Anosmia: loss or absence of the sense of smell

Anatomy and how smell works

Odor detection begins in the nose and relies on specialized receptor cells whose signals travel to the brain. Two cranial nerves play distinct roles: the olfactory nerve carries many odor signals to central processing areas, while the trigeminal nerve senses irritants, cooling and burning sensations. Damage anywhere along this pathway — nasal cavity, olfactory epithelium, cranial nerves, or the brain regions that interpret smell — can reduce or abolish olfaction. See more about these nerves and pathways: olfactory and trigeminal nerves and central processing brain areas for smell.

Common causes

Causes fall into two broad groups: conductive losses, where odorants cannot reach receptors, and sensorineural losses, where receptors or central pathways are damaged. Typical causes include:

• Upper respiratory infections such as the common cold or other viral illnesses; viruses are a well-known trigger of postinfectious anosmia (upper respiratory tract infection, common cold).
• Nasal obstruction from allergies, polyps or structural problems (seasonal allergies, nasal polyps).
• Head trauma causing shearing of the olfactory nerve fibers, for example after a concussion (concussion).
• Neurological disease that affects central olfactory processing, including some neurodegenerative disorders (neurological conditions, Parkinson's disease, Alzheimer's disease).
• Medications, toxic exposures or smoking that impair receptors or mucosal health (certain drugs, poisons, tobacco and other plants plant substances).
• Intracranial mass lesions such as brain tumors which may compress olfactory pathways (brain tumors).

Clinical features, testing and diagnosis

People with anosmia report inability to detect odors and a reduced sense of flavor; basic tastes — sweet, sour, salty, bitter and umami — often remain because they are mediated by taste receptors on the tongue (taste and smell, basic tastes). Clinicians evaluate onset, duration and associated symptoms. Objective smell testing and nasal examination are common first steps. Imaging (for example MRI or CT) may be used when a structural or central cause is suspected.

Treatment, prognosis and safety

Treatment depends on cause. Conductive anosmia due to obstruction often improves with medical or surgical relief of the blockage. Postinfectious and posttraumatic anosmia may recover spontaneously, though recovery can be slow and incomplete. When anosmia results from widespread brain damage it is less likely to be reversible. Very limited or mixed evidence exists for interventions such as olfactory training and some complementary approaches (for example acupuncture has been reported anecdotally) — these should be considered experimental or adjunctive.

Loss of smell carries safety implications: inability to detect smoke, gas leaks or spoiled food increases risk. It also has quality-of-life effects, including reduced enjoyment of food and possible impact on social and emotional wellbeing.

Notable associations and practical notes

Anosmia can be an early sign in some neurodegenerative diseases and therefore may prompt further neurological assessment in the correct clinical context. Sudden onset anosmia during viral outbreaks has drawn attention to the symptom's diagnostic importance. Evaluation typically involves ENT and, when indicated, neurology. For more general information and resources see sense of smell, resources on infections respiratory infection, and guidance about common causes such as the common cold, allergic rhinitis and safe medication use drug side-effects. Additional topics of interest include occupational exposures (toxins), head injury prevention (concussion), intracranial disease (tumors), and the interaction between smell and taste systems (taste). For neuropsychiatric links see schizophrenia, and for lifestyle factors see information about plants and tobacco.

If you suspect new or unexplained anosmia, seek medical assessment to establish cause, discuss safety measures and consider available interventions.