The nasal cavity and paranasal sinuses

Radiology of the bony orbit

Plain films The orbits may be assessed on OF20 and OM projections (see Figs 1. 4 and 1. 8). Asymmetry between the superior orbital fissures is common. The fissure separates the greater wing of sphenoid (below) from the lesser wing (above).

A straight line is seen running through the orbit from the superolateral part of the r im inferiorly and medially. This is caused by the X-ray beam hitting the curving greater wing of sphenoid at a tangent, and is known as the innomi-

nate line.

Owing to its oblique course through the skull base, a specially angulated radiographic projection is required to demonstrate each optic foramen. The normal optic foramen is less than 7 mm in diameter. The dimensions of the foramen should not vary by more than 1 mm. There may, however, be a separate opening for the ophthalmic artery below the foramen, or the foramen may have a keyhole configuration if the foramina are not completely separate. The optic canal is related to the sphenoid sinus and an axial view of this is part of assessment for sinus surgery.

The floor of the orbit and the infraorbital canal are best seen on the OM (see Fig. 1. 8) and OM30 projections.

Computed tomography The bony orbit and its soft-tissue contents are demonstrated very well by CT (see Fig. 1. 29). Axial or coronal images may be obtained. Coronal imaging shows the floor of the orbit and is useful for the assessment of trauma where a fracture is suspected. MRI is more valuable for demonstration of the soft-tissue contents of the orbit than the bone.

THE NASAL CAVITY AND PARANASAL SINUSES

(Figs 1. 8, 1. 11 and 1. 12) The nasal cavity is a passage from the external nose anteriorly to the nasopharynx posteriorly. The frontal, ethmoid, sphenoid and maxillary sinuses form the paired paranasal sinuses and are situated around, and drain into,

1. Nasal septum 8. Ethmoid infundibulum 2. Maxillary sinus 9. Uncinate process 3. Middle nasal turbinate 10. Maxillary ostium 4. Inferior nasal turbinate 11. Maxillary infundibulum 5. Superior meatus 12. Infraorbital nerve G. Middle meatus 13. Alveolar process of maxilla 7. Inferior meatus

the nasal cavity. The entire complex is lined by mucussecreting epithelium.

The nasal cavity

This is divided in two by the nasal septum in the sagittal plane. The nasal septum is part bony and part cartilaginous. The floor of the nasal cavity is the roof of the oral cavity and is formed by the palatine process of the maxilla, w i th the palatine bone posteriorly. The lateral walls of the cavity are formed by contributions from the maxillary, palatine, lacrimal and ethmoid bones. These walls bear three curved extensions known as turbinates or conchae, which divide the cavity into inferior, middle and superior meati, each lying beneath the turbinate of the corresponding name. The space above the superior turbinate is the sphenoethmoidal

recess.

• The sphenoid air cells drain into the sphenoethmoidal recess. • The posterior group of ethmoidal air cells drain into the superior meatus. • The frontal sinus opens in the most anterior opening of the middle meatus. The anterior ethmoidal air cells and maxillary sinus drain into the middle meatus at the hiatus semilunaris, below the ethmoid bulla. • The nasolacrimal duct opens into the inferior meatus, draining the lacrimal secretions.

Blood supply of the nasal cavity The sphenopalatine artery is the terminal part of the maxillary artery. It passes w i th its associated nerves through the sphenopalatine foramen from the pterygopalatine fossa to the nasal cavity posterior to the superior meatus. It has medial branches to the nasal septum and lateral branches to the lateral wall of the nose and turbinates.

The greater palatine artery supplies some of the lower part of the nasal cavity by branches that pass through the incisive foramen in the anterior part of the hard palate.

The superior labial branch of the facial artery supplies some branches to the anteroinferior part of the nasal septum and the nasal alae.

Anterior and posterior ethmoidal branches of the oph-

thalmic artery from the internal carotid artery pass through the cribriform plate to supply the superior part of the nasal cavity.

Little's area is a vascular region of mucosa in the anterior and inferior part of the nasal septum supplied by branches of the sphenopalatine, greater palatine and facial arteries. This is a common site of anterior epistaxis.

The paranasal sinuses

The frontal sinuses These lie between the inner and outer tables of the frontal bone above the nose and medial part of the orbits; they vary greatly in size and are often asymmetrical. They may extend into the orbital plate of the frontal bone.

The ethmoid sinuses These consist of a labyrinth of bony cavities or cells situated between the medial walls of the orbit and the lateral walls of the upper nasal cavity. Enlargements of anterior cells towards the frontal bone are called agger nasi cells, and enlargements of posterior cells below the apex of the orbit are known as Haller's cells.

The sphenoid sinuses These paired cavities in the body of the sphenoid are often incompletely separated from each other, or may be subdivided further into smaller bony cells. They are so closely related to the ethmoid air cell anteriorly that it may be difficult to distinguish a boundary. The anatomical relationships of the sphenoid sinus are of considerable importance. The sella turcica, bearing the pituitary gland w i th the optic chiasm anteriorly, is superior. The cavernous sinus and contents run along its lateral walls. The floor of the sphenoid sinus forms the roof of the nasopharynx (see Fig. 2. 11, pituitary gland).

The maxillary sinuses The maxillary sinuses, or antra, are the largest of the paranasal sinuses. They are sometimes described as having a body and four processes.

The processes comprise: (i) the orbital process, which extends superomedially to contribute to the medial rim of the orbit; (ii) the zygomatic process, which is continuous w i th the zygomatic arch; (iii) the alveolar process, which bears the teeth; and (iv) the palatine process, which forms the roof of the mouth and floor of the nasal cavity.

The body of the maxilla is roughly pyramidal in shape, w i th its apex projecting superomedially between the orbit and nasal cavity. It houses the maxillary sinus. It has an anterior surface that is directed downward and laterally and forms part of the contour of the cheek. It has a curved infratemporal or posterior surface, and this also forms the anterior wall of the infratemporal fossa. Its orbital or superior surface is smooth and triangular and separates the sinus from the orbital cavity. The nasal or medial surface forms the lateral wall of the lower part of the nasal cavity, on to whose middle meatus the sinus drains. The medial wall of the sinus is continued superiorly as a bony projection known as the uncinate process. The maxillary ostium opens superiorly into the infundibulum, which is the channel between the inferomedial aspect of the orbit laterally and the uncinate process medially. The region of the ostium, infundibulum and middle meatus is important clinically and is known as the ostiomeatal

complex.