Learning Objective: By the end of this lesson, the learner will be able to describe the anatomy, locations, drainage pathways, innervation, vascular supply, and clinical relevance of the paranasal sinuses and apply this understanding to clinical scenarios such as sinusitis and the transsphenoidal surgical approach.
The paranasal sinuses are air-filled extensions of the nasal cavity. They occur in four paired groups—frontal, sphenoid, ethmoid, and maxillary sinuses—each housed within a bone of the skull. All sinuses are lined by ciliated pseudostratified epithelium with mucus-secreting goblet cells, allowing continuous mucus movement toward the nasal cavity.
Functions of the Paranasal Sinuses
Although debated, proposed functions include:
- Lightening the skull
- Enhancing immune defense
- Humidifying inspired air
- Resonating the voice
During development, the nasal cavity expands into adjacent bones, forming the sinus cavities. All sinuses drain back into the nasal cavity via openings on the roof and lateral walls.
Frontal Sinuses
- Paired, located within the frontal bone, are the most superior sinuses.
- Drainage: Frontonasal duct → hiatus semilunaris (middle meatus).
- Innervation: Supraorbital nerve (branch of ophthalmic nerve, CN V1).
- Arterial supply: Anterior ethmoidal artery.
Sphenoid Sinuses
- Located within the body of the sphenoid bone.
- Drain into the spheno-ethmoidal recess (superoposterior to the superior concha).
- Innervation: Posterior ethmoidal nerve (CN V1) + branches of maxillary nerve (CN V2).
- Blood supply: Pharyngeal branches of the maxillary artery.
Clinical Relevance –
Transsphenoidal Surgery: Because the sphenoid sinus lies directly below the pituitary gland, surgeons may access the pituitary via an endoscopic transsphenoidal approach, avoiding craniotomy.
Ethmoidal Sinuses
Located within the ethmoid bone and divided into:
- Anterior ethmoidal cells → Drain into hiatus semilunaris (middle meatus)
- Middle ethmoidal cells → Drain into the middle meatus
- Posterior ethmoidal cells → Drain into the superior meatus
Innervation is via nasociliary nerve branches and CN V2 branches. Blood supply is via the anterior and posterior ethmoidal arteries.
Maxillary Sinuses
- The largest paranasal sinuses.
- Located laterally and slightly inferior to the nasal cavity.
- Drain into the hiatus semilunaris, beneath the frontal sinus opening, allowing potential spread of infection from the frontal sinus.
Clinical Relevance – Sinusitis
Because of their continuity with the nasal cavity, infections may spread into the sinuses, causing sinusitis. Maxillary sinusitis often mimics tooth pain, due to shared innervation via the maxillary nerve (CN V2).
