indicates the degree of function of the patient's olfactory pathway. This test is objective, evaluates the olfactory system's integrity, and necessitates sophisticated, pricey equipment.  It is often sensitive to changes in olfactory function and can be helpful in the detection of malingering.  It is unable to identify anomaly areas along the pathway.

IMAGING STUDIES

Rhinitis and rhinosinusitis are examples of inflammatory conditions that CT scans can detect.In terms of assessing the bony structures, it is better than MRI. MRI is ideal for evaluating soft tissues, whereas CT is superior for finding any bone abnormalities of the nose and paranasal sinuses. It is capable of identifying anomalies in soft tissues such as the tract, cortical parenchyma, and olfactory bulb.

With little use, functional magnetic resonance imaging (fMRI), single proton emission computed tomography (SPECT), and positive emission tomography (PET) can detect anomalies.

CAUSES OF SMELL DISTURBANCES

There are two classes of olfactory loss. Conduction or transport impairment: Blockage of the nasal channel as a result of polyposis, persistent nasal irritation, etc. Sensorineural impairment: Damage resulting from viruses, poisons in the air, tumors, convulsions, etc. to the olfactory pathway.

A condition known as hypersomia is characterized by an elevated sensitivity or reactivity to an odorous stimulation. Addison's disease and pregnancy are common causes of this. Dysosmia is a dynamic change in the olfactory neuroepithelium caused by degeneration or regeneration. Odor perception becomes aberrant as a result.

Right inferior temporal lobe lesions can cause olfactory agnosia, an incredibly uncommon occurrence that is frequently associated with prosopagnosia. When a tumor is detected or there are particularly large or resistant polyps, surgery is rarely necessary.  It's crucial to do olfactory testing both before and after the intervention. Anti-epileptic or anti-migraine drugs should be helpful to some degree when epilepsy or migraine is suspected. Drugs that cause olfactory distortions can be stopped and substituted with alternative drugs or contemporary treatments.

Also Read: How does PrepLadder make Rhinology easy for students?

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