Frontal Sinus

What Is the Frontal Sinus?

The frontal sinus is a pair of air-filled cavities in the frontal bone, the bone of the forehead. It is one of the four pairs of paranasal sinuses, together with the maxillary, ethmoid and sphenoid sinuses, and like them it is lined with mucous membrane and opens into the nasal cavity.

The frontal sinus is often described as an ethmoid air cell that has grown upward. It begins in early childhood as an outpouching from the front of the ethmoid region and only later expands into the frontal bone, which is why its drainage pathway stays so closely tied to the ethmoid cells below it.

Explained simply: right behind your eyebrows, the forehead bone is not solid. It holds two small air pockets, one over each eye, with a drain at the bottom of each that empties into your nose.

Where Is the Frontal Sinus Located?

The frontal sinus is located in the lower part of the forehead, inside the frontal bone, behind the brow ridges and above the eye sockets and the root of the nose. On the skull it lies in the flat, upright part of the frontal bone and can extend backward into the roof of each orbit.

The two frontal sinuses sit side by side, separated by a thin bony partition called the intersinus septum. This septum is rarely in the exact midline, so one sinus is usually larger and may even cross over to the other side. Their size varies more than that of any other sinus:

• some reach high into the forehead and spread out over the eyes;
• some stay small, just above the root of the nose;
• a small minority of people have one frontal sinus missing, or both.

Below and behind each frontal sinus lie the anterior ethmoid air cells, and its drainage channel runs down past them to the front of the middle nasal meatus.

Frontal Sinus Anatomy: Walls, Septum and Lining

The frontal sinus has a thick front wall, a thinner back wall, a floor that forms part of the orbital roof and a bony septum between the two sides. Each wall faces a different neighbor, which matters in injuries and in the rare spread of infection.

Anterior and posterior tables

The front wall, called the anterior table, is the thickest wall and forms the bony forehead under the brow. The back wall, the posterior table, is thinner and separates the sinus from the anterior cranial fossa, where the dura mater and the frontal lobes of the brain lie. The floor is the thinnest part; it forms the inner roof of the orbit and slopes down toward the drainage opening.

Lining, vessels and nerves

Like the other sinuses, it is lined by thin ciliated respiratory epithelium with goblet cells, attached directly to the bone as a mucoperiosteum. Blood comes from the supraorbital and anterior ethmoidal arteries, both branches of the ophthalmic artery. Veins drain into the supraorbital and superior ophthalmic veins and into small diploic veins inside the bone, which connect with veins around the brain. Sensation travels in the supraorbital nerve, a branch of the ophthalmic division of the trigeminal nerve.

Where Does the Frontal Sinus Drain?

The frontal sinus drains into the middle meatus of the nasal cavity, through an opening in its floor called the frontal sinus ostium and a narrow passage below it called the frontal recess. Older textbooks call this passage the frontonasal duct, but in most people it is not a true tube.

The pathway is often compared to an hourglass: the funnel-shaped sinus narrows to the ostium, then widens again into the frontal recess, which opens into the front of the middle meatus. Depending on how a thin ridge of bone called the uncinate process attaches, the recess empties either into the ethmoidal infundibulum or directly into the middle meatus. Cilia move mucus toward the exit by sweeping it up the inner wall, across the roof and down the outer wall to the ostium.

In the middle meatus, frontal mucus joins drainage from the maxillary sinus and the anterior ethmoid cells and is carried back to the nasopharynx under the middle nasal concha.

Frontal sinus anatomy classification

The frontal recess is crowded by small anterior ethmoid cells, and their pattern differs from person to person. Surgeons and radiologists describe them with the International Frontal Sinus Anatomy Classification (IFAC), which sorts them into anterior, posterior and medial cells around the drainage pathway, as the second table shows. An older system, the Kuhn classification, grouped frontal cells into four types.

What Is the Function of the Frontal Sinuses?

The frontal sinuses lighten the front of the skull, add mucus to the nasal lining and form part of the resonating spaces of the voice, the same broad jobs shared by all paranasal sinuses. Their exact purpose is still debated, partly because people with very small or absent frontal sinuses live normal lives.

Frontal sinus function in the respiratory system

In the respiratory system, the frontal sinus is a side chamber of the airway rather than part of the main air path. Air inside it exchanges slowly through the ostium, while its mucus drains into the middle meatus and joins the moist film that warms, humidifies and cleans inhaled air before it reaches the pharynx and the lungs. The cilia that clear it belong to the same mucociliary escalator that protects the whole upper airway.

Other proposed roles are specific to the forehead. The air-filled sinus may act as a crumple zone that absorbs some of the force of a blow to the forehead before it reaches the brain, and its growth helps shape the adult brow ridges, which are usually more prominent in men than in women.

Frontal Sinus Anatomy on CT and X-Ray

On imaging, the frontal sinus appears as a dark, air-filled space in the lower forehead, outlined by the white bone of its anterior and posterior tables. On a plain X-ray of the skull taken from the front, such as the occipitofrontal or Caldwell view, the two sinuses show up above the orbits with scalloped upper edges and the off-center septum between them.

Computed tomography (CT) is the standard way to study frontal sinus anatomy in detail. Coronal and sagittal slices show the thickness of each table, the narrow ostium and the cells crowding the frontal recess, which are named with the classification above. On sagittal images the hourglass shape of the drainage pathway is especially clear.

The outline of the frontal sinuses is so variable that it is considered unique to each person, somewhat like a fingerprint. Forensic experts have compared frontal sinus X-rays taken in life and after death to help identify human remains.

How Does the Frontal Sinus Develop?

The frontal sinus is absent at birth and develops during childhood, making it one of the last paranasal sinuses to form. It starts as a small outgrowth from the front of the ethmoid region, near the future frontal recess, and slowly pushes up into the frontal bone.

It usually becomes visible on X-rays during the early school years and keeps enlarging through adolescence, reaching adult size after puberty. Because it develops late, frontal sinus infections are uncommon in young children, whose ethmoid and maxillary sinuses are already present. How far the sinus grows differs widely between people, which explains the large range of adult sizes and shapes. Timing for all four sinuses is outlined on the development of the respiratory system page.

Common Conditions That Affect the Frontal Sinus

The frontal sinus can be affected by inflammation, blocked drainage, injuries and growths. These are described for education only; the respiratory diseases page covers related conditions.

• Frontal sinusitis: inflammation of the lining, often felt as pressure or pain in the forehead above the eyes. See sinusitis.
• Mucocele: a slowly growing, mucus-filled sac that forms when the drainage pathway is blocked. The frontal sinus is its most common site, and it can thin the surrounding bone and push on the eye.
• Osteoma: a benign bony growth found most often in the frontal sinus, usually by chance on a scan.
• Fractures: a hard blow to the forehead can break the anterior table, and more severe injuries can involve the posterior table or the drainage pathway.
• Rare spread of infection: because its veins connect with veins inside the skull and its back wall borders the brain, a frontal sinus infection can, rarely, spread to the bone or to the coverings of the brain.

Marked forehead swelling, changes around the eye, or headache with fever are reasons to see a doctor promptly.