SMELL AND TASTE DISORDERS: CAUSES, SYMPTOMS, DIAGNOSIS, AND TREATMENTS

Medically Reviewed by Dr. Sony Sherpa (MBBS) - August 23, 2024

Smell and taste are two of the most important senses that we possess. They help us enjoy food, detect danger, and communicate. However, some people may experience problems resulting in loss of smell and taste, which can affect their quality of life and well-being. Loss of smell and taste after COVID-19 has become increasingly common around the globe, seen to affect 55-60% of the infected on average.^[1] Smell and taste disorders are also known to affect those with various health conditions, such as Alzheimer’s Disease.

It is important to diagnose and treat smell and taste disorders as soon as possible. Early detection and intervention can lessen the likelihood of developing complications, improve the symptoms, and restore the sense of taste and smell.

In this blog, we will discuss the causes, symptoms, diagnosis, and treatments of smell and taste disorders and provide some tips on how to prevent them.

Symptoms of Smell and Taste Disorders

Smell and taste disorders are conditions that impair the ability to perceive odors and flavors. They can range from mild to severe and can be temporary or permanent.

Some of the common symptoms of smell and taste disorders are^[2]:

• Loss of smell or taste: This is the complete inability to smell (anosmia) or taste (ageusia) anything.
• Reduced ability to smell or taste: This is the partial loss of smell (hyposmia) or taste (hypogeusia).
• Distortion of smell or taste: This is the alteration or distortion of smell (parosmia) or taste (dysgeusia).
• Phantom smells or tastes: This is the perception of smells (phantosmia) or tastes (phantogeusia) that are not present in the environment, sometimes referred to as “phantom” smells or tastes.

Smell and taste disorders can have a significant impact on the quality of life of those who suffer from them. A smell and taste disorder brings along a myriad of challenges as it can be difficult to enjoy food, detect spoiled food, and respond promptly to fire or gas leaks. It also lends one to an increased risk of infection due to missing out on smells associated with sickness, such as those with bad breath or mildew in the environment.

Aside from safety, smell and taste disorders can also affect the appetite, nutrition^[3], social interactions, and mental well-being of the affected individuals.

How Do Smell and Taste Work?

Smell and taste work closely together, which is why it is common to lose both senses to similar underlying causes.

A scent is essentially a stream of molecules that activates smell (olfactory) receptors in specialized cells in the nose called olfactory sensory neurons. These neurons provide a direct link to the brain, which is why our sense of smell often has an immediate effect on our mental and emotional well-being. Once activated, the brain sends signals back within a split second that allows us to experience these molecules as a smell.^[4]

Smell receptors are also located towards the back of the throat, which is why we can often smell what we eat or drink. Those with smell disorders often do not taste their food as much, and it can seem as though their food is bland.^[5]

Taste receptors work in a very similar way. Commonly referred to as taste buds, these reside all over the tongue. Taste receptors have also been found lining the roof of the mouth, the throat and may also line the digestive tract^[6] (as confirmed by in vitro experiments). The most common taste disorders pertain to phantom tastes or distorted tastes and are rarely a loss of taste. A loss of taste is most often caused by a smell disorder.^[7]

The taste and smell receptors lie very close to other sensory receptors, such as those for temperature, touch, pain, and irritation, which is why smell and taste may sometimes be affected by seemingly unrelated factors or sensations.

Causes of Smell and Taste Disorders

There are many possible causes of smell and taste disorders, particularly as they share a great degree of overlap with one another as well as with other sensory receptors.

Some common causes include:

• Infections and Allergies: Colds, flu, sinus infections, and allergies are some of the most common causes of smell and taste disorders. They can cause inflammation, congestion, or discharge in the nasal passages, which can interfere with the transmission of odor molecules to the olfactory receptors in the nose. They can also affect the taste buds on the tongue, which can alter the perception of flavors. Most smell and taste disorders are caused by illnesses and infections.
• Head injuries: Trauma to the nose, ears, or brain can damage nerves involved in transmitting smell and taste. This can result in loss or reduction of smell or taste or distortion of smell or taste. Depending on the severity and location of the injury, smell and taste disorders caused by head injuries can be permanent or reversible. Studies reveal that prior trauma can also increase one’s risk of smell and taste disorders later in life, in some cases affecting more than a third of military veterans.^[8]
• Hormonal changes: Women are more prone to developing smell and taste disorders^[9] than men. Studies have revealed that aging and hormonal decline affect smell and taste and that this may be responsible for the higher proportion of them seen in the elderly. As women experience a more rapid decline during menopause, they more often present with them later in life. The reason hormones affect taste and smell largely has to do with their impact on the regeneration and repair of smell and taste receptors.^[10] Pregnancy and thyroid issues can also reduce sensitivity towards taste and smell due to the way in which they affect the metabolism. The smell or taste issue tends to resolve when hormones reach normal levels.
• Dental or mouth problems: Poor oral hygiene, periodontal disease, tooth decay, dry mouth, and other dental or mouth problems can affect the smell and taste system in a way similar to injury and infection.
• Nasal polyps: Nasal polyps are noncancerous growths in the nasal passage that can block the airflow and reduce the sense of smell. They can also cause nasal congestion, runny nose, sneezing, or postnasal drip, which can affect the taste as well. Allergies, asthma, chronic inflammation, and infections can all lead to nasal polyps.
• Exposure to chemicals: Exposure to irritants, solvents, pesticides, and other chemicals can damage smell and taste receptors, resulting in loss or distortion of smell and taste. Smell and taste disorders induced by chemicals can become permanent if exposure is prolonged or acute.
• Medications: Some medications have side effects that lower the ability to smell and taste or that cause one to smell or taste differently. This occurs due to the way some medications interfere with the production or transmission of odor or taste molecules or the function of receptors. Medications that can cause smell and taste disorders include antibiotics, antihistamines, antidepressants, blood pressure drugs, cholesterol drugs, and diabetes drugs. Symptoms are usually temporary and resolve once the medication is stopped or changed.
• Radiation therapy: Radiation therapy can damage cells in the nose, mouth, or throat and affect the smell and taste system. This is why common side effects of radiotherapy include dry mouth, sore throat, mouth ulcers, infection, or inflammation, all of which interfere with smell and taste. The risk of smell and taste disorders is even higher in chemoradiotherapy. Prolonged therapy can lead to permanent loss or distortion of smell and taste.
• Neurological disorders: Smell and taste disorders often affect those with neurological disorders that affect the function of brain regions or pathways involved in smell and taste. Anosmia and other smell disorders in Alzheimer's disease^[11] and multiple sclerosis have been documented, as well as dysgeusia and taste disorders in Parkinson's disease. Other similar conditions that can affect taste and smell include stroke, brain tumor, epilepsy, and migraine.
• Nutritional deficiencies: Some nutrients are essential for smell and taste to occur. These include zinc^[12], B vitamins, Vitamin A, and Vitamin D. Nutritional deficiencies can cause loss or reduction of smell or taste or distortion of smell or taste.

Diagnosis of Smell and Taste Disorders

If you suspect that you have a smell or taste disorder, you should consult your doctor as soon as possible. The doctor will perform a medical history and a physical examination to determine the possible cause and severity of your disorder.

Your doctor may also order some tests to evaluate your smell and taste function. These tests can help determine the type and extent of your smell disorder and monitor your progress over time:

• Smell identification tests: You may be asked to smell and identify a series of odorants, such as coffee, lemon, or smoke, and to rate the intensity or pleasantness of the odors.
• Taste identification tests: You may be asked to taste a series of solutions, such as sweet, sour, salty, or bitter, and to rate the intensity or pleasantness of the taste.
• Imaging studies (MRI, CT scan): Imaging technologies, like MRIs and CT scanners, can help detect the underlying cause of a physical or structural problem, help rule out other conditions, and provide an accurate diagnosis.
• Neurological evaluation: You may be asked to perform some tasks that assess your cognitive and sensory functions, as well as answer some questions. This test can help determine if your disorder is related to any neurological or psychological factors, such as Alzheimer’s disease, Parkinson’s disease, or depression.

Treatments for Smell and Taste Disorders

The most effective way to treat smell and taste disorders is to treat the underlying cause.^[13] If your disorder is caused by:

• An illness, infection, allergy, or medication, your doctor may prescribe antibiotics, antihistamines, decongestants, corticosteroids or other drugs to treat the condition and relieve the symptoms.
• A nasal polyp, a dental problem, or a head injury, your doctor may recommend surgery to remove the obstruction or repair the damage.
• A nutritional deficiency, your doctor may advise you to take supplements or change your diet to correct the imbalance.
• A neurological disorder, your doctor may refer you to a neurologist or a psychiatrist for further evaluation and treatment.

However, what if you do not know why you have smell or taste loss, or you have had a smell and taste disorder for long enough that treating the cause is not enough to restore you to your senses? It is very easy to understand when you are suffering from congestion, and relatively easy to receive an accurate diagnosis for the cause. Yet, according to experts, viral infections, prior trauma, chronic diseases, or exposure to chemicals are not as easy to pinpoint as an underlying cause.^[14]

In these cases, you may need to start to regenerate and retrain your senses with specific therapies and supplements. The following tips can help minimize the damage of smell and taste loss and might also help one reclaim them in the absence of a cause or an effective treatment option.

Smell Training

Smell training is supposed to be one of the best natural remedies for loss of smell and to regain sense of smell after COVID-19^[15], or any other viral infection. Studies also show that it can help restore smell and taste in those who have suffered from traumatic brain injury or Parkinson’s disease. This technique involves sniffing potent odorants to retrain the olfactory system, twice a day for several weeks or months. The odorants should be strong, pleasant, and familiar to you, such as rose, lemon, clove, or eucalyptus.

This therapy can help stimulate the regeneration of the olfactory receptors, improve the sensitivity and discrimination of smell, and reduce the distortion or phantom smells. Studies show that it may be effective in 34-71% of patients who have lost their smell post-viral infection. This is not as effective as with other uses, such as injury or Parkinson’s disease, where it may improve symptoms in 5-20% of patients.

It may also have some cognitive benefits, as it was shown to increase the volume of the hippocampus, (the brain area involved in memory and learning) in those with mild cognitive impairment^[16].

Smell training can be done at home, with the guidance of your doctor or an ENT specialist capable of performing the therapy.

Taste Therapy

If you have lost both the sense of smell and taste, then first try smell therapy to regain both taste and smell. If your smell is normal and you have an impaired sense of taste, then you might like to try a similar therapy to that of smell therapy. As with smell therapy, one attempts to stimulate the taste buds with potent flavors for each of the 5 tastes: sweet, salty, sour, bitter, and umami.

Taste therapy may help improve the sensitivity and discrimination of taste and reduce taste distortion or phantom tastes. Taste therapy can be done at home or with the guidance of a doctor.

Platelet-Rich Plasma Therapy

If you are battling to regain your sense of taste and smell after losing them, PRP therapy may be an option to try. During the pandemic, a team of researchers tried platelet-rich plasma therapy to improve loss of smell after COVID-19 in a group of patients. In the 6-12 months after the patients received PRP injections, improvements were seen by up to 50%. PRP is a safe regenerative treatment option in which blood plasma is extracted from the patient and made to be concentrated with repair factors, such as platelets and stem cells.^[17]

Pro-Sense Supplements

Some supplements can help treat smell and taste disorders by reducing inflammation, improving blood flow, or modulating the neural activity in the smell and taste system.

Supplements that might help smell and taste are:

• Zinc
• B vitamins
• Vitamin A^[18]
• Vitamin D3
• Omega-3 fatty acids
• Alpha lipoic acid

However, these medications may have side effects or interactions with other drugs, so you should consult your doctor before taking them.

Light Therapy

Photobiomodulation therapy has emerged as a new treatment being tested for loss of taste and smell after COVID-19. In small trial studies, post-covid patients managed to regain their sense of taste and smell after 10 sessions of light therapy.^[19] This involves shining a red light into the nose and mouth using a special type of diode or light device. Doing so helps to stimulate the cells in the nose and mouth, which may support regeneration and lower inflammation. More research is needed to confirm whether this is a viable option for smell and taste disorders in general or not.

Prevention of Smell and Taste Disorders

The best thing you can do to prevent smell and taste disorders is to seek prompt medical attention for infections or injuries and make sure they are treated properly. Early diagnosis and treatment can help prevent or minimize the damage to your smell and taste system and improve your recovery and prognosis.

Certain precautions and healthy habits may also help to protect your sense of smell and taste, especially after an infection or injury. Some of these include:

• Adopting a healthy lifestyle, such as eating a balanced diet, exercising regularly, and getting enough sleep and rest.
• Maintaining good oral hygiene helps prevent dental or mouth problems that can affect your smell and taste system.
• Avoiding smoking, chewing tobacco, or drinking alcohol, which can damage your oral health and impair your smell and taste function.
• Limiting exposure to irritants and chemicals where and if possible.
• Making use of a mask, gloves, or goggles when working with toxic substances.
• Ensuring the air is clean in your home and work environment with adequate ventilation, air purification or humidification.
• Treating and controlling any medical conditions that can affect your smell and taste system, such as allergies, asthma, diabetes, or hypertension, can help prevent or reduce the severity of smell and taste disorders.

Conclusion

Smell and taste disorders are conditions that impair the ability to perceive odors and flavors. They are usually the result of illnesses, injuries, medications, or neurological disorders. Smell and taste disorders can affect the quality of life and well-being of the affected individuals. Therefore, it is important to diagnose and treat smell and taste disorders as soon as possible. There are different tests and treatments available for smell and taste disorders, depending on the cause and severity of the disorder. The best way to regain a lost sense of smell or taste is to treat the underlying cause. Where the cause is not known or the damage is profuse, smell training and supplementation may be able to improve symptoms.

If you think you have a smell or taste disorder, you should consult your doctor and get a professional evaluation and treatment. With proper diagnosis, treatment, and prevention, you can regain or improve your sense of smell and taste, and enjoy the world of odors and flavors again.

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