The Anatomy of the Sinuses

 The paranasal sinuses are four paired, air-filled cavities in the face, arranged around the nasal cavity. All of them drain into the nasal cavity. The four sets of paired sinuses are the maxillary, frontal, sphenoid, and ethmoid sinuses. The drawing below displays them:

The paranasal sinuses develop in the fetus as small outpouchings from the nasal cavity that grow into the surrounding bones. After birth, they continue to develop until early adulthood. The sphenoid sinuses are unique to humans and primates. The maxillary sinuses are the largest, with an average volume of 15 mL in adults. The frontal sinuses are the most superior in position. The ethmoid sinuses are made up of multiple small air cells divided into anterior, middle, and posterior groups, and they vary in size.

The exact functions of the paranasal sinuses are not fully understood, but they have several roles. They help fight infection by producing mucus that drains into the nasal cavity and clears germs. They also add moisture to the air we breathe. They help absorb impact and protect the brain in case of head or face trauma. They also increase the resonance of our voices and, because air is lighter than bone and muscle, they make the head lighter overall.

The maxillary sinuses sit beneath the eyes in the maxillary bones. They are small at birth but grow rapidly between ages 0–3 and again between 6–12 years. In adults, they reach about 15 mL and are the largest sinuses. They are also the most common site of sinusitis. They drain into the nasal cavity through the hiatus semilunaris into the middle meatus. The floor of the maxillary sinuses is formed by the alveolar process of the maxilla and lies close to the roots of the upper teeth, which means that dental infections, such as abscesses, can spread into these sinuses. The osteomeatal complex connects the frontal, anterior ethmoid, and maxillary sinuses to the middle turbinate.

The frontal sinuses are located in the frontal bone above the eyes and have a triangular shape. They are the most superiorly located sinuses. Absent at birth, they begin forming around age two but do not become visible until about ages five or six. Their drainage pathway can vary. Fluid usually flows through the frontal recess, then either into the hiatus semilunaris and middle meatus or directly into the middle meatus. Like the maxillary sinuses, they are also a common site of infection.

The sphenoid sinuses are found behind the eyes and are the deepest sinuses. They occur only in humans and primates. They are absent at birth but begin to appear around age two, maturing between ages twelve and fourteen. They drain through the sphenoethmoidal recess into the nasal cavity. Important structures, such as the internal carotid artery and the optic nerve, lie close to the sphenoid sinuses. Surgeons can pass instruments through the nose into the sphenoid sinus and then into the hypophyseal fossa to treat pituitary adenomas. This approach, called endoscopic trans-sphenoidal surgery, provides a minimally invasive method to remove pituitary tumors.

The ethmoid sinuses are located between the eyes and behind the nose. They appear as small pockets of air cells separated by thin septa. These sinuses are present at birth and mature around age twelve. Their total number varies, ranging from four to seventeen. They are divided into anterior, middle, and posterior groups. The anterior group drains via the hiatus semilunaris into the middle meatus, the middle group drains into the lateral wall of the middle meatus, and the posterior group drains into the lateral wall of the superior meatus.