What causes phantosmia, or smelling things that are not present?

Phantosmia: Causes and Clinical Approach

Phantosmia—smelling odors that are not present—most commonly results from post-viral olfactory damage, head trauma, sinonasal disease, or neurodegenerative conditions, and requires targeted history to distinguish it from parosmia (distorted smell with stimulus present) since the two have different etiologies and management. 1, 2

Key Etiologic Categories

Post-Viral and Infectious Causes

• Viral upper respiratory infections are the most common cause, accounting for 20-25% of specialist clinic presentations with olfactory disturbances 3
• COVID-19 has emerged as a significant cause, with 59-86% of patients experiencing chemosensory dysfunction, though most present with anosmia rather than phantosmia 3
• Post-viral olfactory loss can manifest as phantosmia when incomplete regeneration of olfactory neurons occurs 1

Structural and Traumatic Causes

• Head trauma is a leading cause, as the olfactory nerve is the most commonly disrupted cranial nerve in trauma 4
• Sinonasal inflammatory disease including chronic rhinosinusitis, nasal polyposis, and granulomatous conditions (sarcoidosis, granulomatosis with polyangiitis) 4
• Tumors affecting the cribriform plate: squamous cell carcinomas, meningiomas, esthesioneuroblastomas 4
• Congenital conditions such as cephaloceles and Kallmann syndrome 4

Neurological and Psychiatric Causes

• Temporal lobe dysfunction including seizure disorders can produce olfactory hallucinations 4
• Neurodegenerative diseases: Parkinson disease, Alzheimer disease, and Lewy body dementia may present with phantosmia as an early feature 4, 5
• Psychiatric disorders including schizophrenia can manifest with olfactory hallucinations 6

Other Causes

• Toxic chemical exposures and certain medications 6
• Surgical interventions affecting the olfactory pathway 6

Pathophysiology

The mechanism likely involves either abnormal peripheral signals from damaged olfactory neurons or central processing dysfunction 1, 7:

• Incomplete characterization of odorants due to decreased functioning olfactory neurons 1
• Abnormal signals from primary olfactory neurons or peripheral trigeminal nerves that "trigger" central processing 1
• Central processing problems, as evidenced by delayed return of olfactory function after topical anesthesia and contralateral effects 7

Critical Diagnostic Features to Elicit

History Elements

• Unilateral versus bilateral: Ipsilateral loss of olfactory sensitivity often accompanies phantosmia 1
• Timing: Onset in relation to viral illness, trauma, or neurological symptoms 6, 5
• Associated symptoms: Headaches (may indicate temporal lobe pathology or neurodegenerative disease), cognitive changes, movement disorders 5
• Quality of phantom smell: Foul versus pleasant odors; consistency of the perceived smell 5
• Triggers: Presence or absence of actual odorants (distinguishes from parosmia) 2

Physical Examination

• Thorough nasal endoscopy to identify sinonasal pathology 4
• Complete neurological examination focusing on cranial nerves and signs of neurodegenerative disease 4
• Objective olfactory testing (e.g., UPSIT) to characterize baseline function 4

Imaging Recommendations

Most patients with phantosmia do NOT require imaging unless history or physical examination suggests structural or neurological pathology 4:

When to Image

• MRI orbits, face, and neck (NOT MRI head) is the mainstay for directly imaging the olfactory apparatus, covering the olfactory epithelium, neurons, bulbs at the cribriform plate, and olfactory pathways 4
• CT maxillofacial for suspected sinonasal inflammatory disease, fractures, or bony abnormalities 4
• Contrast-enhanced CT for suspected granulomatous or neoplastic disease 4

When NOT to Image

• CT head, CTA head, MRA head, and FDG-PET/CT have no established role in routine phantosmia evaluation 4
• Functional imaging remains investigative 4

Management Approach

Conservative Management

• Reassurance with observation is appropriate for many cases, as spontaneous resolution occurs frequently 1
• Olfactory training has shown benefit, particularly for post-viral cases 6

Medical Interventions

• Topical medications to the olfactory epithelium (though topical cocaine showed no long-term benefit in one study) 7
• Systemic medications depending on underlying etiology 1

Invasive Options (Rarely Indicated)

• Topical anesthesia to parts of the nose for symptomatic relief 1
• Endoscopic transnasal excision of olfactory neurons in refractory cases, with potential for olfactory recovery post-operatively 1

Common Pitfalls

• Confusing phantosmia with parosmia: Parosmia requires an odorant stimulus present; phantosmia does not 1, 2
• Underestimating incidence: Qualitative olfactory dysfunction is often underreported and requires directed questioning 2
• Missing neurodegenerative disease: Phantosmia may be an early harbinger of Lewy body dementia or Parkinson disease 5
• Ordering inappropriate imaging: MRI head rather than MRI orbits/face/neck, or imaging when clinical features don't warrant it 4
• Assuming all chemosensory complaints are olfactory: True taste dysfunction (mediated by CN VII and IX) is rare; most "taste loss" reflects retronasal olfactory dysfunction 3