The sense of smell is one of the basic human senses. Olfactory disorders can have varying degrees of negative impact on personal safety, career, appetite, nutrition, hygiene, housekeeping, childcare, hobbies, and recreational activities. Severe cases can lead to depression and other psychological problems. For some professions that depend on the sense of smell such as chefs, sommeliers, firefighters, chemists, make-up artists, florists and some industrial workers, it is impossible to perform professional activities without the sense of smell. But the sense of smell is not as valued as the sense of sight and hearing. Health questionnaires in the United States show that self-reported olfactory impairment in adults is 1.4%. A bulk survey in the US with olfactory testing showed a prevalence of olfactory disorders in older adults (53-97 years) of 24.5% (2002). In a Swedish study, the prevalence of olfactory disorders in the general adult population was found to be 19.1% through questionnaires and olfactory tests, of which 13.3% were olfactory decline and 5.8% were complete loss of smell (2004). The survey showed that among the patients with olfactory disorders, 68% thought that olfactory disorders affected their quality of life and 56% thought that olfactory disorders changed their life. 1.Where is the olfactory organ and what does it look like? The olfactory system mainly consists of olfactory epithelium, olfactory bulb and olfactory cortex. The olfactory epithelium is located in the mucosa of the olfactory area at the top of each side of the nasal cavity. The olfactory area contains olfactory receptor cells, and the central protrusions of the olfactory receptor cells are fused into olfactory filaments that pass through the foramen ovale of the top of the nasal cavity and end at the olfactory bulb. The olfactory bulb is located at the anterior skull base and is the first transit point of the olfactory pathway. The axonal fibers of monk’s cap cells and plexiform cells in the olfactory bulb form the olfactory bundle, which travels on the surface of the olfactory grooves on both sides. The olfactory bundle is divided into the lateral olfactory stripe and the medial olfactory stripe. The lateral olfactory stripe enters the pearly cortex and some fibers enter the orbitofrontal gyrus from the pearly cortex. The central contacts of the olfactory system are complex and some of the pathways are not yet fully understood. Figures 1 and 2 list the main afferent pathways of olfaction. 2. How do we smell odors? Firstly, the airflow containing odor molecules passes through the olfactory area at the top of the nasal cavity, the odor molecules dissolve in the mucus of the olfactory area, the odor molecules combine with the odor receptors of the olfactory mucosa and activate the olfactory receptor cells, the olfactory receptor cells send out nerve impulses to the olfactory bulb, the signals are further transmitted to the higher cortex, thus producing the sense of smell. See Figure 3. 3.How many kinds of olfactory diseases are there? Olfactory diseases are divided into hyposmia, complete loss of smell, olfactory hypersensitivity, phantom smell and olfactory inversion. The first three belong to quantitative olfactory disorders and the last two belong to qualitative olfactory disorders. Hyposmia refers to the decrease of olfactory sensitivity. Complete loss of smell is a complete loss of olfactory function. Olfactory hypersensitivity is increased olfactory sensitivity. Phantom smell is the ability to smell when there is no odor molecule stimulation in the environment. The odor is usually unpleasant such as “stinky”, “rotten”, “disgusting”, “burning”, etc. Smell. Olfactory inversion is when the inhaled olfactor is different from the memorized smell of the olfactor, which is usually unpleasant. 4.What are the causes of olfactory impairment? 1）.Nasal obstructive lesions: 23% of the cases are nasal polyps, allergic rhinitis, nasal tumors, nasal septum deviation and other lesions will obstruct the airflow to reach the olfactory area and cause hyposmia or complete loss of smell. 2）.After upper respiratory tract sensation: 19% of cases are caused by viral infection of olfactory epithelium or olfactory nerve, and are more common in women aged 40-60. Most of them are hyposmia and may be accompanied by olfactory inversion. Less than 1/3 of patients can have different degree of recovery, usually takes 3-6 months. (3), Maxillofacial cranial trauma: 15%, 0.5% of cranial trauma is associated with olfactory impairment, the damaged area can be shear injury of olfactory filament, blunt contusion of olfactory bulb, frontal lobe injury or compound injury. It has been reported that 8-39% of patients can have different degrees of recovery of olfactory function. (4), Toxins or drugs: accounting for 3%, a variety of chemicals and certain drugs can damage the sense of smell. 5）.Other: accounting for 21%, including aging, congenital, intracranial skull base tumor, neurodegenerative diseases (idiopathic Parkinson’s disease, Alzheimer’s disease, multiple sclerosis, etc.) endocrine diseases (Adisson’s disease, Turner’s disease, hypothyroidism, etc.), etc. 6), Idiopathic: 21% of them have unknown etiology. 5.How to diagnose olfactory diseases? Based on medical history, onset, course, concomitant symptoms, medication, infection, chemical exposure, etc. Perform a comprehensive ENT-head and neck surgery physical examination and nasal endoscopy if necessary. No routine laboratory tests. For standardized olfactory function test, subjective olfactory test must be performed, which can be done with domestic five-flavor test olfactive solution or Nissan T&T olfactometer, and objective olfactory test – olfactory evoked potential test can be performed if available. Olfactory imaging: routine sinus CT, sinus CT can dog exclude tumor and other occupying disease; if possible, thin layer MRI scan of olfactory nerve can clearly show the olfactory bulb, olfactory bundle and high pear-shaped cortex, hippocampus and other lesions. The newly developed olfactory functional imaging is important for the assessment of olfactory disorders. Sometimes it requires multidisciplinary consultation and treatment by ENT, neurology, neurosurgery and endocrinology physicians. 6.Can olfactory disorders be cured? The treatment should be tailored to the cause. Idiopathic olfactory disorders and congenital olfactory disorders are difficult to treat. 7.What are the new developments in the research of olfactory diseases? In terms of diagnosis, thin-layer MRI scan of olfactory pathway and olfactory function imaging are carried out; Germany, USA and Japan have carried out objective olfactory test – olfactory event-related potential test. In terms of treatment, stem cell therapy is being carried out in foreign countries, but there is no breakthrough in clinical aspects. 8.What should I do if I have olfactory disease? See an ENT doctor as soon as possible. If necessary, a consultation with a neurologist, neurosurgeon or endocrinologist is needed. The nature and degree of olfactory disorder should be clarified, the cause of the disorder should be identified, and treatment should be directed at the cause. For patients with complete loss of smell and hyposmia of unknown cause, personal protection is needed, especially for gas leak and rotten food.