frontal sinus
Meaning of frontal sinus: The air space behind the eyebrows that nobody is born with, and the one whose shape is distinctive enough to identify a person.
Definition of frontal sinus
The frontal sinus is one of a pair of air-filled cavities inside the frontal bone, the bone of the forehead, sitting above and between the eye sockets. Each is lined by the same moist membrane as the nose and drains through a narrow channel into the nasal cavity. The two are rarely the same size and are separated by a thin bony wall.
When does it appear?
Not at birth. The frontal sinus begins as a small pouch that pushes up into the forehead bone during early childhood and keeps enlarging until the skeleton stops growing in the late teens. Adult size varies enormously from one person to the next, and in a small share of people one side or both never form at all. Because the pattern of the bony walls is shaped by that individual growth, no two are alike, and the outline seen on a skull film is used in forensic work to confirm identity.
How does it compare with the other sinuses?
| Sinus | Location | Present at birth |
|---|---|---|
| Frontal | Forehead, above the eyes | No |
| Maxillary | Cheek, above the upper teeth | Yes, small |
| Ethmoid | Between the eyes, a honeycomb of small cells | Yes |
| Sphenoid | Deep in the skull base behind the nose | No |
All four belong to the group called the paranasal sinus cavities, all are lined by the same membrane, and all drain into the nose. They lighten the skull, add resonance to the voice, and help warm and moisten inhaled air. The frontal sinus drains through the ethmoid cells beside it, which is why disease in the two so often travels together.
What goes wrong there?
The drainage channel is narrow and easily swollen shut. When it closes, mucus collects, bacteria multiply in it, and the result is pain and pressure across the forehead that typically worsens on leaning forward. The back wall of the sinus is thin and forms part of the floor of the skull, so an infection that is not controlled has a short route to the tissues around the brain. Infection can also erode forward through the front wall, raising a soft swelling on the forehead over infected bone.
Also known as
Used in a sentence
Imaging showed fluid filling the left frontal sinus.
Good to know
The frontal sinus is absent at birth and develops through childhood, so frontal sinusitis is not expected in young children, and one or both sinuses fail to form at all in a minority of adults.
Word origin
Where “frontal sinus” comes from
Frontal comes from the Latin frons, meaning forehead; sinus is the Latin word for a hollow, a curve or a fold.