Paranasal sinuses

The air spaces located inside the bone are called the paranasal sinuses (sinuses paranasales). They are lined with the respiratory epitheliums which usually consist of pseudostratified columnar ciliated epithelium. The direction of flow created by the ciliary beat is toward the ostium. During inspiration negative pressure is created which  helps to facilitate drainage of the:

  • Maxillary sinus (sinus maxillaris)
  • Frontal sinus (sinus frontalis)
  • Sphenoid sinus (sinus sphenoidalis)
  • Ethmoidal labyrinth (labirynthus ethmoidalis)
  • Mastoid air cells (cellulae mastoideae)

Paranasal sinuses have the following functions:

  • Decreasing of the weight of the skull.
  • Shock absorbing.
  • Resonator of the voice.
  • Warming and humidification of air.
  • Regulation of intranasal pressure and serum gas pressures.
  • Immune defense.

The maxillary sinusis the biggest paranasal sinus and it is located in the body of the maxilla. It is 24 cm3 in size and it secretes serous fluid that is drained by the semilunar hiatus and narrow ethmoidal infundibulum to the middle nasal meatus. The ostium of maxillary sinus is located on the medial wall of the sinus on its upper level which is not a good location to achieve drainage. It also communicates with the infratemporal fossa through the alveolar canals and has a close relationship to the posterior alveolar nerve and infraorbital nerve (superiorly). They may be dehiscent.

Clinical comments

Sinusitis is an inflammation of the paranasal sinuses. It occurs when hypersecretion of mucous fluid collects and the orifices from the sinuses to the nose are blocked (obstruction). Maxillary sinusitis is possible after extraction of the 3rd molars of maxilla. Their roots may be located within the sinus. The infection may extend to the cavernous sinus through the infraorbital vein. The floor of the maxillary sinus in children is usually on the level of nasal cavity so the trocar should not be directed inferiorly during sinus penetration. In adults, the floor of the maxillary sinus is located below the level of nasal cavity at the most inferior level.

The frontal sinus (sinus frontalis) is 5-20 cm3 in size. It secretes serous fluid that is drained into the middle nasal meatus through the frontal recess, and more rarely through the ethmoidal infundibulum. It is absent at birth. The frontal sinus has the:

  • Opening of the frontal sinus.
  • Septum of the frontal sinuses.

Clinical comments

Frontal sinusitis is clinically important because of its connections with the orbit (inferiorly) and anterior cranial fossa (posteriorly). Infections may cause orbital epidural abscess and meningitis. The frontal sinus is innervated by the supraorbital branch of the ophthalmic nerve and may cause pain in the supraorbital region of the forehead.

The sphenoid sinus (sinus sphenoidalis) is located in the body of the sphenoid bone. It is 3 cm3 in size and it secretes serous fluid that drains into the sphenoethmoidal recess of the nasal cavity (superior meatus). The sphenoethmoidal recess is located between the anterior surface of the sphenoid body and superior and supreme nasal concha. The sphenoidal sinuses contain the:

  • Opening of sphenoidal sinus
  • Septum of sphenoidal sinuses.

It lies adjacent to pituitary gland, cavernous sinus, internal carotid artery, middle cranial fossa, optic, and the abducent nerve.

Clinical comments

The infection of sphenoid sinusmay cause cavernous sinus thrombosis, abducent nerve palsy and meningitis.

The ethmoidal labyrinth is 10 cm3 in size. It consists of the anterior, middle and posterior ethmoidal cells. Its serous secretion is drained from the posterior ethmoidal cells into the sphenoethmoidal recess, to the superior or supreme meatus of the nasal cavity and from the anterior, middle ethmoidal cells into the middle nasal meatus.

Clinical comments

The ethmoidal infectionsmay cause retrobulbar neuritis of the optic nerve.

The mastoid air cells (cellulae mastoideae)are located in the mastoid process. They communicate with the middle ear.