What could be causing a patient's persistent halitosis (bad breath) that smells like onion?

Persistent Onion-Like Breath: Diagnostic Approach and Management

Breath that continuously smells like onion most likely represents a qualitative olfactory disorder (phantosmia or parosmia) rather than true halitosis, and requires formal psychophysical olfactory testing with nasal endoscopy to differentiate between peripheral and central causes. 1, 2

Initial Diagnostic Evaluation

The evaluation must begin with clarifying whether this is:

• True halitosis (others can smell it) versus pseudo-halitosis/halitophobia (only the patient perceives it) 3
• Phantosmia (smelling odors that aren't present) versus parosmia (distorted perception of actual odors) 2

Key History Elements

• Document the onset timing and any temporal relationship to recent viral infections, particularly COVID-19, as post-viral olfactory dysfunction accounts for 20-25% of specialist presentations and can manifest as qualitative disorders 1, 4
• Ask specifically if others can smell the onion odor—if no one else perceives it, this strongly suggests a qualitative olfactory disorder rather than genuine halitosis 3, 5
• Inquire about recent upper respiratory infections, head trauma, or new medications, as these are common triggers for olfactory distortions 1, 2

Mandatory Objective Testing

• Perform rigid nasal endoscopy to identify sinonasal inflammatory disease, polyps, or masses that could cause conductive olfactory loss 4, 1
• Conduct validated psychophysical testing using UPSIT or Sniffin'Sticks (4-25 minutes), as patients cannot accurately assess their own olfactory function severity 4, 1
• The combination of endoscopy and objective testing differentiates between conductive (sinonasal obstruction), sensorineural (olfactory epithelium/nerve damage), and central (brain) causes 4, 2

When True Halitosis is Confirmed

If others can genuinely smell the onion-like odor, consider:

Oral Causes (80-90% of cases)

• Examine for poor oral hygiene, tongue coating, periodontal disease, and dental calculus, as these account for the vast majority of genuine halitosis 5, 6, 7
• Look specifically for deep periodontal pockets, unclean dentures, faulty restorations, and food impaction 5, 8
• Note that foetor ex ore correlates strongly with calculus, plaque, and infrequent dental visits 6

Extraoral Causes (10-20% of cases)

• In patients with chronic kidney disease on dialysis, elevated blood urea nitrogen leads to high salivary urea that converts to ammonia, causing characteristic bad breath in one-third of hemodialysis patients 4
• Consider other systemic diseases, medications, and metabolic disorders if oral examination is unremarkable 8, 7

When Phantosmia/Parosmia is Suspected

Advanced Imaging Indications

• Order MRI orbits/face/neck with and without contrast only when psychophysical testing severity doesn't correlate with endoscopic findings, raising suspicion for occult sinonasal or skull base tumors 1, 2
• Consider imaging for persistent or progressive symptoms without clear inflammatory cause, or when neurological signs suggest temporal lobe pathology or neurodegenerative disease 2
• Avoid CT head or vascular imaging, as these have no established role in olfactory evaluation 2

Common Etiologies to Rule Out

• Post-viral olfactory dysfunction (including COVID-19) is the most common cause of qualitative disorders 4, 1
• Sinonasal inflammatory disease causing conduction loss 4, 2
• Neurodegenerative disorders (Alzheimer's, Parkinson's, Lewy body dementia) causing central phantosmia 2
• Tumors affecting the cribriform plate (meningiomas, esthesioneuroblastoma) 2

Treatment Algorithm

For Post-Viral Qualitative Disorders

• Initiate olfactory training immediately and continue for minimum 3-6 months, referring to validated resources like www.fifthsense.org.uk 1, 2
• Recovery occurs in 44-73% of COVID-19 patients within the first month, with mean improvement time of 7.2 days, though some develop persistent dysfunction 4, 1

For Genuine Halitosis

• Implement tongue brushing, scaling and root planing, and improved oral hygiene as first-line treatment 5, 8
• Prescribe antimicrobial oral rinses for non-responders 8
• Address any underlying periodontal disease or dental pathology 6, 7

For Pseudo-Halitosis/Halitophobia

• Refer to psychological specialist, as these conditions are outside the treatment realm of dental practitioners and require psychiatric management 3
• Avoid repeated dental treatments that will not address the underlying psychological condition 3

Follow-Up Protocol

• Re-evaluate at 1 month, 3 months, and 6 months after initiating treatment 1
• Repeat psychophysical testing at follow-up visits to objectively document changes 1
• Refer to otolaryngologist or specialized smell/taste clinic if no improvement after 3-6 months of olfactory training 1

Critical Pitfall to Avoid

The most common error is assuming all patients complaining of persistent bad breath have genuine halitosis requiring dental treatment, when many actually have qualitative olfactory disorders (phantosmia/parosmia) that require completely different evaluation and management 1, 2, 3. Always clarify whether others can perceive the odor before proceeding with extensive dental workup.