Nasal cavity and paranasal sinuses

Nasal Cavity Proper - The Bony Framework

• Roof: Formed by nasal, frontal, ethmoid (cribriform plate), and sphenoid bones.
• Floor (Hard Palate): Palatine process of maxilla and horizontal plate of palatine bone.
• Medial Wall (Nasal Septum): Vomer, perpendicular plate of the ethmoid, and septal cartilage.
• Lateral Wall: Complex and features three bony projections (conchae).
  • Superior & Middle Conchae: Part of the ethmoid bone.
  • Inferior Concha: A separate bone.
  • Contributing bones: Maxilla, lacrimal, ethmoid, inferior nasal concha, palatine, sphenoid.

⭐ Clinical Pearl: The lamina papyracea of the ethmoid bone is a paper-thin plate forming part of the lateral wall, separating the ethmoid sinus from the orbit. Fractures can lead to orbital cellulitis from sinus infections.

Vasculature & Innervation - Wires and Pipes

• Arterial Supply:
  • Sphenopalatine Artery (Maxillary a.): Main supply to posterior nasal cavity.
  • Ethmoidal Arteries (Ophthalmic a.): Anterior & Posterior, supply the roof and anterior septum.
  • Kiesselbach's Plexus: Anastomosis on the anterior septum; common site of epistaxis (nosebleeds).
• Venous Drainage:
  • Follows arteries to pterygoid plexus, facial vein, & cavernous sinus.
  • ⚠️ Veins from the "danger triangle" of the face can drain to the cavernous sinus, risking infection spread.
• Innervation:
  • General Sensation: Trigeminal N. (CN V) -> V1 (anterior) & V2 (posterior).
  • Special Sensation (Smell): Olfactory N. (CN I) to olfactory mucosa.
  • Autonomic (Parasympathetic):

⭐ Epistaxis: Over 90% of nosebleeds occur in Kiesselbach's plexus (Little's Area) on the anteroinferior nasal septum.

Paranasal Sinuses - The Echo Chambers

• Function: Air-filled spaces that lighten the skull, add resonance to the voice, and produce mucus.

• Four Sinuses: 📌 Frontal, Ethmoidal, Sphenoidal, Maxillary (Mnemonic: "F-E-S-Ma").

• Drainage Pathways:

  • Middle Meatus: Drains frontal, maxillary, & anterior ethmoidal sinuses.
  • Superior Meatus: Drains posterior ethmoidal sinus.
  • Sphenoethmoidal Recess: Drains sphenoidal sinus.
  • Inferior Meatus: Drains nasolacrimal duct (tears).

⭐ The maxillary sinus is the largest and most commonly infected. Due to its high ostium, drainage is poor, and infections can arise from dental roots of adjacent maxillary molars.

Clinical Correlates - When Anatomy Goes Awry

• Sinusitis: Inflammation of paranasal sinuses, most often viral.

  • Maxillary Sinus: Most commonly infected. Its drainage ostium is high on its medial wall, impeding gravity-dependent drainage into the middle meatus.
  • Complications: Infection can spread through thin sinus walls.
    • Orbit: Orbital cellulitis (via lamina papyracea of ethmoid).
    • Cranial Cavity: Meningitis, cavernous sinus thrombosis (via valveless ophthalmic veins from the "danger triangle").

• Epistaxis (Nosebleed):

  • Anterior Bleeds (90%): Kiesselbach's plexus on the anteroinferior septum.
  • Posterior Bleeds: Woodruff's plexus (sphenopalatine artery branches); more severe.

⭐ Maxillary sinusitis can present as toothache. The roots of the maxillary molars/premolars often project into the sinus floor, and both structures share sensory innervation from the V2 nerve (trigeminal).

High‑Yield Points - ⚡ Biggest Takeaways

• Kiesselbach's plexus, located on the anterior nasal septum, is the most common source of epistaxis.
• Severe posterior nosebleeds typically involve the sphenopalatine artery, a branch of the maxillary artery.
• The maxillary sinus is the most frequently infected sinus; its proximity to maxillary molars can cause dental-related sinusitis.
• Infection of the ethmoid sinuses can erode the thin lamina papyracea, leading to orbital cellulitis.
• The middle meatus is the primary drainage site for the frontal, maxillary, and anterior ethmoid sinuses.
• The sphenoid sinus offers surgical access to the pituitary gland.