Anosmia

"Anosmia - absence of all olfactory function."

• Cummings Otolaryngology Head and Neck Surgery, Table 36.1

Examples of Anosmia:

"Unilateral anosmia can sometimes be demonstrated... if unilateral, usually is not recognized by the patient."

• Adams and Victor's Principles of Neurology, 12th Ed.

Parosmia

"Parosmia - qualitative dysfunction in the presence of an odorant (i.e., distorted perception of an odor stimulus)."

• Cummings Otolaryngology Head and Neck Surgery, Table 36.1

"These terms refer to distortions of odor perception where an odor is present... parosmia is in the nature of an olfactory illusion."

• Adams and Victor's Principles of Neurology, 12th Ed.

Examples of Parosmia:

"Often mistaking pleasant or neutral odors for unpleasant ones."

• KJ Lee's Essential Otolaryngology

"For instance, warm cookies from the oven - which smell sweet and delicious to most people - might smell unpleasant and rotten to people with parosmia."

• Cleveland Clinic

Causes of Parosmia:

• Post-infectious (most common today): COVID-19, upper respiratory infections - parosmia typically starts 1-3 months after the infection, often as olfaction is beginning to recover
• Head injury with partial damage to olfactory bulbs
• Psychiatric conditions: Severe depression and psychosis - patients may report all food smells unbearable (termed cacosmia), often accompanied by taste distortion (cacogeusia)
• Nasal sinus infections and chronic rhinosinusitis
• Partial nerve recovery: Incomplete regrowth of olfactory sensory neurons causes "miswiring" and distorted signal encoding - like a piano with some strings cut, producing a discordant sound instead of a pleasant one

Pathophysiology of Parosmia:

Related Olfactory Disorders (Comparison Table)

Classification Framework

1. Quantitative abnormalities - loss or reduction (anosmia, hyposmia)
2. Qualitative abnormalities - distortions or illusions (parosmia/dysosmia)
3. Olfactory hallucinations - from temporal lobe disorders or psychiatric disease (phantosmia, uncinate fits)
4. Higher-order loss - olfactory agnosia (inability to recognize/name odors despite detecting them)

Recent Research on Parosmia Treatment Options and Outcomes

1. Olfactory Training (OT) - First-Line Therapy

• A 2025 systematic review and meta-analysis (Chen et al., Rhinology) pooled 9 studies covering 179 patients with chronic COVID-19-related olfactory dysfunction. It found significant post-training improvements in all dimensions of the Sniffin' Sticks test: threshold, discrimination, identification, and total TDI score - suggesting both top-down (central) and bottom-up (peripheral) mechanisms are involved. [PMID: 40465208]
• However, a 2025 RCT (Rhinology) found patients who had smell problems for an extended time showed no difference vs. an odorless placebo after 12 weeks of training - suggesting timing of treatment is critical. Delayed treatment appears to reduce benefit.
• Mayo Clinic data indicate OT alone recovers smell in ~26% of patients; combining OT with nasal steroid irrigation doubles recovery to ~50%.

2. Platelet-Rich Plasma (PRP) Injections - Promising Emerging Therapy

• Lechien & Saussez, 2025 (Otolaryngology - Head & Neck Surgery, [PMID: 39888025]) - the largest study to date: 514 COVID-19 patients at two centers compared PRP + OT vs. OT alone. Results:
  • All PRP subgroups (anosmia, hyposmia, parosmia) showed significantly higher TDI score increases than controls
  • Parosmia patients had greater quality-of-life improvement than other groups following PRP
  • Older age and longer duration of OD negatively predicted outcomes
  • Parosmia patients had the highest baseline burden on quality of life
• Fieux et al., 2025 (Int Forum Allergy Rhinol, [PMID: 39740091]) - prospective cohort RCT on long-term outcomes of PRP for post-viral olfactory loss, further supporting the approach.
• A phase 3 RCT (NCT07151703, Taiwan) is currently comparing intranasal injection vs. topical PRP delivery, with completion estimated in 2028.

3. Stellate Ganglion Block (SGB) - Does NOT Work

• 3-month response rate: 43% SGB vs. 38% placebo - no significant difference
• No difference in clinical global impression of improvement
• Conclusion: SGB is not superior to placebo and should NOT be recommended for COVID-19-induced parosmia [PMID: 40504522]

4. Corticosteroids - Limited Evidence

• Nasal steroid irrigation when combined with OT does boost recovery rates (Mayo Clinic data)
• Oral/systemic steroids are no longer routinely recommended for post-infectious parosmia

5. Transcranial Electrical Stimulation - Exciting but Early

• 63% achieved complete recovery after 10 sessions over 2 weeks
• The control group (sham stimulation) showed no such improvement

6. Other Agents - Mostly Ineffective or Unproven

7. Prognosis and Natural Recovery

• Most post-COVID parosmia is temporary. The 2024 systematic review (Bischoff et al.) notes spontaneous recovery occurs in a significant proportion, making placebo-controlled trials essential for accurate assessment.
• Parosmia appearing during recovery from anosmia is considered a positive prognostic indicator - it signals the olfactory system is regenerating.
• Outcomes worsen significantly if treatment is delayed beyond 6 months (Mayo Clinic). Early referral is key.
• Parosmia patients report disproportionately high quality-of-life burden - food aversion, weight loss, depression, and social anxiety are common.

Summary of Evidence Grades