Anatomy of the Olfactory Epithelium

The olfactory epithelium is membranous tissue located inside the nasal cavity. It measures about 3 square centimeters (cm²) in adults. Containing olfactory receptor cells, it is involved in the sense of smell.

Olfactory disorders can range from a mild decrease in the ability to smell to a complete loss of smell. A loss of smell can indicate a mild illness or injury or something more serious. A decrease in the ability to smell can be an early indicator of Parkinson’s disease and Alzheimer’s disease.

Olfactory dysfunction is also associated with some psychiatric conditions, such as schizophrenia, mood disorders, and anxiety disorders, as well as some illnesses, including COVID-19.

Anatomy

The olfactory epithelium is a thin, cellular tissue that runs along the roof of the nasal cavity. In adults, it is situated about 7 centimeters (cm) behind the nostrils. It is part of the nasal septum and the superior turbinate bones.

Three cell types make up the olfactory epithelium: basal, supporting, and olfactory. Olfactory receptor cells have hairlike extensions called cilia.

It used to be thought that the olfactory epithelium developed separately from the olfactory placode. More recent studies, however, have found that it develops from neural crest cells, which form in embryos.

Anatomical Variations

The nasal epithelium can be affected by congenital conditions (those present at birth). Kallmann syndrome, for example, is a genetic disorder in which the area of the brain called the hypothalamus and the olfactory neurons do not fully develop. This can result in the inability to smell (anosmia). Kallman syndrome can also delay puberty if a hormone deficiency of nIHH (normosmic idiopathic hypogonadotropic hypogonadism) is present.

Another genetic disorder that can impair smell is ciliopathy. In ciliopathy, the body's ability to make cilia is impaired. When cilia are not there or they are malformed, a person does not have the ability to smell. 

Function

The olfactory epithelium is part of the olfactory sensory system, whose role is to pass along smell sensations to the brain. It does this by trapping odors that pass across the cilia before sending the information about those odors to the olfactory bulb.  

The olfactory bulb is located in the front of the brain. After the olfactory bulb receives information from the cells in the nasal cavity, it processes the information and passes it to other parts of the brain. 

Associated Conditions

Damage to the olfactory epithelium also can cause a person to lose the sense of smell. Damage is often caused by toxic fumes, physical trauma, blockage in the nasal passage, tumors in the brain, allergies, or infections. Olfactory disorders can be temporary, but in some cases, they are permanent.

The loss of smell is a spectrum, with problems ranging from a change or distortion of smell (dysosmia) to a diminishment of smell (hyposmia) to the complete loss of smell (anosmia). Loss of smell is not uncommon in people with traumatic brain injury. One study found that 15% to 35% of people with traumatic brain injury sustained a loss of smell.

Since smell and taste are so closely linked, the loss of smell often contributes to an inability to taste, making eating less enjoyable. Also, there are some dangers to not being able to smell, such as not being able to detect something burning. Lack of smell can contribute to mood disorders, too, like anxiety and depression.

Presbyosmia is an age-related loss of smell. According to one study, up to 39% of people over the age of 80 have olfactory dysfunction. Presbyosmia occurs gradually and is not preventable. It may be related to a loss of nerve endings and mucus production as people age. An increased use of medication as well as neurological disorders also may contribute to age-related loss of smell.

Cancer of the nasal cavity is rare. Squamous cell cancer is the most common cause of nasal and paranasal cancers, followed by adenocarcinoma.

Nasal cancer symptoms may include a runny nose, congestion, and a sensation of fullness or tenderness in the nose. Problems with your ability to smell can also arise. Later stages may involve nose bleeds, facial and tooth pain, and eye problems.

Early symptoms of some neurodegenerative disorders, like Parkinson’s disease and Alzheimer’s disease, include a decreased ability to smell. Some psychiatric illnesses, like schizophrenia, mood disorders, and anxiety disorders, are also associated with olfactory dysfunction. Loss of smell is one of the key symptoms of COVID-19. 

Tests

Testing for Kallmann syndrome may include blood tests to check hormone levels. Magnetic resonance imaging (MRI) of the head and nose may be done to look for anatomical abnormalities of the nose, hypothalamus, and pituitary gland. Molecular genetic testing may also be performed to identify gene mutations.

Treatment for Kallmann syndrome and nIHH usually involves hormone replacement therapy. Medication to strengthen the bones may also be used, as the absence of puberty hormones can weaken them.

Advances in DNA testing (genetic testing) using RNA sequence analysis may help diagnose ciliopathy. Ciliopathy is considered incurable, but recent advances in gene therapy could mean treatment options may be available in the future.

Your healthcare provider may order certain scans to diagnose loss of smell, like computed tomography (CT scan), MRI, or an X-ray. Nasal endoscopy may be used to look inside your nose. A test known as the University of Pennsylvania Smell Identification Test (UPSIT) may be done to assess the degree to which your smell is affected.

Treatment of anosmia, dysosmia, and hyposmia will depend on what is believed to be the cause of the olfactory dysfunction. Your healthcare provider may prescribe decongestants, antihistamines, or nasal sprays for allergies. If an infection is the cause, antibiotics may be in order.

Smoking is known to impair the ability to smell, so quitting smoking may also help. If there is a blockage in the nasal passage, your healthcare provider may discuss surgically removing a polyp or tumor.

Nasal cancer is diagnosed by an ear, nose, and throat specialist (otolaryngologist). Your healthcare provider will first need to get a good look at the tumor and may do this through certain tests, like a nasal endoscopy, CT scan, MRI, X-ray, or positron emission tomography (PET scan).

To definitively determine if the tumor is malignant (cancerous) or benign (noncancerous), your healthcare provider will take a biopsy of the tumor. Treatment options for nasal cancers include surgery, chemotherapy, radiation therapy, targeted therapy, and palliative care (to ease symptoms).