The sinuses are air-filled cavities within the bones of the face and skull, collectively known as the paranasal sinuses. They serve functions such as lightening the skull, increasing voice resonance, and producing mucus to humidify the air. These hollow spaces are situated around the nasal cavity. While the left and right sinuses are separated by bony structures, they share a single, central drainage pathway that allows for an indirect connection.
Mapping the Sinus Pairs
The human head contains four pairs of sinuses, existing symmetrically on the left and right sides. The largest are the maxillary sinuses, located in the upper jawbone, behind the cheeks, and beneath the eyes. Above the eyes, within the forehead bone, are the frontal sinuses, separated by a thin wall of bone.
The ethmoid sinuses are found between the eyes and the upper part of the nose, forming a honeycomb of small air cells within the ethmoid bone. Deep within the skull, behind the ethmoid sinuses, is the fourth pair, the sphenoid sinuses. These four pairs are named for the specific facial bones in which they reside.
Direct vs. Indirect Connection
The left and right sinuses lack a direct physical connection, as they are separated by bony partitions and the nasal septum. The left and right frontal sinuses, located in the forehead, are divided by a bony wall known as the interfrontal sinus septum. Similarly, the sphenoid sinuses, situated deep in the skull, are separated by their own thin intersinus septum.
The true link between the left and right sides is an indirect one, established by the central nasal cavity. All four sinus pairs—maxillary, frontal, ethmoid, and sphenoid—drain their mucus through small openings called ostia into this shared nasal passage. The nasal cavity acts as the main collection point for both sides.
Why Sinus Symptoms Are Often Bilateral
This bilateral manifestation occurs because the shared nasal cavity is the primary site of inflammation and blockage. When a person catches a cold or has an allergy flare-up, the lining of the entire nasal passage and all the ostia can become swollen.
This widespread inflammation can cause the drainage openings for both the left and right sinuses to become blocked at the same time. When the ostia are obstructed, mucus becomes trapped within the sinus cavities, leading to pressure, congestion, and pain across the forehead, cheeks, and eyes. Furthermore, viral or allergic processes that cause rhinosinusitis are systemic, meaning the entire respiratory mucosal lining is affected.