What is the workup for a patient experiencing phantosmia (smelling smoke) without an apparent source?

Workup for Phantosmia (Smelling Smoke)

Begin with rigid nasal endoscopy and standardized psychophysical olfactory testing (UPSIT or Sniffin'Sticks), reserving MRI orbits/face/neck for cases with discordant findings, progressive symptoms, or neurological signs. 1

Initial Clinical Evaluation

History Taking

• Onset and temporal pattern: Document whether phantosmia is persistent, episodic, or associated with headaches (which may suggest Lewy body dementia or other neurodegenerative disease) 2
• Unilateral vs bilateral: Unilateral phantosmia may suggest peripheral pathology amenable to local treatment 3
• Associated symptoms: Screen for neurological signs (cognitive decline, parkinsonism, visual hallucinations suggesting Lewy body dementia), psychiatric symptoms (schizophrenia), or other olfactory dysfunction 4, 2
• Medication review: Identify potential causative agents, though medications more commonly cause dysgeusia than phantosmia 5
• Recent infections: COVID-19 and other upper respiratory infections can cause qualitative olfactory disorders 6
• Trauma history: Head injury is a recognized cause of phantosmia 4

Physical Examination

• Rigid nasal endoscopy: Essential to differentiate conduction loss from sinonasal obstruction, inflammatory disease (rhinosinusitis, nasal polyps), or tumors 6, 1
• Standardized olfactory testing: Use validated instruments like UPSIT or Sniffin'Sticks to objectively characterize olfactory function and distinguish phantosmia from parosmia 6, 1, 4

Indications for Imaging

When to Order MRI

MRI orbits/face/neck with and without contrast is indicated when: 1

• Discordance exists between subjective symptoms and endoscopic findings
• Progressive or persistent symptoms without clear inflammatory or obstructive cause
• Associated neurological signs suggesting CNS pathology (cognitive changes, focal deficits, seizures)
• Endoscopy raises suspicion for sinonasal or skull base tumors not fully visualized

Avoid Unnecessary Imaging

• Do not routinely order imaging for all phantosmia cases, as most do not require advanced imaging unless clinical examination warrants it 1
• CT head or vascular imaging has no role in olfactory evaluation and should be avoided 1
• CT maxillofacial is reserved specifically for evaluating fractures, paranasal sinus inflammatory disease, and bony anatomy when clinically indicated 1

Differential Diagnosis to Consider

Peripheral Causes

• Sinonasal inflammatory disease: Rhinosinusitis and nasal polyposis causing conduction loss 6, 1
• Post-infectious: Upper respiratory infections including COVID-19 6, 4
• Tumors: Squamous cell carcinomas, esthesioneuroblastoma, meningiomas affecting the cribriform plate 1
• Inflammatory lesions: Sarcoidosis, granulomatosis with polyangiitis 1

Central Causes

• Neurodegenerative disorders: Alzheimer disease, Parkinson disease, Lewy body dementia (phantosmia may precede typical hallucinations) 1, 2
• Temporal lobe pathology: Seizures, tumors, or other structural lesions 1
• Psychiatric conditions: Schizophrenia and other primary psychiatric disorders 4, 7

Evidence for Central Processing

Research suggests phantosmia may relate to central processing problems rather than purely peripheral dysfunction, as demonstrated by delayed return of olfactory ability after topical cocainization and contralateral effects 8

Treatment Considerations

When Peripheral Etiology is Confirmed

• For unilateral peripheral phantosmia refractory to conservative management, surgical excision of olfactory epithelium has been reported successful with preservation of olfactory function 3
• Topical cocainization is not effective for long-term treatment of phantosmia 8

When Central Etiology is Suspected

• Functional MRI can objectively demonstrate phantom activation in sensory-specific brain regions 7
• Antipsychotic medications (thioridazine, haloperidol) have shown efficacy in primary phantageusia/phantosmia cases, with objective fMRI confirmation of reduced activation 7

Post-Infectious Cases

• Olfactory training should be offered for post-infectious olfactory dysfunction, including COVID-19 cases, due to its simplicity and safety 6
• Modified olfactory training has demonstrated effectiveness for COVID-19-induced parosmia 4
• Evidence is insufficient to recommend intranasal or systemic corticosteroids for post-COVID olfactory dysfunction 6

Critical Pitfalls to Avoid

• Do not assume all phantosmia requires imaging—clinical examination should guide this decision 1
• Do not confuse phantosmia with parosmia—the former occurs without external stimuli, while the latter is distortion of actual odors 4
• Do not overlook neurodegenerative disease—persistent phantosmia, especially with headaches or other neurological symptoms, warrants evaluation for Lewy body dementia and other central pathology 2
• Do not order CT head for olfactory evaluation—it has no established role 1