Olfactory groove meningioma is the most commonly missed intracranial cause of anosmia — it grows silently, reaching large sizes before symptoms beyond anosmia occur, and is misattributed to ageing or sinusitis for years. Foster Kennedy syndrome (anosmia + ipsilateral optic atrophy + contralateral papilloedema) is the classic triad. Any unexplained bilateral anosmia + any neurological symptom = MRI brain. The anosmia-PD prodromal relationship is one of the most evidence-based early markers in neurodegenerative disease — 90% of PD patients have confirmed olfactory loss at diagnosis, predating motor symptoms by years. The combination of anosmia + REM sleep behaviour disorder + constipation has >80% predictive value for PD development within 15 years.

Post-COVID anosmia mechanism: SARS-CoV-2 preferentially targets sustentacular (support) cells of the olfactory epithelium via ACE2 receptors, causing massive local inflammation that disrupts olfactory receptor neuron function. Crucially, olfactory receptor neurons themselves are relatively spared (explaining the high recovery rate). Recovery occurs through neurogenesis from basal stem cells — facilitated by olfactory training. Parosmia (distorted smell — familiar things smelling like burning, chemicals, or faeces) during recovery is a positive prognostic sign indicating nerve regeneration and synapse remodelling.

Sniffin' Sticks is the most practical validated olfactory test for primary care — it takes 10 minutes, is validated against electrophysiological measures, and documents objective impairment essential for monitoring and medicolegal purposes. For practices without formal testing: a gross 3-item screen (coffee, vanilla, eucalyptus) takes 2 minutes. Inability to identify any of the three = significant anosmia. CT sinuses is the first-line imaging for conductive anosmia — it shows polyp extent, sinus opacification, bony erosion (malignancy), and Lund-Mackay score (influences surgical decision-making). MRI is reserved for soft tissue characterisation, olfactory bulb visualisation (Kallmann), and intracranial pathology.

The unilateral vs bilateral distinction is the most useful first branch point in anosmia assessment. Bilateral anosmia is more likely to be systemic (post-viral, nutritional, neurodegenerative, bilateral polyps). Unilateral anosmia is almost always structural — tumour, polyp affecting one side disproportionately, or trauma. Unilateral anosmia mandates imaging (CT sinuses at minimum) and 2WW ENT referral because of the significant probability of sinonasal malignancy or intracranial structural pathology. There is essentially no common benign cause of unilateral anosmia — unlike bilateral anosmia where post-viral is the most common cause.

Dupilumab (Dupixent) for severe CRS with nasal polyps (NICE TA655, 2021) is one of the most effective treatments for polyp-related anosmia — it is an IL-4/IL-13 receptor antagonist (same biologic as for severe atopic eczema) that dramatically reduces polyp size and restores olfaction in patients with severe CRSwNP uncontrolled by steroids or surgery. Eligibility: severe bilateral CRSwNP, inadequate response to systemic steroids, previous FESS or contraindicated. 300 mg SC every 2 weeks, specialist-initiated. Olfactory improvement is measurable within 4 weeks of initiation. GPs should refer appropriate refractory polyp patients to ENT for dupilumab assessment.

Olfactory training (OT) mechanism — repeated exposure to odorants activates residual olfactory receptor neurons and promotes axonal regeneration via neuroplasticity, helping re-establish olfactory neuron-to-bulb synaptic connections. The conscious attentional focus is part of what drives neuroplasticity — passive sniffing is less effective than actively trying to recall what the smell used to be like. High-dose training (switching to a second set of odorants at 3 months) provides additional benefit by preventing olfactory adaptation and stimulating different receptor populations. NNT approximately 4 for measurable improvement at 12 weeks. Fifth Sense charity (fifthsense.org.uk) provides training kits and patient resources.

Alpha lipoic acid (ALA) 600 mg OD is a safe, readily available antioxidant supplement with modest but genuine evidence in post-viral anosmia — it reduces oxidative stress in the olfactory epithelium and may support neuronal recovery. While not a primary treatment, it can be recommended as a safe adjunct to olfactory training in patients who are motivated to try every available option. ALA is widely available OTC, has an excellent safety profile, and costs approximately £15/month. Parosmia education is crucial — the single biggest source of distress in patients with post-COVID anosmia is misinterpreting parosmia as their condition worsening, when it is actually the most positive prognostic sign (indicating active nerve regeneration). The reassurance message — 'the distortion means your nerves are growing back and forming new connections' — is genuinely therapeutic.

Food safety education for anosmic patients is one of the most clinically important aspects of management — anosmia represents a genuine public health safety risk because smell is the primary early warning system for fire (smoke), gas leaks, CO, and food spoilage. Fitting a CO detector is especially important because CO poisoning itself causes anosmia (and the anosmic patient cannot smell the gas that killed their sense of smell in the first place). The Equality Act 2010 implication is worth documenting — persistent anosmia (lasting >12 months, affecting daily activities including cooking, eating, work, and safety) may meet the disability definition, entitling the patient to reasonable adjustments at work. A GP letter documenting the condition and its functional impact supports the patient's claim.

The 3-month review point for post-viral anosmia is the clinical decision gate — patients with consistent olfactory training who show no improvement at 3 months (unchanged Sniffin' Sticks score) should be referred to ENT rhinology for further investigation (CT sinuses, PRP assessment, theophylline consideration). Setting realistic long-term expectations is important: approximately 10–20% of post-COVID anosmia patients will have persistent measurable impairment at 12 months. These patients need long-term support: food safety counselling, nutritional monitoring, psychological support, and signposting to Fifth Sense charity — rather than repeated investigation.