Frontal Sinus: Structure, Function & Diseases

The frontal sinus is one of the sinuses. It is located under the frontal bone, at the level of the eyebrows or slightly above. The frontal sinus is lined with mucosa and has the function of ventilating the respiratory system as well as heating, humidifying and purifying the air we breathe.

What characterizes the frontal sinus?

The frontal sinus is a bony cavity in the skull, located in the frontal bone (Os frontale) above the root of the nose. Its interior is covered with mucosa. The frontal sinus occurs in pairs and belongs to the paranasal sinuses, which are connected to the nasal cavity. It is a part of the respiratory system. Since it is very variable in shape and size and is created individually in each person, it is also called the fingerprint of the skull. The frontal sinus is not yet present at birth; it only forms later. From the eruption of the permanent teeth until the end of puberty is the period of the strongest growth. Only when the growth in size of the body is complete, the frontal sinus has reached its final dimensions.

Anatomy and structure

The two frontal sinuses in the skull bone are air-filled cavities. That is why they are called pneumatization spaces (from the old Greek pneuma = air). They are completely lined with mucosa, the respiratory ciliated epithelium. This consists, among other things, of mucus-producing goblet cells and kinocilia. The latter distribute the mucus toward the pharynx by ciliated movements. In addition to the connection between the frontal sinus and the other paranasal sinuses, there is also a channel to the nasal cavity, the sinus passage. The size of the frontal sinus varies from person to person, and its shape is not always the same. Thus, in Europeans, a distinction is made between four different forms. The mitral form is the most common variant, while the bean form is rather rare. The leaf shape and the pyramidal shape are roughly in between in terms of frequency. The frontal sinus is usually asymmetrical and its two portions are separated by the septum interfrontale, a cartilaginous septum that is not centrally located in most people. The frontal sinus is bounded by the roof of the orbit and by the anterior fossa, a depression in the base of the skull.

Function and tasks

The evolutionary origin of the paranasal sinuses, including the frontal sinus, is attributed to the fact that this allows the skull bone to reach an appropriate size without being burdened by excessive weight. Originally, it was assumed that the sinuses were involved in voice production by serving as resonant cavities. However, various experiments showed that this assumption was incorrect. Important functions of the sinus are humidification, warming and purification of the air we breathe. During inhalation, the air flows through the frontal sinus and the other sinuses. This allows microscopic foreign bodies and microorganisms to enter. The ciliated epithelium lining the frontal sinus consists of various glands and cells. The so-called goblet cells are glands responsible for the formation of mucus and are located between the other cells of the ciliated epithelium. Under the microscope, it can be seen that this mucosa looks like a lawn with constantly moving – ciliated – blades of grass. The goblet cells continuously produce mucus and release it to the surface. The kinocilia ensure that this mucus is transported further. This removes foreign bodies and microorganisms from the respiratory tract. They are pushed with the mucus toward the pharynx and can then drain away. This self-cleaning mechanism of the ciliated epithelium is called mucociliary clearance (MCC). In addition to cleaning the air we breathe, the sinus serves to warm and humidify the air. As the air we breathe passes through the sinuses, its temperature is warmed to that of the body. This is why it is so important to breathe through the nose when the outside temperature is cold, so that cold air does not enter the lungs. Another task is humidification. The ongoing release of mucus in the sinus increases the humidity of the air. If the breath is too dry, this irritates the mucous membranes and can lead to infections.

Diseases and complaints

The most common disease of the frontal sinus is sinusitis frontalis, or sinusitis. It occurs in both acute and chronic forms. Often not only the frontal sinus is affected by the inflammation, but also other paranasal sinuses.Acute sinusitis usually develops as a result of a cold. The inflammation of the mucous membranes of the nasal cavity spreads to the frontal sinus or other sinuses. The mucous membrane swells and can no longer fulfill its task of moistening, warming and purifying the air breathed. Mucus accumulates and, in the case of bacterial inflammation, also pus (frontal sinus empyema). As a result, the frontal sinus is blocked and the mucus can no longer drain away. The accumulation creates pressure that manifests itself in a headache. A good way to tell if the sinus is inflamed is a pressing headache when the upper body is bent down. Lowering the head increases the pressure on the sinuses, which causes throbbing pain that is clearly noticeable in the forehead area if inflammation is present. In chronic sinusitis, the inflammation recurs and does not heal, while the acute form occurs only for a short time. In some people, the frontal sinus forms insufficiently (“frontal sinus hypoplasia”), resulting in more frequent inflammation. Injuries caused by great force in the forehead area can lead to fracture of the frontal bone. If the posterior wall of the frontal sinus also breaks in the process, inflammation of the brain structures is possible.