Olfactory Dysfunction: Anosmia, Hyposmia, Parosmia — Diagnosis and Treatment

Olfactory receptor neurons (ORNs) lie in the olfactory epithelium at the upper nasal vault (~1-2 cm²). Their axons traverse the cribriform plate to the olfactory bulb. Bulb → olfactory tract → primary olfactory cortex (piriform, entorhinal) → secondary processing (orbitofrontal cortex, amygdala, hippocampus). This pathway also accounts for much of the "taste" experience — retronasal smell.

Key ORN feature: dual compartment — olfactory mucosa is the only direct link between the peripheral nervous system and the CNS. This is a potential CNS entry route for viruses and toxins. It is also one of the few neural tissues with stem-cell-driven regenerative capacity — the basis of rehabilitation.

Terminology: anosmia — absence of smell (TDI <16). Hyposmia — reduced (16-30). Normosmia — normal (>30). Parosmia — distorted perception of odours, often unpleasant (coffee smells like burning plastic). Phantosmia — perceiving odour without stimulus (usually unpleasant). Specific anosmia — insensitivity to a single group (e.g. musk).

Categories: 1) Conductive — odorant cannot reach the olfactory epithelium (nasal obstruction, polyps, septal deviation, mucus). 2) Sensorineural — ORN or central processing dysfunction (post-infectious, post-traumatic, neurodegenerative, congenital, toxic). 3) Mixed.

Prevalence: up to 20% of adults have some olfactory reduction; severe anosmia 1-3%. Rises with age — 25-30% of those 65+ have clinically significant loss. No sex difference. COVID-19 caused a dramatic transient global rise. We expand on the clinical framework in our general ENT services.

Sinonasal disease (most common — 60-70%): chronic rhinosinusitis (especially with nasal polyps — CRSwNP), allergic rhinitis, acute viral rhinitis, septal deviation, sinonasal tumour. Often treatable — functional endoscopic sinus surgery (FESS) + intranasal steroid frequently gives marked improvement.

Post-infectious olfactory dysfunction (PIOD): anosmia/hyposmia for weeks to months after upper respiratory infection. Recognised before COVID-19 (rhinovirus, coronavirus, parainfluenza); dramatic rise after COVID-19. Mechanism: olfactory epithelium support cell involvement, local inflammation, ORN apoptosis. Recovery: 75-90% partial or full at 6-12 months, 10-25% persistent.

COVID-19 specific: acute anosmia (often an early sign), parosmia during recovery (coffee, meat, onion, garlic randomly perceived as foul). Mechanism: damage to olfactory sustentacular (support) cells + local inflammation; "miswiring" during ORN regeneration causes parosmia.

Post-traumatic: head trauma (especially frontal/occipital — shearing forces tear olfactory nerves at the cribriform plate). Partial improvement first 6 months; less in next 12; plateau at 2 years. Full recovery is rare.

Neurodegenerative: in Parkinson's disease, hyposmia/anosmia often precedes motor symptoms by years — an early sign. Similar in Alzheimer's. Unexplained persistent anosmia at young age may warrant neurology referral.

Other: intracranial lesion (frontal lobe tumour, meningioma — especially near planum sphenoidale), iatrogenic (cribriform plate injury during FESS, septal surgery), toxin (formaldehyde, solvents, heavy metals), drugs (some antibiotics, antihistamines), endocrine (hypothyroidism, Cushing, diabetes — subclinical), congenital (Kallmann syndrome — anosmia + hypogonadotropic hypogonadism).

Structured history: timing, trigger (infection, trauma, surgery, toxin), type (loss/distortion/phantom), relation to a stimulus (one group vs all), associated nasal symptoms, systemic (neurologic, endocrine, malignancy), medication and substance use, occupational exposure. Functional impact — food enjoyment, safety (gas, smoke, spoiled food), work and social life.

Examination: anterior rhinoscopy + nasal endoscopy (rigid or flexible; middle meatus, upper cavity, polyps, mucus flow, septum, choanae). Oral cavity and pharynx. Cranial nerves.

Validated olfactory testing: subjective "I can't smell" is insufficient — objective testing essential. Sniffin' Sticks (Europe) measures three dimensions: Threshold (T) — n-butanol increasing concentrations; Discrimination (D) — 16 triplets with one different odour; Identification (I) — 16 common odours recognised. TDI total score; <16 anosmia, 16-30 hyposmia, >30 normosmia. UPSIT (US) — 40-odour scratch-and-sniff. Brief Smell Identification Test (BSIT) — rapid 12-odour version.

Imaging: paranasal sinus CT — for sinonasal aetiology; mucosal thickening, polyps, bony anomaly, olfactory cleft. Brain MRI — anosmia with neurological signs, unexplained persistent anosmia, intracranial lesion suspicion — particularly olfactory bulb volume (bulbar atrophy is prognostic).

Ancillary tests — by indication: allergy tests (CRSwNP eosinophilia, IgE), thyroid, glucose/HbA1c, vitamin (B12, D), drug/toxin screening (occupational), brain MRI + IGF/LH (Kallmann suspicion), early neurodegenerative assessment (motor + cognitive — neurology).

Taste assessment: smell and taste are often confused. Pure taste (sweet, salty, sour, bitter, umami) preserved, with "tasteless" actually representing retronasal smell — reflecting olfactory dysfunction. Specific taste disturbance (cranial nerves 7, 9, 10) is rare. For the related clinical reference, see sinusitis (FESS) page.

Sinonasal aetiology: intranasal corticosteroids (mometasone, fluticasone — daily; often months of therapy), saline irrigation (high-volume — Neti, NeilMed), budesonide irrigation in selected cases. If allergic rhinitis: antihistamine, consider immunotherapy. In CRSwNP unresponsive to medical therapy — functional endoscopic sinus surgery (FESS) with polyp removal, sinus ventilation, careful opening of the olfactory cleft. Biologics (dupilumab, omalizumab, mepolizumab) provide notable olfactory improvement in eosinophilic CRSwNP — robust evidence.

Post-infectious / post-COVID olfactory training: evidence-based first-line. Method: 4 essential odours (rose, lemon, clove, eucalyptus — Hummel protocol) in separate vials, twice daily (morning and evening), sniff each for 15-20 seconds with cognitive engagement (visual/memory cueing). 12-24+ weeks (better outcomes beyond 24 weeks; some studies extend to 48).

Olfactory training outcomes: 30-60% achieve clinically significant gain (TDI +5 or category change). Early start, motivated adherence, control of concurrent inflammation improve results. Effective also for parosmia (cortical "re-learning").

Adjuncts: omega-3 (1-2 g/day — limited evidence, low risk), intranasal vitamin A (5000 IU for 8 weeks — some positive trials), oral steroid 1-2 weeks + topical thereafter — especially in early post-infectious if not contraindicated. Theophylline and alpha-lipoic acid used but evidence weak.

Post-traumatic: spontaneous recovery is low; olfactory training can be tried but response is less. Anosmia after cranial surgery is usually permanent. Patient expectation management is important.

Neurodegenerative: olfactory rehabilitation has limited gains; treatment of the primary disease (Parkinson — dopaminergic; Alzheimer — cholinergic) does not directly restore smell.

Coping with life: gas detector, smoke detector (fire safety), food expiry tracking, body hygiene routine (including perfume), taste-enhancing strategies (umami-rich foods, spices, temperature contrast). Psychological support — anosmia significantly affects quality of life and increases depression risk.

Parosmia: existing odours are perceived consciously but "wrong" — distorted, often unpleasant. Post-COVID it has been extra common; 30-50% of patients transition from anosmia to parosmia (typically 2-6 months later — paradoxically during "recovery").

Classic triggers: coffee, meat, onion, garlic, eggs, peppers, perfume, toothpaste, body odours — described as burnt, rotting, chemical, sewage, rubber. Inability to eat and weight loss are major problems. Social isolation (declining meal invitations).

Mechanism: during olfactory epithelial regeneration, "miswiring" — regenerating ORNs target wrong glomeruli in the olfactory bulb; the odour-to-cortex map is disrupted.

Treatment: olfactory training works in parosmia too (perhaps even more — re-learning process). Not avoidance but gradual re-introduction of triggers. Visual cueing (looking at a picture of what you smell + saying its name + recall as concept) helps.

Nutrition strategies: temporary elimination of triggers (coffee, onion, garlic, meat — short term), bland tastes (oats, rice, fish, legumes), cold foods (heat releases more odour — cold less triggering), neutral aromas (mushroom, tofu, flatbread). Trigger identification diary.

Duration: parosmia resolves or attenuates in most cases within 6-24 months. 10-20% persist beyond 2 years. Prolonged duration increases depressive symptom risk — psychology and psychiatry support if needed.

Counselling: patients may feel they are "going crazy" because of the experience; validation is important. Process information, evidence-based strategies, realistic expectation management are key parts of care. Related reading: our Istanbul ENT services.