You have four pairs of sinuses in your face, totaling eight individual cavities. They’re called the paranasal sinuses because they all surround and connect to your nasal cavity. Each pair is named after the bone it sits in: the maxillary, frontal, ethmoid, and sphenoid sinuses. Together, they form a connected drainage system lined with mucus-producing tissue that empties into the back of your nose.
Maxillary Sinuses
The maxillary sinuses are the largest of the four pairs. They sit inside the bones of your upper jaw, beneath your eyes and on either side of your nose. Each one is roughly pyramid-shaped. Because they’re so close to your upper teeth, an infection in these sinuses often causes pain in the cheeks just below the eyes and can even feel like a toothache.
One quirk of the maxillary sinuses is that their drainage opening sits near the top of the cavity wall rather than the bottom. That’s a poor setup for gravity-assisted drainage, and it’s one reason these sinuses are especially prone to congestion. Mucus has to be pushed upward by tiny hair-like structures in the lining before it can exit into the nasal passage.
Frontal Sinuses
The frontal sinuses are in your forehead, just above your eyebrows. They’re the last sinuses to develop. On CT scans, they barely begin to form around age 2 and don’t reach their full adult size until the late teens or early twenties. Their average adult volume is small, roughly 3.5 cubic centimeters per side, about the size of a large marble. Some people have frontal sinuses that are noticeably asymmetric, and in rare cases one side never develops at all.
When the frontal sinuses become inflamed or infected, the pain tends to concentrate in the forehead directly above the eyes. They drain downward through a narrow channel that connects to the same middle passage of the nose used by most of the other sinuses.
Ethmoid Sinuses
Rather than being a single open chamber on each side, the ethmoid sinuses are a cluster of small air-filled pockets, often called ethmoid air cells, nestled between your eyes behind the bridge of your nose. They’re organized into front, middle, and rear groups, and each group drains into a slightly different spot inside the nasal cavity. The front and middle cells drain into the middle region of the nasal passage, while the rear cells drain higher up.
Of all four sinus types, the ethmoid cells are the earliest to appear. They’re already partially present at birth. Because they sit so close to the eye sockets, ethmoid infections produce a distinctive pain pattern: pressure behind and between the eyes, sometimes with tearing, and a headache often described as a splitting sensation across the forehead.
Sphenoid Sinuses
The sphenoid sinuses are the most deeply buried. They sit behind the eyes, roughly in the center of your skull, inside the sphenoid bone. Their location makes them neighbors to several critical structures, including the optic nerves, the pituitary gland (which sits in a bony saddle just above), and major blood vessel channels on either side.
That proximity is why sphenoid sinus disease, though less common than infections in the other sinuses, can sometimes cause vision-related symptoms. The pain from sphenoid sinusitis is also harder to pinpoint. It doesn’t concentrate in one obvious spot the way maxillary or frontal pain does. Instead it may be felt vaguely in the front or back of the head.
How the Sinuses Work
All four pairs are lined with the same type of tissue: a thin membrane covered in mucus-producing cells and microscopic hair-like projections called cilia. The cilia beat in coordinated waves, sweeping mucus steadily toward the drainage openings and into the nasal cavity. Your nasal lining produces roughly 20 to 40 milliliters of mucus per day, a few tablespoons, most of which you swallow without noticing.
The coordination of cilia doesn’t have to be perfect across the entire surface. Research shows that cilia beat in sync only in local patches of neighboring cells, and a moderate amount of variation in their direction actually helps move mucus efficiently. Problems arise when too many cilia stop beating, die off, or become severely misaligned, all of which can happen during chronic inflammation or certain genetic conditions. When mucus clearance slows down, fluid pools in the sinuses and creates a breeding ground for infection.
Where Each Sinus Causes Pain
Because the four sinus pairs occupy different areas of the face and skull, each one produces a recognizable pain pattern when inflamed:
- Maxillary: aching in the cheeks below the eyes, sometimes mimicking a toothache in the upper jaw.
- Frontal: headache concentrated across the forehead above the eyebrows.
- Ethmoid: pressure and pain behind and between the eyes, sometimes with watery eyes.
- Sphenoid: diffuse, hard-to-locate pain that can radiate to the front or back of the head.
Knowing which area hurts most can give you a reasonable guess about which sinuses are involved, though infections often affect more than one pair at a time.
Common Anatomical Variations
Not everyone’s sinuses look the same on a scan. Certain structural variations are surprisingly common and can influence how well the sinuses drain.
A concha bullosa is one of the most frequently seen variants. It happens when one of the bony ridges inside the nose (called a turbinate) becomes partially air-filled, essentially forming an extra small sinus. If it’s large enough, it can narrow the drainage pathways nearby. Haller cells are small air pockets that develop in the floor of the eye socket, right next to the maxillary sinus drainage opening. When present, they can crowd that opening and make the maxillary sinus more prone to blockage.
Onodi cells are a variation of the ethmoid air cells in which a rear ethmoid pocket extends far enough back and sideways to wrap around the optic nerve. This is clinically important because it means the nerve sits closer to the sinus wall than expected. A deviated nasal septum, where the central wall of the nose leans to one side, is another extremely common finding that can affect airflow and drainage on the narrower side.