What is the approach to diagnose and manage a patient with insidious onset of loss of taste and smell?

Approach to Insidious Onset Loss of Taste and Smell

Test for COVID-19 immediately, perform nasal endoscopy to rule out chronic rhinosinusitis or masses, and obtain objective olfactory testing (UPSIT or Sniffin'Sticks) to quantify dysfunction, as insidious onset suggests either post-viral, inflammatory sinonasal disease, or structural/neurologic pathology rather than acute infection. 1, 2

Initial Diagnostic Workup

COVID-19 Testing

• Test all patients with new-onset chemosensory dysfunction for SARS-CoV-2, as 59-86% of COVID-19 patients develop loss of taste and smell, and isolated anosmia can be the sole presenting feature in up to 22% of cases 3, 1
• Note that traditional nasal symptoms (congestion, rhinorrhea) are often absent in COVID-19-related chemosensory loss, unlike other viral upper respiratory infections 3
• COVID-19 has high specificity (98.7%) for olfactory dysfunction as a screening criterion, though sensitivity is lower (22.7%) 3

Nasal Endoscopy (Essential)

• Rigid nasal endoscopy is mandatory to differentiate between obstructive causes (polyps, masses, tumors) and non-obstructive inflammatory causes 3, 2
• Chronic rhinosinusitis accounts for a significant proportion of chronic olfactory dysfunction and requires direct visualization 2
• If endoscopy reveals polyps or masses, this explains the smell decline, though the mechanism may be mixed (obstructive, inflammatory, or epithelial damage) 3

Objective Olfactory Testing

• Perform standardized psychophysical testing (UPSIT or Sniffin'Sticks extended version) to objectively quantify dysfunction, as patients commonly underestimate their impairment 3, 2
• One study showed 98.3% had objective olfactory dysfunction by UPSIT even when only 35% self-reported complaints 3, 1
• Testing takes 4-25 minutes depending on the test chosen and provides test-retest reliability for monitoring treatment response 3

Key Clinical Distinctions

Understanding "Taste Loss"

• Approximately 95% of reported "taste loss" actually reflects loss of retronasal olfaction (flavor perception) rather than true gustatory dysfunction 1, 4
• True taste dysfunction affects only the five basic tastes (sweet, bitter, sour, salty, umami), while flavor perception requires intact smell 1, 4

Timing and Pattern Recognition

• Insidious onset over weeks-to-months suggests metabolic, structural, or chronic inflammatory causes rather than post-viral causes, which typically have more acute onset 2
• Post-viral olfactory loss accounts for 20-25% of specialist clinic presentations, but usually follows a clear temporal relationship with upper respiratory infection 1, 5

Differential Diagnosis by Etiology

Most Common Causes (in order of frequency)

1. Chronic rhinosinusitis with or without polyps (30% of cases) - requires endoscopy and potentially CT sinuses 4, 6
2. Post-viral olfactory loss (19-27% of cases) - though insidious onset makes this less likely 4, 7, 6
3. Idiopathic causes (16-26% of cases) - diagnosis of exclusion 7, 8, 6
4. Head trauma - specifically inquire about remote head injury, as olfactory nerve is most commonly disrupted cranial nerve in trauma 5, 4, 7
5. Medications - common offenders in taste dysfunction 4, 9

Red Flags Requiring Advanced Imaging

• If subjective and psychophysical smell loss does not correlate with endoscopic findings, suspect sinonasal or skull base tumor 3
• MRI brain with olfactory protocol is indicated if neurologic signs present, symptoms persist beyond 6 months despite treatment, or discordance between testing and endoscopy 2, 5
• MRI should cover olfactory epithelium, neurons, bulbs at cribriform plate, and olfactory pathways 5
• CT sinuses indicated if chronic rhinosinusitis suspected on endoscopy 2, 5

Neurologic Examination Requirements

Perform focused cranial nerve examination (CN I, VII, IX, X) to assess:

• Olfactory nerve function (CN I) 4
• Facial nerve for taste from anterior 2/3 tongue (CN VII) 4
• Glossopharyngeal for posterior 1/3 tongue taste (CN IX) 4
• Vagus nerve function (CN X) 4
• Screen for neurodegenerative disease (Parkinson's, Alzheimer's, Lewy body dementia), as chemosensory dysfunction may be an early feature 5, 4

Immediate Management

Olfactory Training (Start Immediately)

• Initiate olfactory training regardless of etiology, using a protocol of sniffing four strong-smelling substances for 20 seconds each, twice daily 2
• Continue for at least 3-6 months before assessing efficacy 2
• This should be started while diagnostic workup proceeds, as up to 50% of patients with olfactory dysfunction improve over time 4

Follow-up Schedule

• Re-evaluate at 1,3, and 6 months after initiating treatment 2
• Refer to otolaryngology or specialized smell/taste clinic if no improvement after 3-6 months of olfactory training 2
• Recovery is inversely correlated with severity and duration of loss, age, smoking, and male sex 4

Common Pitfalls to Avoid

• Do not rely on patient self-report alone - objective testing reveals much higher rates of dysfunction than subjective complaints 3, 1
• Do not assume all "taste loss" is gustatory - screen for olfactory dysfunction first, as this is the cause in 95% of cases 1, 4
• Do not skip nasal endoscopy - visual inspection is essential to identify treatable sinonasal pathology 3, 2
• Do not delay olfactory training - start immediately while pursuing diagnosis, as early intervention may improve outcomes 2
• Do not forget medication review - drugs are common offenders in chemosensory dysfunction 4, 9