Loss of Smell: What Could Be Causing It

Why Loss of Smell Happens: The Olfactory System in 60 Seconds

Olfactory neurons sit exposed at the roof of the nasal cavity, making them uniquely vulnerable to injury. Airborne molecules dissolve in nasal mucus, bind receptors on these neurons, and send signals through the cribriform plate to the olfactory bulb. Anything that blocks airflow, damages those neurons, or disrupts central processing can reduce or eliminate smell.

The distinction matters clinically. Conductive losses (from swelling or polyps that block airflow) are often reversible. Sensorineural losses (from viral damage to the olfactory epithelium or shearing of nerve fibers after trauma) take longer to recover and sometimes become permanent. A 2019 review in Nature Reviews Neurology noted that olfactory neurons are among the few in the adult brain that regenerate throughout life, which explains why even severe post-viral anosmia can resolve over months [1]. Central causes, where the brain itself cannot process smell signals, carry the most concerning differential diagnoses, including early Parkinson disease and Alzheimer disease [1].

According to NHANES data analyzed by the National Institute on Deafness and Other Communication Disorders, approximately 13.3 million American adults aged 40 and older have measurable olfactory dysfunction, yet fewer than a quarter of them report being aware of the deficit [2]. That gap between objective impairment and self-reported awareness is a recurring challenge in clinical practice.

Upper Respiratory Infections: The Single Most Common Cause

Viral upper respiratory infections (URIs) cause 18-45% of all anosmia cases seen in rhinology clinics [3]. The mechanism involves direct viral cytotoxicity to olfactory sensory neurons and supporting sustentacular cells. Common culprits include rhinoviruses, parainfluenza, and coronaviruses.

The prognosis is generally favorable. A prospective German study of 542 post-infectious anosmia patients found that 32% recovered completely within 12 months, another 35% showed meaningful improvement, and 33% had persistent dysfunction at the one-year mark [3]. Age over 60 and complete (rather than partial) initial loss predicted worse outcomes.

"Post-infectious olfactory loss remains the most common cause of anosmia presenting to smell and taste clinics worldwide," stated the European Position Paper on Rhinosinusitis and Nasal Polyps (EPOS 2020), which also recommended olfactory training as the initial management step [4]. Recovery depends partly on how quickly the olfactory epithelium regenerates, a process that can take 8 to 24 weeks even in otherwise healthy adults.

Short-duration smell loss during a cold or flu (lasting days to two weeks) is almost always conductive, caused by mucosal swelling and excess mucus. This type resolves as the infection clears. The cases that persist beyond four weeks after symptom resolution indicate actual neuronal damage and warrant a more structured evaluation.

Post-COVID Anosmia: What the Data Show Three Years Later

SARS-CoV-2 brought olfactory dysfunction into public awareness at a scale no prior virus had achieved. Early pandemic data from a European multicenter study of 1,420 patients found that 85.6% of mild-to-moderate COVID-19 cases self-reported olfactory dysfunction, while objective testing confirmed impairment in 54.7% [5]. The virus enters olfactory supporting cells via the ACE2 receptor, and damage to sustentacular cells disrupts the local environment that olfactory neurons need to function [6].

Recovery rates have been reassuring. A 2022 BMJ meta-analysis pooling data from 18 studies and 3,699 patients found that 95.3% of individuals recovered olfactory function within six months [7]. A smaller subset, estimated at 5-7%, developed persistent dysfunction lasting beyond 12 months. Among those with prolonged loss, parosmia (distorted smell, often describing pleasant odors as rotten or chemical) affected approximately 43% and was frequently more distressing than the anosmia itself [7].

The Omicron and subsequent variants have shown lower rates of anosmia compared to the original Wuhan strain and Delta. A UK study published in The Lancet found that the odds of reporting smell or taste loss dropped by 62% with Omicron BA.1 compared to Delta (adjusted OR 0.38, 95% CI 0.36-0.40) [8]. Vaccination status also modifies risk: a CDC analysis showed vaccinated individuals who developed breakthrough infections were 44% less likely to report olfactory dysfunction than unvaccinated patients [9].

Chronic Sinusitis and Nasal Polyps

Chronic rhinosinusitis (CRS), particularly with nasal polyps (CRSwNP), is the second-most-common cause of olfactory dysfunction in referral settings. The EPOS 2020 guidelines report that 60-80% of CRSwNP patients experience some degree of hyposmia [4]. The mechanism is twofold: polyps physically obstruct airflow to the olfactory cleft, and chronic eosinophilic inflammation damages the olfactory epithelium over time.

This distinction explains why surgical removal of polyps does not guarantee smell recovery. A systematic review of 2,190 post-surgical CRSwNP patients found that 63% reported improved olfaction after endoscopic sinus surgery, but only 23% achieved full normalization [10]. Patients with higher tissue eosinophilia and longer disease duration before surgery had worse olfactory outcomes.

Medical management with intranasal corticosteroids improves smell in many CRS patients. Mometasone furoate nasal spray 200 mcg daily produced statistically significant improvements in olfaction compared to placebo in a randomized trial of 310 CRSwNP patients (P<0.001) [10]. Newer biologics targeting type 2 inflammation have shown striking results. Dupilumab, an anti-IL-4/IL-13 monoclonal antibody, improved UPSIT scores by a mean of 10.5 points over placebo in the SINUS-24 trial (N=276), with 27.2% of dupilumab-treated patients achieving a normal smell score versus 5.9% on placebo [11].

Head Trauma and Toxic Exposures

Traumatic brain injury accounts for 10-20% of chronic anosmia cases [1]. The mechanism is typically shearing of olfactory nerve fibers as they pass through the cribriform plate during acceleration-deceleration injuries. Even mild concussions can produce measurable olfactory deficits. A longitudinal study of 268 TBI patients found olfactory dysfunction in 25.8% at initial assessment, with only partial recovery in most: just 36% of those initially affected showed improvement at one year [12].

The prognosis is worse than post-infectious anosmia. Younger patients fare somewhat better, likely due to greater neural plasticity, but complete recovery after traumatic anosmia is uncommon when the initial loss is total.

Occupational and environmental exposures also damage the olfactory system. Chronic exposure to cadmium, formaldehyde, solvents, and heavy metals is associated with measurable olfactory decline [1]. Cigarette smoking reduces olfactory function in a dose-dependent manner, and while the effect is partially reversible, former smokers who quit after 15+ pack-years still showed worse olfactory scores than never-smokers in a cross-sectional NHANES analysis [2].

Medications can impair smell as well. Angiotensin-converting enzyme (ACE) inhibitors, methotrexate, intranasal zinc gluconate (removed from the U.S. market by the FDA in 2009 after reports of persistent anosmia), calcium channel blockers, and certain antibiotics have all been implicated [1]. The American Academy of Otolaryngology's clinical practice guideline recommends medication review as a standard step in evaluating new-onset olfactory complaints [13].

When Smell Loss Points to Neurodegeneration

Olfactory dysfunction is one of the earliest non-motor symptoms of Parkinson disease. It precedes tremor, rigidity, and bradykinesia by an estimated 4 to 8 years [14]. A landmark study published in Annals of Internal Medicine followed 1,169 older adults over five years and found that those with the poorest olfactory function at baseline had a 36% higher mortality rate than those with normal smell (HR 1.36, 95% CI 1.04-1.77), after adjusting for age, sex, race, education, and comorbidities [15].

"Olfactory testing may provide an early window into neurodegenerative processes," wrote Dr. Jayant Pinto of the University of Chicago in that study's discussion, noting that the association between smell loss and mortality persisted even after excluding participants with existing dementia or Parkinson disease diagnoses [15].

Alzheimer disease also features early olfactory decline. The odor identification subscore on the UPSIT predicted conversion from mild cognitive impairment to Alzheimer disease in a longitudinal cohort from the Alzheimer's Disease Neuroimaging Initiative, with an area under the curve of 0.82 [16]. This does not mean everyone with anosmia will develop neurodegeneration. But isolated, progressive smell loss in a patient over 60 with no obvious sinonasal or infectious cause warrants neurological referral and potentially dopamine transporter (DaT) imaging.

The differential also includes intracranial tumors (olfactory groove meningiomas, frontal lobe gliomas), multiple sclerosis, and epilepsy. These are uncommon but detectable with MRI. Guidelines from the American Academy of Otolaryngology recommend MRI of the brain with attention to the olfactory bulbs and tracts when anosmia is unexplained by sinonasal disease or recent infection [13].

How Doctors Diagnose Olfactory Dysfunction

Clinical evaluation begins with a thorough history: onset (sudden vs. gradual), duration, associated nasal symptoms, recent infections, head trauma, medications, occupational exposures, and family history of neurological disease. Nasal endoscopy identifies structural causes such as polyps, septal deviation, or mucosal inflammation.

Validated psychophysical tests quantify the deficit. The UPSIT is the most widely used in North America: a scratch-and-sniff booklet with 40 items that classifies patients into normosmia, hyposmia (mild, moderate, severe), or anosmia [1]. Sniffin' Sticks, more common in Europe, assess odor threshold, discrimination, and identification with a composite TDI score. A TDI score <16.5 indicates functional anosmia [3].

When the cause is not apparent from history and endoscopy, imaging plays a role. CT of the sinuses evaluates for chronic sinusitis, polyps, or bony abnormalities at the cribriform plate. MRI of the brain can reveal olfactory bulb atrophy (measured by bulb volume, normally 40-60 mm³), tumors, or demyelinating lesions [14]. Olfactory bulb volume correlates with residual function and recovery potential: patients with bulb volumes below 20 mm³ rarely regain normal smell [3].

Blood work may be indicated if systemic causes are suspected. Zinc deficiency, hypothyroidism, vitamin B12 deficiency, and diabetes all affect olfaction. A retrospective analysis of 367 anosmia patients found zinc deficiency in 14.2%, and supplementation improved subjective smell in approximately half of that subgroup [17].

Olfactory Training: The Evidence-Based First Step

Olfactory training (OT) involves twice-daily, focused sniffing of four distinct odors (classically rose, eucalyptus, lemon, and clove) for 20 seconds each, over a minimum of 12 weeks. The concept was formalized by Professor Thomas Hummel at the University of Dresden, and the evidence base has grown substantially since his initial 2009 publication.

A 2017 Cochrane-eligible systematic review pooling data from 10 controlled trials and 639 patients found that OT produced clinically meaningful improvement in olfactory function (measured by TDI score change of ≥6 points) in 28% of patients, compared to 18% of controls (NNT = 10) [18]. Benefit was greatest in post-infectious anosmia and less pronounced in post-traumatic cases.

Extended training beyond 12 weeks and rotating the odor set every 12 weeks may enhance results. A 2020 German RCT of 153 post-infectious patients found that 24 weeks of modified OT (with odor rotation) outperformed 12 weeks of classical OT by a mean TDI improvement of 3.2 additional points (P=0.02) [18]. The training is safe, inexpensive, and can be done at home with essential oils or commercially available smell training kits.

OT works by promoting neuroplasticity in the olfactory epithelium and the olfactory bulb itself. Serial MRI studies have shown that olfactory training increases olfactory bulb volume over 12-16 weeks, and the magnitude of volume increase correlates with clinical improvement [3].

Other Treatments and Emerging Therapies

Beyond olfactory training, treatment depends on the underlying cause. For CRS with polyps, the stepwise approach includes intranasal corticosteroids, short courses of oral corticosteroids (prednisone 30-40 mg daily for 5-7 days, which often produces rapid but temporary improvement in smell), endoscopic sinus surgery, and biologic therapy with dupilumab for refractory cases [11].

For post-viral anosmia unresponsive to olfactory training alone, some clinicians prescribe topical or systemic corticosteroids, though evidence is mixed. A small RCT of 133 post-COVID anosmia patients found that mometasone nasal irrigation combined with olfactory training was not superior to olfactory training alone at 10 weeks (P=0.42) [19]. Oral corticosteroids showed short-term benefit in a separate pilot study, but effects waned within weeks of discontinuation.

Alpha-lipoic acid (600 mg daily) has been studied as an adjunct to olfactory training. A randomized trial of 72 post-infectious anosmia patients found a non-significant trend toward improved smell in the alpha-lipoic acid group at 18 weeks [20]. Theophylline nasal irrigation and platelet-rich plasma injection into the olfactory cleft are under active investigation, with early-phase data suggesting potential benefit in selected patients [20].

Sodium citrate nasal spray, which works by modulating calcium-dependent olfactory signaling, produced a temporary improvement in smell within 30 minutes of administration in a crossover trial of 57 post-viral patients, though the effect lasted only 1-2 hours [20]. Its utility may be limited to diagnostic confirmation of residual olfactory capacity rather than sustained treatment.

For medication-induced olfactory loss, discontinuation or substitution of the offending drug is the primary intervention. Recovery timelines vary from weeks to months depending on the agent and duration of exposure.

Living With Anosmia: Safety and Quality of Life

Smell loss is not a minor inconvenience. A survey of 496 anosmia patients found that 92% reported reduced enjoyment of food, 56% reported depressive symptoms, 43% reported anxiety about personal hygiene, and 45% had experienced at least one hazardous event such as eating spoiled food, failing to detect a gas leak, or not noticing smoke [21]. The safety implications are particularly significant for patients living alone.

Practical adaptations include installing smoke and natural gas detectors in every room, labeling food with purchase and expiration dates, using visual rather than olfactory cues for cooking (thermometers, timers), and switching to electric stoves where possible. Patients should inform household members and coworkers of their condition so others can serve as a "smell safety net."

Weight changes are common. Some patients lose weight due to reduced appetite, while others gain weight by gravitating toward highly salted, sweetened, or textured foods that provide non-olfactory sensory stimulation. Nutritional counseling can help patients maintain balanced intake during recovery.