Paranasal Air Sinuses

Anatomical Overview

Common Features of All Paranasal Sinuses

1. Lined by respiratory mucosa - ciliated and mucus-secreting
2. Open into the nasal cavities via ostia
3. Innervated by branches of the trigeminal nerve [CN V]

Size Order (Largest to Smallest)

1. Maxillary sinus
2. Frontal sinus
3. Sphenoid sinus
4. Ethmoid sinus (cells)

1. Maxillary Sinuses

• Location: Largest sinuses; completely fill the bodies of the maxillae
• Shape: Pyramidal - apex directed laterally, base forming the medial wall (adjacent to nasal cavity)
• Opening: Near the top of the base, into the center of the semilunar hiatus in the middle nasal meatus (an unfavorably high position that impedes drainage when supine)
• Relationships:
  • Roof (superolateral surface): Related to the orbit above
  • Anterolateral surface: Related to roots of upper molar and premolar teeth, and the face anteriorly
  • Posterior wall: Related to the infratemporal fossa
• Nerve supply: Infraorbital and alveolar branches of the maxillary nerve [V2]
• Blood supply: Branches of the infraorbital and superior alveolar branches of the maxillary arteries

2. Frontal Sinuses

• Location: Most superior sinuses; one on each side in the frontal bone under the forehead
• Shape: Triangular - base oriented vertically at the midline above the bridge of the nose, apex directed laterally about one-third of the way along the upper margin of the orbit
• Variable in size; can be asymmetric or absent
• Opening: Drains onto the lateral wall of the middle meatus via the frontonasal duct, which penetrates the ethmoidal labyrinth and continues as the ethmoidal infundibulum at the front end of the semilunar hiatus
• Nerve supply: Branches of the supraorbital nerve (from ophthalmic nerve [V1])
• Blood supply: Branches of the anterior ethmoidal arteries

3. Ethmoidal Cells (Ethmoid Sinuses)

• Location: Fill the ethmoidal labyrinth on each side, between the orbit laterally and the nasal cavity medially
• Structure: Not a single cavity - a variable number of individual air chambers (cells)
• Separated from the orbit by the thin orbital plate (lamina papyracea) of the ethmoid bone
• Three groups based on drainage location:
  • Anterior ethmoidal cells - open into the ethmoidal infundibulum or frontonasal duct (middle meatus)
  • Middle ethmoidal cells - open onto the ethmoidal bulla or the lateral wall above it (clinically grouped with anterior cells)
  • Posterior ethmoidal cells - open into the superior nasal meatus
• Walls may be completed by adjacent bones (frontal, maxillary, lacrimal, sphenoid, palatine)
• Nerve supply: Anterior and posterior ethmoidal branches of the nasociliary nerve [V1], plus maxillary nerve [V2] via orbital branches from the pterygopalatine ganglion
• Blood supply: Anterior and posterior ethmoidal arteries
• Notable cell - Agger nasi: The most anterior ethmoidal cell; projects anterior to the axilla of the middle turbinate, forming a bulge in the lateral nasal wall. Its posterior limit forms the anterior border of the frontal recess.

4. Sphenoidal Sinuses

• Location: Paired, within the body of the sphenoid bone
• Opening: Via apertures high on their anterior walls, into the sphenoethmoidal recess (posterior roof of nasal cavity)
• Relationships: Clinically the most dangerous - closely related to:
  • Above: Cranial cavity, pituitary gland, optic chiasm
  • Laterally: Cavernous sinuses, internal carotid arteries
  • Below and in front: Nasal cavities
• Nerve supply: Posterior ethmoidal branch of ophthalmic nerve [V1]; maxillary nerve [V2] via pterygopalatine ganglion
• Blood supply: Branches of the pharyngeal arteries from the maxillary arteries

Clinical point - Transsphenoidal approach to the pituitary: Because only thin shelves of bone separate the sphenoidal sinuses from the nasal cavities below and hypophyseal fossa above, the pituitary gland is surgically approached through the nasal roof → anteroinferior sphenoid bone → sphenoidal sinus → top of sphenoid → hypophyseal fossa. - Gray's Anatomy for Students, p. 1215

Drainage Summary (Ostia)

Development and Embryology

• Altering the size and shape of the face during infancy and childhood
• Adding resonance to the voice during adolescence

Functions (Proposed)

• Vocal resonance
• Reduction of skull weight (pneumatization)
• Humidification and air conditioning of inspired air
• Heat insulation for the brain
• Physical buffer against facial injury
• Nitric oxide (NO) production - the respiratory epithelium contains large amounts of nitric oxide synthase; NO produced in the sinuses plays a role in ciliary beat frequency and host defense

Clinical Relevance

1. Sinusitis: Infections of the nasal mucosa can spread through the sinus ostia. Poorer circulation and unfavorably placed openings (e.g., the maxillary ostium near the roof of the sinus) impair drainage and promote chronic inflammation.
2. Orbital complications: Inflammation of the ethmoidal cells can spread to the orbit by penetrating the thin orbital plate (lamina papyracea) of the ethmoid bone.
3. Sphenoidal sinus surgery: Surgical access to the pituitary gland uses a transsphenoidal approach through the sinus.
4. Dental sinusitis: The close relationship of maxillary sinus floor to roots of upper molars/premolars means dental infections can cause maxillary sinusitis (and vice versa).
5. Frontal sinus absent: The frontal sinus may be absent entirely - a normal variant of no clinical significance.