Olfactory Disorders

Olfactory Pathway - Nose Knows How

• Olfactory Epithelium: Located in the nasal cavity roof, superior septum & superior concha.
  • Cell Types:
    • Olfactory Receptor Neurons (ORNs): Bipolar neurons; site of odor detection.
    • Basal Cells: Stem cells; responsible for regenerating ORNs.
    • Sustentacular Cells: Provide structural and metabolic support.
• Odorant Reception Mechanism:
  • Odorants dissolve in mucus, bind to Odorant Binding Proteins (OBPs).
  • OBP-odorant complex activates G-protein coupled receptors (GPCRs, specifically Golf) on ORN cilia.
  • Signal Transduction: ↑cAMP → opens ion channels → depolarization → action potential.
• Central Olfactory Pathway: 📌 CN I
  • ORN axons (unmyelinated) pass through cribriform plate, synapse in Olfactory Bulb (glomeruli).
  • Olfactory Tract projects to Primary Olfactory Cortex (bypasses thalamus initially):
    • Piriform cortex (main olfactory discrimination)
    • Amygdala (emotional response to odors)
    • Entorhinal cortex (olfactory memory)

⭐ Olfactory receptor neurons are unique as they are bipolar neurons that regenerate throughout life (approximately every 30-60 days).

Smell Saboteurs - Types & Triggers

• Classification of Olfactory Disorders:

  • Anosmia: Total smell loss.
  • Hyposmia: Reduced smell.
  • Hyperosmia: Heightened smell.
  • Dysosmia: Distorted smell perception.
    • Parosmia: Misperception of an odor (e.g., pleasant odor perceived as foul).
    • Phantosmia: Olfactory hallucination (perception of odor when none is present).
  • Agnosia: Inability to identify or classify odors, despite intact smell detection.

• Etiology: Conductive vs. Sensorineural Loss

  Type	Mechanism	Examples
  Conductive	Impaired airflow to olfactory epithelium	Nasal polyps, rhinitis (allergic/viral), septal deviation, tumors.
  Sensorineural	Damage to neuroepithelium/olfactory pathways	Viral URI, head trauma, neurodegenerative (Parkinson's, Alzheimer's), toxins, congenital (Kallmann syndrome).

• Major Etiological Categories:

⭐ Upper Respiratory Tract Infection (URTI) is the most common cause of persistent olfactory loss in adults.

Sniff Sleuth - Testing Smells

• Clinical Approach:
  • History: Detail onset, duration, severity (anosmia/hyposmia), associated symptoms (nasal obstruction, rhinorrhea, neurological deficits), history of URTI, head trauma, toxin exposure.
  • Examination: Nasal endoscopy for sinonasal pathology (polyps, inflammation); focused neurological exam (esp. CN I).
• Psychophysical Smell Tests: Standardized objective evaluation.
  • Qualitative: Basic identification of common familiar odors (e.g., coffee, soap).
  • Quantitative:
    • UPSIT (University of Pennsylvania Smell Identification Test): 40-item "scratch-and-sniff" forced-choice booklet. Scoring norms available.

      ⭐ The University of Pennsylvania Smell Identification Test (UPSIT) is a widely used 40-item scratch-and-sniff test.

    • Sniffin' Sticks: Pen-like devices testing Odor Threshold, Discrimination, and Identification (TDI score).
• Imaging Studies:
  • CT Sinuses: Indicated for suspected sinonasal disease (e.g., CRS, tumors).
  • MRI Brain/Olfactory Bulb: For suspected central causes or unexplained olfactory loss.

Specific Disorders & Management - Scent Setbacks

High‑Yield Points - ⚡ Biggest Takeaways

• Anosmia is the complete loss of smell; hyposmia is reduced smell.
• Most common cause of anosmia/hyposmia is viral URI, followed by head trauma.
• Kallmann syndrome: congenital anosmia with hypogonadotropic hypogonadism.
• Olfactory neuroblastoma (esthesioneuroblastoma) from olfactory epithelium; presents with nasal obstruction, epistaxis.
• COVID-19 is a significant cause of sudden onset anosmia/hyposmia.
• Zinc deficiency can cause olfactory dysfunction.
• Olfactory groove meningiomas can cause anosmia by compressing olfactory nerves_