Congenital anosmia and other closely related genetic disorders affecting the sense of smell are estimated to occur in 1 in 10,000 people. Only 15 people have been described as having congenital anosmia unrelated to Kallmann syndrome or other rare genetic disorders (Kallmann syndrome affects 1 in 10,000 men and 1 in 50,000 women).

An individual living with congenital anosmia can experience loss of smell as either an isolated incident or as the symptom of another rare disorder. Isolated congenital anosmia (ICA) refers to the lack of only one’s sense of smell from birth. Isolated anosmia is not associated with other abnormalities, nor is the exact genetic cause known. It is thought to be related to defects during the development of the olfactory bulbs in the brain (neural structure involved in producing a sense of smell) prior to birth, or the replacement of the cells specialized in detecting smells in the nasal cavity in the airway. The human sense of smell is actually a process of recognizing compounds with an odor and signaling the brain to detect a scent. The odor compounds enter the nostril(s) of the nose, are absorbed into the skin (epithelium) layer in the nasal passage, recognize and bind to specific proteins on the surface of olfactory cells called olfactory receptors, and the olfactory receptor produces an electrochemical signal that passes to the brain via neuron cell signaling. ICA can be caused by disrupted activity or development in three levels of smell signal transduction from the nose to the brain:

1. Defects in the transportation of odors to the cells in the nasal cavity that detect the presence of odorous molecules and send nervous signals to the brain (olfactory neuroepithelium) caused by congenital malformations in the nasal cavity that obstruct the nose

2. Disruption of signal transduction or propagation in olfactory cells that carry the nervous signal from the nasal cavity to the brain

3. Malformation of olfactory regions of the brain

Congenital anosmia may also be associated with other abnormalities in development or senses. While the exact genetic mutation causing congenital anosmia is unknown, studies have indicated that CNGA2 gene mutations may lead to the reported symptoms. These genetic mutations may also play a role in the development of other rare diseases that cause congenital anosmia as one of the symptoms. A case study involving twin boys with diagnosed congenital anosmia determined that they each had a shortened, damaged copy of CNGA2 on the X-chromosome. A shortened gene lacks important information to form a protein, leading to a non-functional protein product. The CNGA2 gene encodes a part of an olfactory channel located in olfactory sensory neuron cells that is responsible for responding to odors and transmitting messages to the brain to perceive “smell”. There is some evidence that the genes GNA1, ADCY3, and CNGA2 may be involved in detecting smell and that mutations may cause anosmia. Several syndromes in which congenital anosmia presents as one of the traits include Kallmann syndrome, congenital insensitivity to pain, CHARGE syndrome, Bardet -Biedl syndrome, Leber congenital amaurosis, Refsum disease and additional syndromes.

An individual affected by congenital anosmia will have no ability to smell odors. This lack of a sense of smell is present from birth, and often the individual will be unaware of odors. Due to lack of smell, individuals with anosmia have reported that their sense of taste is also diminished or affected. Food may be distinguished by general qualities such as texture and strong flavors, but lack subtle differences such as the difference in sweetness between fruit and chocolate. This can cause individuals to lose interest in food. 

A diagnosis of congenital anosmia involves ruling out all other known conditions that cause lack of smell (injury, lack of development of nasal cavities, blockage, etc.). For example, hyposmia is the loss of a sense of smell during one’s lifetime, usually due to accidents, injury, or in the temporary case congestion. Typically, a person with congenital anosmia will have no known recollection of smell. This will result in administration of different tests to determine the level of affliction and the possibility of other related inherited disabilities.

In order to determine that a person experiencing anosmia has isolated congenital anosmia, a doctor may prescribe one or several tests. These diagnostic tests are designed to evaluate the individual’s sense of smell and whether other factors are contributing to its loss. To test an individual for ability (if any) to smell:

1. Smell tests (Sniffin’ Sticks or UPSIT test): The University of Pennsylvania developed a smell identification test to determine the smallest amount of odor that a person is capable of detecting. The individual is instructed to “scratch and sniff” a number of surfaces with varying odors and concentrations and then select from a list of multiple options matching the odor description and/or source.

2. Odorant confusion matrix: An individual is exposed to 10 random smells a total of 10 times, and instructed to identify the odor. Anyone scoring less than 10% of correct answers is believed to have total anosmia

3. Threshold testing: Designed to test the degree of anosmia that someone experiences, this test exposes a person to two sprays: one with an odorant (often rubbing alcohol) and one with only water. They must identify which spray contains the odorant. With each round, the concentration of odorant in water will be decreased to determine the minimum amount necessary for the individual to effectively smell. 

To test an individual for abnormalities in the olfactory region (area of the brain that receives signals from the nose and translates them to smell) or other areas of the nasal passage:

1. CT scan: A CT scan of the brain may or may not involve the use of a contrast agent (a harmless chemical injected through IV designed to allow better visualization of an organ in the body). The individual’s brain is examined using x-rays to determine whether their olfactory region has abnormalities or lack of development.

2. Nasal endoscopy: A small tube and camera is inserted through the nostril into the nasal passage to evaluate it for abnormalities.

3. Olfactory nerve test: While the olfactory bulb may have developed normally, disruptions in the neural pathways to the proper processing centers in the brain would result in the inability to discern smells. This test can be conducted by exposing an individual to a smell while blindfolded (inhibiting other senses) or while keeping their nose pinched (testing changes caused by blockage).

There is currently no treatment available to individuals with isolated congenital anosmia. The primary method to cope with this lack of sense is to find ways to interact with one’s surroundings without relying on smell. Individuals that have been born without a sense of smell or lose it during their lifetime attempt to enjoy food in other ways, such as overseasoning or focusing on the sensation or “mouth-feel” when eating. 

While individuals with isolated congenital anosmia may not have symptoms of discomfort or other related health problems, they may lack the ability to discern certain situations involving smell. This may present dangers that go unnoticed such as a gas burner left on the stove, smoke from a nearby fire, spoiled or rotten food, and other signals meant to stimulate the brain and warn the person. A patient should be mindful of this lack of smell and rely on other senses and the help of others. In addition, patients with an affected sense of taste as a result of anosmia should seek other ways to enjoy food and eating. If the congenital anosmia is associated with another underlying syndrome, the prognosis would vary and may affect the quality of life or lifespan due to factors contributed by the underlying condition.

Individuals with anosmia may find it helpful to seek enjoyment in food in new and different ways. The website Anosmic Chef contains recipes written by a person with anosmia for others living without a lack of smell or taste https://anosmicchef.wordpress.com/

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