Phantosmia Workup
Most patients with phantosmia (olfactory hallucinations) do not require imaging unless history or physical examination reveals concerning features such as unilateral symptoms, neurological deficits, or symptoms refractory to conservative management. 1
Initial Clinical Assessment
History Taking
- Determine laterality: Unilateral (unirhinal) vs. bilateral (birhinal) phantosmia, as these represent distinct clinical entities with different etiologies 2
- Characterize the odor: Specific, unpleasant burning smells suggest migraine-associated phantosmia 3
- Timing and triggers: Note if phantosmia can be initiated by coughing, sneezing, laughing, or vigorous nasal breathing, or inhibited by Valsalva maneuvers or sleep—these features suggest cyclic unirhinal phantosmia 4, 2
- Associated symptoms: Headaches (particularly migraine), taste disturbances, cognitive changes, or parkinsonian features 5, 3
- Temporal relationship: Recent viral infection, head trauma, or upper respiratory illness 1
- Duration: Phantosmia lasting 5-60 minutes occurring with headache onset suggests migraine aura 3
Physical Examination
- Thorough nasal examination: Assess for sinonasal pathology or obstruction 1
- Complete neurological examination: Screen for cognitive impairment, parkinsonism, or other neurodegenerative signs 1, 5
- Olfactory testing: Assess for concurrent hyposmia—present in birhinal phantosmia with associated symptomology (BPAS) but typically absent in cyclic unirhinal phantosmia (CUP) 2
Diagnostic Testing
When Imaging is NOT Required
- Clear temporal relationship to viral infection with no neurological red flags 1
- Migraine-associated phantosmia with typical features (brief duration, burning smell, occurs with headache) 3
- Symptoms responsive to conservative management 1
When to Order Imaging
MRI brain with olfactory protocol is indicated when: 1
- History or examination suggests central nervous system pathology
- Unilateral symptoms without clear benign etiology
- Progressive neurological symptoms
- Symptoms persist beyond 6 months despite appropriate therapy
- Concern for tumors affecting the cribriform plate (meningiomas, esthesioneuroblastoma, squamous cell carcinoma) 1
MRI specifications should include: 1
- T2-weighted sequences
- Fluid-attenuated inversion recovery (FLAIR)
- Three-dimensional isotropic heavily T2-weighted sequences for cisternal segments
- Contrast-enhanced T1-weighted images for intradural and foraminal segments
Additional Testing
- EEG: Consider in unirhinal phantosmia, as ipsilateral sharp waves are frequently present in cyclic unirhinal phantosmia 2
- CT maxillofacial: Only if sinonasal inflammatory disease or bony pathology is suspected clinically 1
- Formal olfactory testing (UPSIT, Sniffin' Sticks): To objectively characterize degree of olfactory dysfunction 1, 2
Common Pitfalls
- Do not routinely order CT head—there is no evidence supporting its use in olfactory nerve evaluation 1
- Do not dismiss phantosmia as purely psychiatric—it may herald neurodegenerative disease like Lewy body dementia, particularly when associated with headaches or cognitive changes 5
- Do not overlook migraine as a cause—phantosmias occur in approximately 0.66% of headache clinic patients and respond to migraine prophylaxis 3
- Recognize that unirhinal phantosmia produces bihemispheric brain activation regardless of which nostril is affected, so laterality of symptoms does not predict laterality of pathology 4
Treatment Considerations Based on Etiology
- Migraine-associated: Initiate standard migraine prophylaxis—phantosmias typically diminish or disappear with headache treatment 3
- Cyclic unirhinal phantosmia: Consider GABAergic agents (e.g., thioridazine has been shown to inhibit brain activation in phantosmia) 4, 2
- Post-viral: Reassurance and observation, as many cases resolve spontaneously 1
- Refractory peripheral cases: Surgical excision of olfactory epithelium may be considered as an alternative to olfactory bulbectomy, with potential preservation of olfactory function 6