Olfactory Bulb: Characteristics and Dysfunction
Also known as: Anosmia, Hyposmia, Parosmia, Phantosmia, Loss of smell, Smell disorders, Olfactory dysfunction
The olfactory bulb plays a key role in our sense of smell. Its dysfunction can significantly affect quality of life and deserves proper medical attention.
What is the olfactory bulb?
The olfactory bulb is a brain structure responsible for processing smell information. It sits at the base of the skull, in the anterior cranial fossa, resting on the cribriform plate just beneath the frontal lobes.
Olfactory sensory neurons in the lining of the nose detect odor molecules and send their axons up into the bulb, where they connect with mitral and tufted cells. From there, signals travel on to the piriform cortex (the primary olfactory cortex), the amygdala, and the parahippocampal gyrus. This wiring explains why smell is so tightly bound to memory, emotion, and taste: the olfactory pathway feeds directly into the brain regions that handle all three.
Smell also does more than provide pleasure. It protects you, by flagging spoiled food, smoke, and gas leaks before anything else does.
Causes and risk factors
Several factors can disrupt how the olfactory bulb and the wider olfactory system work:
- Viral infections (cold, flu, COVID-19)
- Head trauma
- Neurodegenerative diseases (Parkinson’s, Alzheimer’s)
- Nasal polyps or chronic sinusitis
- Exposure to toxic or irritating chemicals
- Certain medications
- Chemotherapy or radiation therapy to the head and neck, since cancer and its treatment can change taste and smell
- Smoking
- Hormonal changes and dental problems
- Natural aging, as most people lose some ability to smell as they get older
Smell problems become more common with age and are more common in men than in women. About 1% to 2% of North Americans report a problem with their sense of smell. Among older adults the figures are much higher: nearly one quarter of men ages 60 to 69 had a smell disorder, compared with about 11% of women in the same age range.
Symptoms
Olfactory bulb dysfunction can show up as:
- Anosmia: the complete inability to detect odors
- Hyposmia: a reduced ability to detect odors
- Parosmia: a change in the normal perception of odors, such as when the smell of something familiar becomes distorted
- Phantosmia: the sensation of an odor that isn’t there
- Difficulty recognizing tastes, since flavor depends heavily on smell and food tends to taste bland without it
These issues can be temporary or permanent depending on the cause. Losing your sense of smell is not a trivial problem: it can degrade the quality of your diet, drive weight loss or weight gain, and push people to over-salt their food, which matters a great deal if you have high blood pressure.
Treatments
Treatment depends on the underlying cause:
- Treating the underlying infection or nasal inflammation
- Smell training, a gradual retraining of the sense of smell with a set of different scents
- Topical (intranasal) corticosteroids for inflammatory forms. A steroid rinse has shown significant improvement when used alongside smell training. Oral steroids are a different story: a randomized trial of 115 patients with post-COVID-19 smell loss found no benefit from systemic steroids
- Surgery for polyps or obstruction
- Neurological follow-up if a degenerative disease is involved
Smell training is the first-line treatment recommended by expert consensus after a viral infection, and it is the one treatment whose effectiveness is most solidly demonstrated. It is also simple, low cost, free of side effects, and can be done at home. In the classic protocol, patients use four odorants and spend about 10 seconds consciously smelling each one, twice a day, for three months. Patients with parosmia are more likely to see meaningful improvement in identifying and discriminating odors than those with smell loss alone.
Recovery often takes time, and a good deal of it happens on its own, though many people are still affected months later. After COVID-19, the reported prevalence of ongoing smell dysfunction is 27% to 60% at six months and 26.5% to 46% at one year, dropping to about 8.3% at two years. Roughly 85% of patients no longer report a distorted sense of smell after two years.
When should you see a doctor?
It’s a good idea to see a doctor if you experience:
- Persistent loss of smell after an infection
- Sudden onset of smell problems with no obvious cause
- Perceiving unusual or unpleasant smells
- Loss of smell along with neurological symptoms
An ENT and neurological workup may be needed to identify the cause. Smell disorders are typically diagnosed by an otolaryngologist (an ENT doctor) through a physical examination, a review of your health history, and smell testing, which may measure the smallest amount of odor you can detect or use scratch-and-sniff booklets.
Do not brush off a lasting change in your sense of smell. It can be an early sign of a serious condition, including Parkinson’s disease, Alzheimer’s disease, and multiple sclerosis.
Prevention
Certain steps can help protect your sense of smell:
- Protect yourself against viral infections (vaccination, hand hygiene)
- Avoid exposure to irritating chemicals
- Don’t smoke, since smoking is a recognized cause of smell problems
- Wear a properly fitting helmet when biking, skateboarding, or playing contact sports like hockey and football, since head injury is a recognized cause of smell loss
- Treat sinusitis and nasal polyps promptly
Sources
- National Institute on Deafness and Other Communication Disorders (NIDCD), U.S. national institute data on smell disorders, definitions, and prevalence
- MedlinePlus, U.S. National Library of Medicine information on taste and smell disorders
- MedlinePlus, Head and Brain Injuries, helmet use and other measures to prevent head injuries
- Neuroanatomy, Cranial Nerve 1 (Olfactory), StatPearls, NCBI Bookshelf, olfactory bulb anatomy and central connections
- Post-viral olfactory loss and parosmia, PMC (NIH), evidence on smell training, steroids, and recovery
- National Cancer Institute, taste and smell changes during cancer treatment