Olfactory groove meningioma is an important intracranial cause of anosmia — it can grow 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. Anosmia with new neurological symptoms or signs needs urgent same-day assessment (999/ED for new focal signs, seizure, meningism or reduced consciousness). NICE NG127: consider neuroimaging for adults with unexplained loss of smell lasting more than 3 months; do not routinely refer adults with loss of smell and normal neuroimaging. Hyposmia is common in Parkinson's disease but is non-specific — it also occurs with ageing, rhinitis, prior viral infection, smoking and Alzheimer's disease. NICE advises that objective smell testing should not be used for routine differential diagnosis of parkinsonian syndromes, except in research.

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) can occur during recovery, but it does not prove recovery and may be persistent or severely disabling.

Formal tests such as Sniffin' Sticks or UPSIT belong in specialist smell or research services; they are not a routine GP investigation. 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. NICE NG127: consider neuroimaging for unexplained loss lasting more than 3 months; do not routinely refer adults with normal neuroimaging. Do not routinely request MRI for all unexplained cases.

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 symptoms are more concerning when accompanied by unilateral nasal obstruction, epistaxis or blood-stained discharge, crusting, facial pain or swelling, a visible mass or polyp, orbital symptoms, cranial-nerve signs, or persistent progressive symptoms — these need ENT assessment through the local rhinology/head-and-neck pathway. An isolated unilateral reduction without nasal or neurological findings is not automatically malignancy, but should be reviewed if persistent or progressive.

Dupilumab (Dupixent) for severe CRS with nasal polyps (NICE TA1134, 2026) 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. NICE TA1134 eligibility (adults): add-on to intranasal corticosteroids, not controlled by systemic corticosteroids or sinus surgery, at least 1 previous sinus surgery, SNOT-22 score of at least 50, and only if the company provides it according to the commercial arrangement. 300 mg SC every 2 weeks, specialist-initiated — this is specialist treatment, not a primary-care option. 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. Olfactory training is reasonable and low risk; the exact odours and switching interval are not essential. Continue for at least 12 weeks, and longer if there is gradual improvement. Fifth Sense charity (fifthsense.org.uk) provides training kits and patient resources.

Evidence for supplements and pharmacological treatments (including alpha-lipoic acid, PEA/luteolin and zinc) is limited or insufficient, so they should not be offered routinely; PRP and theophylline remain experimental/specialist options. Parosmia education matters — it can occur during recovery and may improve over time, but it is not a reliable marker of recovery and can be persistent or severely disabling. Monitor nutrition, mood and quality of life, and refer to a smell clinic or ENT if it persists or affects nutrition.

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 is odourless and a patient with smell loss may also miss smoke or gas leaks. 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 — for unexplained loss lasting more than 3 months, consider neuroimaging or specialist ENT/neurology assessment according to local pathway (NICE NG127); do not routinely refer adults with normal neuroimaging. Setting realistic long-term expectations is important: a minority of post-COVID anosmia patients will have persistent impairment. These patients need long-term support: food safety counselling, nutritional monitoring, psychological support, and signposting to Fifth Sense charity — rather than repeated investigation.