Paranasal Sinuses Structure and Function

Paranasal Sinuses: Structure and Function

• Paranasal sinuses are hollow cavities within certain facial bones, lined with respiratory mucous membrane.
• They connect to the nasal cavity via small openings (ostia).
• Prolonged nasal intubation can block these openings, leading to sinusitis.
• Their function is largely unknown; however, they lighten the facial bones.
• They also influence voice resonance and may act as insulation against cold air.
• Shaped to likely influence orbital cavity position.
• Lined with respiratory mucous membrane with a sensory nerve supply, concentrated around the ostium.
• Mucus is moved by cilia towards the ostium; gravity is a minor factor in drainage.
• Four pairs of paranasal sinuses: maxillary, ethmoidal, sphenoidal, and frontal.

Maxillary Sinus

• Located within the maxilla bone (formerly called maxillary antrum).
• Pyramidal shape; base at lateral nasal wall, apex in zygomatic process.
• Roof is the floor of the orbit.
• Floor is the alveolar part of the maxilla (tooth-bearing area).
• Contains ridges, one consistently at the roof-anterior wall junction created by the infraorbital nerve.
• Present at birth as a shallow slit.
• Size variability; larger sinuses can extend into zygomatic and alveolar processes, potentially encompassing tooth roots.
• Roots typically encased in bone; if not, root apices touch the sinus mucous membrane. Tooth extraction in these cases can result in a fistula that generally heals naturally.
• Ostium located high up and back on the nasal wall (2-4mm diameter); a secondary, smaller ostium might also exist posteriorly.
• Opens at posterior end of semilunar hiatus in middle meatus.
• Blood supply from facial, maxillary, infraorbital, and greater palatine arteries. Drains into facial vein and pterygoid plexus.
• Lymph drainage primarily via infraorbital foramen or ostium to submandibular nodes.
• Innervated by maxillary nerve branches (superior alveolar; greater palatine; infraorbital). Anterior superior alveolar nerve supplies canine and incisor teeth and adjacent nasal wall. Middle superior alveolar nerve, which sometimes is absent, supplies premolar teeth.

Ethmoidal Sinus

• Located within ethmoidal labyrinth (lateral portion of ethmoid bone); not a singular cavity, divided into multiple air cells.
• Variable number of cells (3-18), termed anterior, middle, or posterior based on drainage location.
• Lateral wall (lamina papyracea) is thin, enabling visibility of air cells.
• Superior and middle nasal conchae project from medial wall.
• Anterior ethmoidal air cells are located anteriorly, draining into anterior semilunar hiatus.
• Middle ethmoidal air cells drain into middle meatus (ethmoidal bulla may protrude at lateral nasal wall).
• Posterior ethmoidal air cells drain into superior meatus and may extend near the optic nerve.
• Blood supply from supraorbital, anterior and posterior ethmoidal, and sphenopalatine arteries (both internal and external carotid systems).
• Lymph drainage to submandibular and retropharyngeal nodes.
• Innervation by various nerves (including supraorbital, anterior ethmoidal, posterior ethmoidal, etc); significant feature—posterior cells can receive input from both ophthalmic and maxillary branches, leading to potential referred pain.

Sphenoidal Sinus

• Located in body of sphenoid bone; two sinuses separated by a septum often off-center.
• Variable size; can extend into basiocciput and greater wing of sphenoid or pterygoid process for larger ones.
• Located above the pituitary fossa and middle cranial fossa; laterally to cavernous sinus and internal carotid artery; posteriorly to posterior cranial fossa and pons; inferiorly to roof of nasopharynx.
• Internal features include indentations or ridges from neighboring structures.
• Ostium in anterior wall, opening into the sphenoethmoidal recess (behind superior concha).
• Blood supply from posterior ethmoidal and sphenopalatine branches of maxillary artery.
• Lymph drainage to retropharyngeal nodes.
• Innervation by posterior ethmoidal nerve and pterygopalatine ganglion (maxillary nerve branch).

Frontal Sinus

• Appear in the second year; excavations in the diploe of frontal bone; likely from migrating anterior ethmoidal air cells.
• Extend above the medial end of eyebrow, into the squamous part of the frontal bone and back into orbital roof.
• Unequal in size; separated by a midline septum.
• Anterior cranial fossa and orbit are key relations.
• Drain through ostium at the lower medial corner; likely into semilunar hiatus via ethmoidal infundibulum or frontonasal duct.
• Blood supply from supratrochlear, supraorbital, and anterior ethmoidal arteries; venous drainage to diploic and superior ophthalmic veins.
• Lymph drainage to submandibular nodes (eyebrow skin drains differently to preauricular nodes).
• Innervation from supraorbital and supratrochlear nerve branches piercing the frontal bone.

Development

• Sinuses are rudimentary or absent at birth.
• Continue enlarging throughout life; more rapid enlargement around 6–7 years of age and postpuberty.
• Growth at different stages differs; in youth, it's bone enlargement; in old age, it's bone resorption.
• Early enlargement coincides with second dentition eruption.

Explore the anatomy and role of paranasal sinuses in this quiz. Learn about their structure, function, and the impact of conditions like sinusitis. This quiz covers key concepts related to maxillary and other sinuses relevant to respiratory health.