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Sinus

From Wikipedia, the free encyclopedia

Sinus (anatomy)

Sinus is Latin for “bay”, “pocket”, “curve”, or “bosom”. In anatomy, the term is used in various contexts.

A sinus is a sack or cavity in any organ or tissue, or an abnormal cavity or passage caused by the destruction of tissue. In common usage, “sinus” usually refers to the paranasal sinuses, which are air cavities in the cranial bones, especially those near the nose and connecting to it.

The term is also used for a chronically infected tract such as a passage between an abscess and the skin. It is however distinct from a fistula, which is a tract connecting two epithelialised surfaces. It can also affect the chest and lungs.

Sinusitis

The sinuses can become inflamed, which leads to an infection called sinusitis. Sinusitis is an inflammation of the sinuses and nasal passages (WebMD). This inflammation causes pressure. The pressure is often experienced in the cheek area, eyes, nose, on one side of the head, and can result in a severe headache.

According to WebMD, the human skull contains 4 major pairs of hollow air-filled sacks called sinuses that connect the space between the nostrils and the nasal passage. These sinuses reduce the skulls weight, help insulate the skull, and allow the voice to resonate within it (WebMD). These sinuses are:

  • Frontal sinuses (in the forehead)
  • Maxillary sinuses (behind the cheek bones)
  • Ethmoid sinuses (between the eyes)
  • Sphenoid sinuses (behind the eyes)

Sinusitis is either acute, which is usually the result of a viral infection in the upper respiratory tract, or chronic. Allergens, which are allergy causing substances, can also cause acute sinusitis. A person with this infection often has nasal congestion with thick nasal secretions, fever, and cough (WebMD). Patients can be treated by “reducing the swelling or inflammation in the nasal passages and sinuses, eliminating the infection, promoting drainage from the sinuses, and maintaining open sinuses” (WebMD). Sinusitis can be treated with medications and can also be eliminated by surgery.

The cause of sinus infection is bacteria in the lining cells of the sinuses. The bacteria, that is in the nasal passage, somehow enters the sinuses and results in inflammation. Normally the sinuses contain defenses against bacteria, germs, and other foreign viruses. “Bacteria that normally cause acute sinusitis are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis (WebMD) These microorganisms, along with Staphylococcus aureus and some anaerobes (bacteria that live without oxygen), are involved in chronic sinusitis (WebMD).” Fungi can also cause chronic sinusitis.

Outlook on Sinusitis

Sinusitis or sinus infections usually clear up if treated early and appropriately. Aside from those who develop complications, the outlook for acute bacterial sinusitis is good. People may develop chronic sinusitis or have recurrent attacks of acute sinusitis if they have allergic or structural causes for their sinusitis.

 

Paranasal sinuses

Paranasal sinuses are a group of four paired air-filled spaces, that surround the nasal cavity (maxillary sinuses), above and between the eyes (ethmoid sinuses and frontal sinuses), and behind the ethmoids (sphenoid sinuses). The sinuses are named for the facial bones behind which they are located.

Humans possess a number of paranasal sinuses, divided into subgroups that are named according to the bones within which the sinuses lie:

  • the maxillary sinuses, also called the maxillary antra and the largest of the paranasal sinuses, are under the eyes, in the maxillary bones.
  • the frontal sinuses, superior to the eyes, in the frontal bone, which forms the hard part of the forehead.
  • the ethmoid sinuses, which are formed from several discrete air cells within the ethmoid bone between the nose and the eyes.
  • the sphenoid sinuses, in the sphenoid bone at the center of the skull base under the pituitary gland.

The paranasal air sinuses are lined with respiratory epithelium (ciliated pseudostratified columnar epithelium).

Development

Paranasal sinuses form developmentally through excavation of bone by air-filled sacs (pneumatic diverticula) from the nasal cavity. This process begins prenatally, and it continues through the course of an organism’s lifetime.

Ailments

For more details on this topic, see Sinusitis.

The paranasal sinuses are joined to the nasal cavity via small orifices called ostia. These become blocked easily by allergic inflammation, or by swelling in the nasal lining which occurs with a cold. If this happens, normal drainage of mucus within the sinuses is disrupted, and sinusitis may occur.

These conditions may be treated with drugs such as pseudoephedrine, which causes vasoconstriction in the sinuses, reducing inflammation, by traditional techniques of nasal irrigation, or by corticosteroid.

Malignancies of the paranasal sinuses comprise approximately 0.2% of all malignancies. About 80% of these malignancies arise in the maxillary sinus. Men are much more often affected than women. They most often occur in the age group between 40 and 70 years. Carcinomas are more frequent than sarcomas. Metastases are rare. Tumours of the sphenoid and frontal sinuses are extremely rare.

Maxillary sinus

The maxillary sinus (or antrum of Highmore) is the largest of the paranasal sinuses, and is pyramidal in shape.

General characteristics

Found in the body of the maxilla, this sinus has three recesses: an alveolar recess pointed inferiorly, bounded by the alveolar process of the maxilla; a zygomatic recess pointed laterally, bounded by the zygomatic bone; and an infraorbital recess pointed superiorly, bounded by the inferior orbital surface of the maxilla. The medial wall is composed primarily of cartilage. The ostia for drainage are located high on the medial wall and open into the semilunar hiatus of the lateral nasal cavity; because of the position of the ostia, gravity cannot drain the maxillary sinus contents when the head is erect. The sinus is lined with mucoperiosteum, with cilia that beat toward the ostia. This membrane is also referred to as the “Schneiderian Membrane”, which is histologically a bilaminar membrane with ciliated columnar epithelial cells on the internal (or cavernous) side and periosteum on the osseous side. The size of the sinuses varies in different skulls, and even on the two sides of the same skull.

The infraorbital canal usually projects into the cavity as a well-marked ridge extending from the roof to the anterior wall; additional ridges are sometimes seen in the posterior wall of the cavity and are caused by the alveolar canals.

The mucous membranes receive their postganglionic parasympathetic nerve innervation for mucous secretion originating from the greater petrosal nerve (a branch of the facial nerve). The superior alveolar (anterior, middle, and posterior) nerves, branches of the maxillary nerve provide sensory innervation.

Nasal wall/base

Its nasal wall, or base, presents, in the disarticulated bone, a large, irregular aperture, communicating with the nasal cavity.

In the articulated skull this aperture is much reduced in size by the following bones:

  • the uncinate process of the ethmoid above,
  • the ethmoidal process of the inferior nasal concha below,
  • the vertical part of the palatine behind,
  • and a small part of the lacrimal above and in front.

The sinus communicates through an opening into the semilunar hiatus on the lateral nasal wall.

Posterior wall

On the posterior wall are the alveolar canals, transmitting the posterior superior alveolar vessels and nerves to the molar teeth.

Floor

 

The maxillary sinus can normally be seen above the level of the premolar and molar teeth in the upper jaw. This dental x-ray film shows how, in the absence of the second premolar and first molar, the sinus became pneumatized and expanded towards the crest of the alveolar process (location at which the bone meets the gum tissue).

The floor is formed by the alveolar process of the maxilla, and, if the sinus is of an average size, is on a level with the floor of the nose; if the sinus is large it reaches below this level.

Projecting into the floor of the antrum are several conical processes, corresponding to the roots of the first and second molar teeth; in some cases the floor is perforated by the apices of the teeth.

Pathology

Maxillary Sinusitis

Maxillary sinusitis is inflammation of the maxillary sinuses.

Maxillary sinusitis is common due to the close anatomical relation of the frontal sinus, anterior ethmoidal air sinus and the maxillary teeth, allowing for easy spread of infection. Furthermore, the drainage orifice lies near the roof of the sinus, and so the maxillary sinus does not drain well, and infection develops more easily.

Treatment

The treatment of acute maxillary sinusitis is usually perscription of a broad-spectrum cephalosporin antibiotic resistant to beta-lactamase, administered for 10 days.

Frontal sinus

Sinuses are mucosa-lined airspaces within the bones of the face and skull. The frontal sinuses, situated behind the superciliary arches, are absent at birth, but are generally fairly well developed between the seventh and eighth years, only reaching their full size after puberty. The frontal bone is membranous at birth and there is rarely more than a recess until the bone tissue starts to ossify about age two. Consequently this structure does not show on radiographs before that time. Frontal sinuses are rarely symmetrical and the septum between them frequently deviates to one or other side of the middle line. Sinus development begins in the womb, but only the maxillary and ethmoid sinuses are present at birth. Approximately 5% of people have absent frontal sinuses.[1]

Their average measurements are as follows: height 28 mm, breadth 24 mm, depth 20 mm, creating a space of 6-7 ml.[2]

Each opens into the anterior part of the corresponding middle meatus of the nose through the frontonasal duct which traverses the anterior part of the labyrinth of the ethmoid. These structures then open into the hiatus semilunaris in the middle meatus.

The mucous membrane in this sinus is innervated by the supraorbital nerve, which carries the postganglionic parasympathetic nerve fibers for mucous secretion from the ophtalmic nerv and supplied by the supraorbital artery and anterior ethmoidal artery.

Through its copious mucus production, the sinus is an essential part of the immune defense/air filtration carried out by the nose. Nasal and sinal mucosae are ciliated and move mucus to the choanae and finally to the stomach. The thick upper layers of nasal mucus trap bacteria and small particles in tissue abundantly provided with immune cells, antibodies, and antibacterial proteins. The layers beneath are thinner and provide a substrate in which the cilia are able to beat and move the upper layer with its debris through the ostia toward the choanae.

Inflammations of the frontal sinus can give rise to serious complications, as it is in close proximity to the orbit and cranial cavity (orbital cellulitis, epidural and subdural abscess, meningitis).

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