Olfactory perceptual changes can be quantitative (smell loss) or qualitative (smell distortions). Olfactory dysfunction is a very common condition with a reported prevalence between 4 and 25%. Men are more likely to suffer from it than women and smoking, working in a factory environment, low level of education, and having a low household income have been reported as risk factors. Olfactory dysfunction, like visual and auditory impairment, becomes more prevalent with increasing age.

There are many causes of olfactory dysfunction. The three most common causes are sinonasal disease like nasal polyps or chronic inflammation of the nasal passages and/or paranasal sinuses (rhinitis, sinusitis, rhinosinusitis), upper respiratory infection, and head trauma. 

Olfactory dysfunction can be quantitative and/or qualitative. 

  • Quantitative smell disorders refer to disorders in which there is complete or partial loss of olfaction. Anosmia, the complete loss of olfaction, and hyposmia, the partial loss of olfaction are the two disorders classified as quantitative because they can be measured. In recent decades, several olfactory tests were designed in various countries. Some measured identification only, some sensitivity only, others combined both, and one added a discrimination measurement. Thus, the different commercially available tests do not specifically measure the same olfactory competencies, but all are designed to detect hyposmia or anosmia.
  • Qualitative smell disorders can’t be measured and refer to disorders in which there is alternation or distortion in the perception of smell. The term dysosmia refers to a qualitative olfaction disorder and include both parosmia (the inability of the brain to properly identify an odor's "natural" smell) and phantosmia ((olfactory hallucination). 

Olfactory dysfunction including anosmia, hyposmia, and dysosmia can be either bilateral or unilateral on either nostril. Anosmia only on the left nostril would be termed unilateral left anosmia while bilateral anosmia would be termed total anosmia.
Olfactory training is a promising therapeutic treatment for olfactory loss and is particularly successful in patients with smell loss after upper respiratory tract infection. Olfactory training means that people exposed themselves several times a day to some odors. They sniff each odor for at least 15 seconds and repeat it. It is recommended to do this at least four times a day. The training takes about 16 weeks and then these odours are switched to the same ones, but with lower concentrations. Studies have shown, that after 18 weeks, patients are able to recognize even the lowest concentration of an odor. The olfactory system holds the unique ability to be activated by the sensorimotor act of sniffing, which is characterized by a short and deep intake of breath through the nose, without the presentation of an odor. 

Disturbed sense of smell is associated with several disieases including hypogonadostropic hypogonadism and Kallmann’s syndrome.

Hypogonadotropic hypogonadism

Its origin is due to a lack of hormones from pituitary gland that normally stimulate the ovaries or testes: follicle stimulating hormone (FSH) and luteinizing hormone (LH). The decrease or lack of smell is an early symptom in hypogonadotropic hypogonadism associated with disturbances of smell therefore we consider that it`s necessary to include an smell test in childhood, at early ages. In front of the suspect and diagnosis the treatment will be in charge of endocrinology and pediatrics, mainly based on hormone replacement therapy to induce puberty and improve fertility.

Kallmann's syndrome

Kallmann's syndrome is caused by the failure of olfactory axons (olfactory sensory neurons project slender projection of a nerve cell to the brain within the olfactory nerve) and gonadotropin-releasing hormone (GnRH) neurons to enter the embryonic forebrain, resulting in anosmia and sterility.

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