Paranasal Sinuses - Comprehensive Overview

Definition

The Four Pairs

1. Maxillary Sinuses (Largest)

• Location: Body of the maxillary bone, lateral to the nasal cavity, inferior to the orbit
• Size: ~15 mL at maturity; largest of all sinuses
• Drainage: Middle meatus via the maxillary ostium (hiatus semilunaris, ostiomeatal complex)
• Key anatomy: The floor of the sinus is closely related to the roots of the upper premolars and molars (especially 2nd premolar, 1st and 2nd molars) - clinically important for dental infections spreading to sinuses
• Ostium position: Located high on the medial wall - poor dependent drainage, making it prone to stasis
• Nerve supply: Infraorbital nerve (V2), anterior and posterior superior alveolar nerves

2. Frontal Sinuses

• Location: Frontal bone, above the orbits
• Development: Absent at birth; begin developing at ~2 years, reach adult size by puberty
• Drainage: Middle meatus via the frontonasal duct (nasofrontal duct), which empties into the infundibulum or middle meatus
• Variation: Highly variable in size; may be absent (aplastic) or asymmetric
• Clinical note: Separated from the anterior cranial fossa by a thin posterior bony wall - frontal sinusitis can spread intracranially

3. Ethmoid Sinuses (Ethmoidal Air Cells)

• Location: Ethmoid bone, between the orbits (medial wall of orbit = lamina papyracea)
• Structure: Multiple small air cells (3-18 per side), divided into:
  • Anterior cells - drain into the middle meatus (infundibulum)
  • Middle cells - drain into the middle meatus (above the bulla ethmoidalis)
  • Posterior cells - drain into the superior meatus
• Development: Present at birth; grow throughout childhood
• Key relations: Orbit laterally (lamina papyracea - very thin), optic nerve and anterior cranial fossa superiorly, anterior ethmoidal artery runs through the roof

4. Sphenoid Sinuses

• Location: Body of the sphenoid bone, most posteriorly placed
• Drainage: Sphenoethmoidal recess (above the superior concha), NOT into the middle meatus
• Key relations (most important surgically):
  • Optic nerve (superolateral wall)
  • Internal carotid artery (lateral wall)
  • Pituitary gland / sella turcica (superior)
  • Cavernous sinus (lateral)
  • Posterior clinoid process (roof)
• Clinical note: Transsphenoidal hypophysectomy uses the sphenoid sinus as a surgical corridor to the pituitary

Drainage Summary Table

Development

Blood Supply and Nerve Supply

• Arterial supply: Branches of the ophthalmic artery (anterior and posterior ethmoidal arteries) and maxillary artery (sphenopalatine, descending palatine, infraorbital arteries)
• Venous drainage: Facial vein, pterygoid plexus, and ophthalmic veins (which communicate with the cavernous sinus - important route for intracranial spread)
• Nerve supply: Branches of CN V (trigeminal) - V1 (ophthalmic) for ethmoid/frontal, V2 (maxillary) for maxillary/sphenoid

Functions (Debated)

1. Reduction of skull weight - replacing bone with air
2. Voice resonance - contributing to timbre
3. Humidification and warming of inspired air
4. Thermal insulation of sensitive structures (orbit, brain)
5. Crumple zone - buffer against facial trauma
6. Immunological defense - mucus secretion and mucociliary clearance

Ostiomeatal Complex (OMC)

• Maxillary ostium
• Infundibulum
• Hiatus semilunaris
• Ethmoid bulla
• Middle meatus and middle turbinate

Important Anatomical Variants (Clinically Relevant)

Clinical Conditions

Sinusitis

• Acute (<4 weeks): Usually viral (common cold), secondary bacterial infection by Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis
• Chronic (>12 weeks): Mucosal inflammation, polyps, anatomical obstruction
• Most commonly affected: Maxillary sinus (due to high ostium causing poor drainage), followed by ethmoid cells

Complications of Sinusitis (Spread of Infection)

• Orbital: Preseptal/periorbital cellulitis, orbital abscess (via lamina papyracea from ethmoid)
• Intracranial: Meningitis, brain abscess, cavernous sinus thrombosis, epidural/subdural empyema (via frontal sinus)
• Osteomyelitis: Pott's puffy tumor (frontal bone osteomyelitis with subperiosteal abscess)

Carcinoma of Paranasal Sinuses

• Rare; most common in the maxillary sinus
• More common in men; associated with occupational exposure (nickel refining, woodworking, chromate exposure)
• Often presents late (invades orbit, palate, cheek) - poor prognosis
• Most common histology: squamous cell carcinoma
• Ohngren's line: Imaginary line from medial canthus to angle of mandible - tumors above and posterior (superoposterior) have worse prognosis (infrastructure vs. suprastructure tumors)

Mucocele

• Chronic cystic dilation of a sinus due to ostial obstruction; most common in the frontal sinus
• Can erode bone and expand into orbit or anterior cranial fossa

Radiology

• X-ray (Water's view) - Best for maxillary sinus (OM view = occipitomental); shows fluid level or opacification
• CT scan - Gold standard for sinus disease; shows bony anatomy, OMC, and variants
• MRI - Better for soft tissue/tumor extension, fungal sinusitis, intracranial complications