Sense of smell (lost/changed)

Loss or Change in Sense of Smell (Olfactory Dysfunction)

Introduction

The sense of smell (olfaction) plays a vital role in human life by enhancing flavor perception, detecting hazards (such as smoke or gas), and contributing to social and emotional well-being. Disorders of smell are broadly classified into:

• Anosmia: Complete loss of smell.

• Hyposmia: Reduced ability to smell.

• Parosmia: Distorted smell perception.

• Phantosmia: Perception of smell without stimulus.

Loss or change in smell can be temporary (e.g., during viral infections) or persistent (e.g., neurodegenerative disorders). Since smell and taste are closely linked, patients often present with a combined complaint of “loss of taste,” though true gustatory function may remain intact.

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Anatomy and Physiology of Smell

• Olfactory epithelium: Located in the upper nasal cavity; contains olfactory receptor neurons.

• Olfactory bulb and tract: Transmit signals to the brain.

• Central processing: Primary olfactory cortex, amygdala, orbitofrontal cortex.

Disruption at any of these levels (epithelial, neural, or central) can cause olfactory dysfunction.

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Etiology of Smell Loss/Change

1. Nasal and Sinonasal Causes

• Upper respiratory tract infections (URTIs): Viral infections (including COVID-19, influenza, rhinovirus).

• Allergic rhinitis: Nasal congestion blocks airflow to olfactory epithelium.

• Chronic rhinosinusitis (with or without nasal polyps): Inflammatory obstruction and epithelial damage.

• Structural abnormalities: Deviated septum, nasal tumors.

2. Neurological and Neurodegenerative Disorders

• Parkinson’s disease: Olfactory loss often precedes motor symptoms.

• Alzheimer’s disease: Early sign; linked to neurofibrillary tangles in olfactory bulb.

• Multiple sclerosis, epilepsy, head trauma: Damage to olfactory bulb or tract.

3. Post-Traumatic Causes

• Skull base fractures or head injuries damaging olfactory nerves.

4. Infectious and Inflammatory Conditions

• Meningitis, encephalitis.

• Chronic sinus infections.

5. Toxic and Medication-Related Causes

• Smoking.

• Occupational exposure (solvents, heavy metals).

• Medications:

  • ACE inhibitors (enalapril, lisinopril).

  • Statins (simvastatin).

  • Antibiotics (clarithromycin).

  • Antihistamines and decongestants (drying effect).

6. Metabolic and Systemic Causes

• Hypothyroidism.

• Diabetes mellitus.

• Malnutrition (zinc and vitamin A deficiency).

7. Aging (Presbyosmia)

• Progressive decline with age due to neuronal loss.

8. Psychiatric Causes

• Olfactory hallucinations in psychotic disorders.

• Distorted smell in depression and anxiety.

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Clinical Presentation

• Primary symptom: Loss or alteration in ability to detect odors.

• Associated features:

  • Nasal congestion, rhinorrhea, sneezing (rhinitis/sinusitis).

  • Headache, facial pressure (sinusitis).

  • Neurological symptoms (memory loss, tremors in Parkinson’s).

  • Taste disturbances (due to impaired retronasal olfaction).

  • Phantom smells (phantosmia).

Red flags: Unilateral anosmia, blood-stained nasal discharge, or associated neurological deficits (possible tumor).

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Diagnostic Evaluation

1. History

• Onset: sudden (post-viral, trauma) vs gradual (neurodegenerative, chronic sinusitis).

• Associated nasal, systemic, or neurological symptoms.

• Medication and toxin exposure.

2. Physical Examination

• Nasal endoscopy: Evaluate for polyps, septal deviation, inflammation.

• Neurological exam: Memory, motor function, cranial nerve assessment.

3. Laboratory Investigations

• CBC, ESR, CRP (infection/inflammation).

• Thyroid function tests.

• Vitamin A, zinc levels.

4. Imaging

• CT scan of sinuses (chronic rhinosinusitis, structural abnormalities).

• MRI brain (olfactory bulb, neurodegenerative disease, tumors).

5. Olfactory Function Tests

• University of Pennsylvania Smell Identification Test (UPSIT).

• Sniffin’ Sticks test.

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Management

Treatment depends on the underlying cause.

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1. General Supportive Measures

• Smoking cessation.

• Avoid exposure to irritants and toxins.

• Ensure good nasal hygiene (saline irrigations).

• Use of flavor enhancers and food safety measures for patients with anosmia.

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2. Pharmacological Management

A. Allergic Rhinitis / Chronic Rhinosinusitis

• Intranasal corticosteroids:

  • Fluticasone propionate 50 mcg per spray; 1–2 sprays per nostril once daily.

  • Mometasone furoate 50 mcg per spray; 2 sprays per nostril once daily.

• Oral antihistamines:

  • Loratadine 10 mg orally once daily.

  • Cetirizine 10 mg orally once daily.

• Leukotriene receptor antagonists (for nasal polyps):

  • Montelukast 10 mg orally once daily.

B. Post-Viral Olfactory Dysfunction (e.g., COVID-19 related)

• Olfactory training: Exposure to different odors (rose, lemon, clove, eucalyptus) twice daily for ≥12 weeks.

• Short course of corticosteroids (if severe inflammation):

  • Prednisone 30–40 mg orally once daily for 7–10 days (taper as needed).

C. Bacterial Sinusitis

• Antibiotics if bacterial infection suspected:

  • Amoxicillin-clavulanate 875/125 mg orally twice daily for 5–10 days.

  • Doxycycline 100 mg orally twice daily (if penicillin allergy).

D. Neurodegenerative Disorders

• No curative therapy for smell loss, but symptomatic treatment of underlying disease:

  • Levodopa-carbidopa 100/25 mg orally three times daily (Parkinson’s).

  • Donepezil 5–10 mg orally once daily (Alzheimer’s).

E. Nutritional Deficiencies

• Zinc sulfate: 220 mg orally once daily.

• Vitamin A: 10,000–25,000 IU orally daily (under supervision).

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3. Non-Pharmacological Therapies

• Olfactory training: Evidence-based therapy for post-viral and idiopathic anosmia.

• Endoscopic sinus surgery (ESS): For chronic rhinosinusitis with polyps not responding to medical therapy.

• Cognitive/occupational therapy: For patients with neurodegenerative-associated smell loss.

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Precautions and Patient Counseling

• Counsel patients on safety risks: inability to detect smoke, gas leaks, spoiled food. Install smoke/gas detectors at home.

• Encourage adherence to nasal sprays (proper technique critical).

• Stress that recovery after viral or trauma-related smell loss may be slow or incomplete.

• Highlight importance of early ENT or neurology referral if symptoms persist.

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Drug Interactions

• Intranasal corticosteroids (fluticasone, mometasone): Possible systemic absorption; interactions with CYP3A4 inhibitors (ketoconazole, ritonavir) → risk of Cushing’s syndrome.

• Oral antihistamines (loratadine, cetirizine): Additive sedation with alcohol, benzodiazepines.

• Montelukast: Caution with CYP2C9 and CYP3A4 inducers (rifampin, phenytoin) → reduced efficacy.

• Prednisone: Interaction with NSAIDs (GI bleeding risk); enhances hypokalemia with diuretics.

• Antibiotics:

  • Amoxicillin-clavulanate interacts with warfarin (increased INR).

  • Doxycycline interacts with antacids, iron (reduced absorption).

• Zinc supplements: Reduce absorption of tetracyclines and fluoroquinolones.

• Vitamin A: Toxicity risk if combined with retinoids (isotretinoin).

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Prognosis

• Temporary smell loss due to viral infections or allergic rhinitis often resolves.

• Chronic rhinosinusitis with polyps may relapse even after surgery; long-term intranasal steroids often needed.

• Post-traumatic anosmia often permanent due to nerve damage.

• Neurodegenerative-related anosmia is progressive and irreversible.

• Post-COVID anosmia shows partial to full recovery in most patients within 6–12 months, though some remain with persistent symptoms.