Radiography

Many different radiographic views are used to demonstrate individual structures or regions within the skull. The basic views are the lateral, dorsoventral or ventrodorsal, left and right lateral oblique, rostrocaudal, and occlusal (intraoral). General anesthesia is recommended for all skull radiographs. Obtaining well-positioned radiographs of diagnostic quality is difficult or impossible with sedation only. For intraoral, rostrocaudal, and open-mouth views, anesthesia is required.

Cross-sectional imaging in three planes using computed tomography (CT) and magnetic resonance imaging (MRI), where available, is a superior technique in localizing and defining bone and soft tissue pathology.

Lateral View

The patient is placed in lateral recumbency. A foam wedge is placed under the animal’s nose and mandible so that the sagittal plane of the skull is parallel to the tabletop. The beam is centered midway between the ear and the eye, dorsal to the zygomatic arch (LeRtL or RtLeL). The jaws should be opened if the temporomandibular joints are the areas of interest (Figure 5-1, A, B, and I).

Figure 5-1 Normal skull. A and B, Lateral view. Normal skull. C and D, Ventrodorsal view. E, Dorsoventral view. F, Oblique. Made in right lateral recumbency, the oblique view demonstrates the right temporomandibular joint (black arrow). The osseous bulla (open arrow) is more clearly visible than on the true lateral view. Normal skull. G and H, Rostrocaudal views. G, Normal frontal sinus in a dog. This is a rostrocaudal skyline view. Both frontal sinuses are air filled. The frontal bone is of uniform opacity and has a slightly scalloped inner margin. H, Open-mouth view showing the osseous bullae (arrows). I and J, Lateral and ventrodorsal views of the skull of a cat. Normal skull. K, Normal tympanic bulla—oblique view. Note the thin wall and the air-filled cavity (arrow). L, An occlusal view of the maxilla. M, Normal tympanic bullae in a cat. This is a rostrocaudal open-mouth view. The bullae have thin, well-defined, bony walls. Feline bullae have medial and lateral compartments that are separated by a thin wall of bone. An endotracheal tube is seen superimposed on the skull between the two bullae. Normal skull. N, Normal tympanic bullae in a cat. This is a rostrocaudal view with the mouth closed and the head tilted 10 to 15 degrees dorsally (backward) from the vertical plane. The x-ray beam is centered just ventral to the mandibular symphysis. The bullae are projected ventral to the mandibles. O, Normal tympanic bullae in a dog. On this transverse CT image, the two tympanic bullae are seen as thin-walled bony structures containing air ventral to the petrous temporal bones of the caudal skull. Air is seen within the nasopharynx between the bullae and dorsal to the endotracheal tube. The two small round bony structures ventral and lateral to the endotracheal tube are parts of the hyoid apparatus.

Ventrodorsal View

The animal is placed in dorsal recumbency. A radiolucent block is placed under the neck behind the skull. The occipitoatlantal articulation is extended so that the hard palate lies parallel to the film. The x-ray beam is centered between the eyes and the ears on the midline. The sinuses are more clearly seen on this view than on the dorsoventral view, although it is more difficult to achieve symmetry in the ventrodorsal position. This view is best for demonstrating the cranium because the calvarium is closer to the film (Figure 5-1, C and D).

Intraoral film or a film-screen combination in a flexible envelope can be used to demonstrate the mandibles. The ventrodorsal position can be used with an intraoral film to demonstrate the mandibular incisor teeth. The x-ray beam is directed rostrocaudally at an angle of 20 degrees (V20°R-DCdO). With the animal’s mouth opened wide, the maxilla parallel to the tabletop, and the same beam angle centered on the maxilla, the nasal and ethmoid regions can be demonstrated.

Dorsoventral View

The animal is placed in sternal recumbency with the head resting on the cassette so that the hard palate lies parallel to the tabletop. This position can be maintained by a bandage passed across the neck, behind the skull, and fixed to the table. On occasion it may be easier to position the head if the cassette is raised off the tabletop on a support. The x-ray beam is centered between the eyes and the ears on the midline. It is easier to achieve symmetry in this position than it is in the ventrodorsal position, but the calvarium is farther from the film and is therefore more distorted on this view. However, this is of no practical significance (Figure 5-1, E).

The maxillary nasal turbinates can also be examined in this position by introducing the intraoral film corner first and placing it as far back in the mouth as possible. Alternatively, a high-detail single-screen film combination in a plastic lightproof envelope can be used (Figure 5-1, L). The x-ray beam is centered over the nasal septum. This position can be used with intraoral film to demonstrate the maxillary incisor teeth. The x-ray beam is directed rostrocaudally at an angle of 20 degrees (D20°R-VCdO).

Oblique Views

Oblique views enable some structures to be demonstrated without superimposition of the contralateral side. Oblique views are used to study the temporomandibular joints, the osseous bullae, the frontal sinuses, and the dorsal edge of the orbit. In the open-mouth position, the maxillary and mandibular dental arcades are profiled by using oblique views. The structure profiled varies with the oblique study selected (Figure 5-1, F and K).

Specific oblique views for various anatomic and pathologic abnormalities are discussed in subsequent sections.

Rostrocaudal (Frontal) View

The patient is placed in dorsal recumbency, and the neck is flexed so that the hard palate lies perpendicular to the film. The patient’s head is held in position with a bandage or tape around its nose. The beam is directed at right angles to the tabletop, along the line of the hard palate and centered between the eyes (RCd). The frontal sinuses, the odontoid process or dens, and the foramen magnum can be demonstrated on this view. In cats, a similar technique can be used to demonstrate the tympanic bullae, which lie ventral to the mandibles. The patient is placed in dorsal recumbency, and the head is flexed. Rather than being oriented with the hard palate perpendicular to the tabletop, the head is tilted slightly dorsal so that the hard palate is at an angle of approximately 70 to 80 degrees to the tabletop and the beam centered just ventral to the mandibular symphysis (Figure 5-1, N). This frontal view, with the animal’s mouth opened, can be modified to demonstrate the osseous bullae and temporomandibular joints, in which case the beam is directed rostrocaudally at an angle of 20 to 30 degrees to the hard palate (Ro20°V-CdDO). By varying the angles of the hard palate to the tabletop, the frontal view can also be used to outline the calvarium (Figure 5-1, G and M; also see Figure 5-13, C and F).

Caudorostral View

With the animal in sternal recumbency, the head is supported above the level of the neck with the hard palate parallel to the tabletop. A horizontal beam, directed caudorostrally, is used parallel to the tabletop and centered on the skull with the cassette placed in front of the animal’s nose. The frontal sinuses are profiled. An advantage of the caudorostral view is that it will demonstrate fluid levels in the frontal sinuses.

Fractures

Skull fractures in dogs are not very common except for fractures of the mandibles. The superimposition of bones makes fractures difficult to demonstrate. Lateral oblique views are useful in outlining the mandibular rami. The mandibular symphysis is frequently the site of a separation injury in cats that have fallen from a height (Figure 5-2, G). Fractures involving the calvarium, frontal bones, or nasal bones are often depression fractures, and lesion-oriented oblique (tangential) views of the area may be necessary to demonstrate them. Overriding of fracture fragments may cause a linear opacity, whereas a fracture with displaced ends causes a linear radiolucent defect. Soft tissue swelling is often present. Fractures may be the cause of focal cranial nerve dysfunction. Fractures involving the nasal or frontal bones may be accompanied by hemorrhage into the frontal sinus or nasal cavity. The hemorrhage causes a soft tissue opacity within the air-filled cavity (Figure 5-2).

Figure 5-2 Fractures. A, Fracture of the rostral mandibles and maxillae. There is also a fissure fracture (arrow) of the nasal bone. B and C, Fracture of the horizontal ramus of the mandible. The displacement in the lateral plane is seen as a step defect (arrow) on the ventrodorsal view. D, Fracture of the right zygomatic arch. The temporozygomatic suture, which is a late-closing suture, should not be mistaken for a fracture line. E, Depressed fracture of the nasal bone (arrows). The rostral mandible is also fractured. A dorsoventral view of the skull (F) and a ventrodorsal intraoral view of the mandibular symphysis (G) of a cat that had fallen from a height. The mandibular symphysis is separated and minimally displaced. This was not visible on the dorsoventral view.

Suture lines should not be mistaken for fractures. In the dog and cat, suture lines close within a few weeks after birth, although in some small breeds the suture lines may remain open permanently, as may the fontanelles (areas of unossified tissue found at the junction of several suture lines). In these breeds, such as the Maltese and Chihuahua, no frontal sinus may be evident.

Temporomandibular Dislocation

The temporomandibular joint is formed by the articulation between the condyle of the mandible and the mandibular fossa of the squamous part of the temporal bone. The rostral surface of the retroglenoid process forms part of the mandibular fossa.

Diagnosis of dislocation of the temporomandibular joint can be problematic radiologically. The following views are useful:

Figure 5-3 A, Normal temporomandibular joint. B and C, Dislocation of the right temporomandibular joint. The displaced mandibular condyle (straight arrows) can be seen both on the oblique and ventrodorsal views. The right external auditory canal is almost obliterated because of swelling in that area (curved arrow). The left mandible is fractured (open arrow). D, Temporomandibular luxation in a cat. The right mandible is luxated rostrally. The right mandibular condyle is rostral to the mandibular fossa of the temporal bone (arrows), with which it should articulate. The left temporomandibular joint appears normal. (D, Courtesy Dr. W. H. Rhodes.) E and F, A 3-year-old Golden Retriever presented with pain on opening its jaw. Lateral oblique radiographs of the right temporomandibular joint (arrows) in the open- and closed-mouth positions. E, The temporomandibular joint is normal on the closed study. F, On the open-mouth study the joint is subluxated. Diagnosis: temporomandibular subluxation.

Traumatic luxation (Figure 5-3, D) of the temporomandibular joint is not very common in dogs. It may be associated with mandibular fracture, particularly in cats. Clinically, affected animals hold the mouth open, and manipulation of the mandible is painful. The dislocation is usually unilateral. The displaced mandibular condyle can be seen radiographically on the appropriate view. It is seen to be displaced rostrally and dorsally from the retroglenoid (retroarticular) process. If the dislocation is unilateral, comparison with the opposite side is helpful. The mandibles are displaced toward the normal side. There may be an associated fracture. If the retroglenoid process is fractured, the condyle may displace caudally.

Temporomandibular Dysplasia

Temporomandibular dysplasia is a congenital condition described in young red Irish Setters and Bassett Hounds and some other breeds. The mandibular fossa is shallow. If the mouth is fully opened, subluxation of the temporomandibular joint may occur. The coronoid process of the mandible on one side becomes fixed lateral to the zygomatic arch, resulting in an open-mouth locked jaw. Clinically this occurs when the animal yawns or grabs a moving object. In some cases the condition resolves spontaneously. Other patients are treated surgically by removing part of the zygomatic arch or part of the coronoid process of the mandible. The condition is best demonstrated on open- and closed-mouth oblique studies.

Temporomandibular Arthrosis

Osteoarthrosis of the temporomandibular joint is occasionally seen. It may be the result of trauma, or it may be a sequela to conditions such as craniomandibular osteopathy, otitis media, or temporomandibular dysplasia. Periarticular osteophytes are seen on lateral oblique and dorsoventral views. Narrowing of the joint space may also be seen.

Foreign Bodies

Radiopaque foreign bodies are readily seen within the skull. They are usually located in the mouth, pharynx, or nasal chambers. Radiolucent bodies may require contrast medium to outline them (Figures 5-4 and 5-5; see Figure 5-11).

Figure 5-4 A and B, Foreign body. A needle in the pharyngeal region of a cat.

Figure 5-5 A and B, Foreign body. An air gun pellet within the skull shadow of a cat.

Infection

Any of the bones of the skull may become infected. However, infection is uncommon except as an extension from infection in a nasal chamber, frontal sinus, or tooth root or as a result of direct trauma.

Neoplasia

Primary neoplasia of skull bones is not common. The dog is more often affected than the cat. Osteosarcoma may affect any of the bones of the skull. Its appearance is that of a destructive lesion, usually accompanied by a profuse and aggressive periosteal reaction. If the tumor is superficial, there will be an associated soft tissue swelling. Proliferative changes and sclerosis are more prominent than are erosive changes when the cranial vault is involved. Other neoplasms, such as fibrosarcoma, chondrosarcoma, and osteochondroma, are occasionally seen.

Differentiation of primary bone tumors is not easy. Osteomas are occasionally seen and are benign. They appear as dense and circumscribed areas provoking little if any adjacent reaction. Multiple myeloma has been reported in the dog, showing the typical punched-out lesions described in human patients. Soft tissue tumors, such as squamous cell carcinoma, malignant melanoma, and fibrosarcoma, frequently invade and destroy adjacent skull bones. Metastases to regional lymph nodes are common with squamous cell carcinoma. Malignant melanoma may metastasize early to lymph nodes and the lungs.

Fibrosarcoma rarely metastasizes. Exophthalmos may occur as a result of displacement by a neoplasm when it is in the vicinity of the orbit. Teeth may be lost or displaced. Biopsy is needed for a definitive diagnosis.

An effort should be made to rotate the skull for a lesion-orientated view so that the mass lesion is as near to the film as possible and there are few superimposed structures (Figure 5-6).

Figure 5-6 Neoplasia. A, Gross destruction of the frontal and nasal bones from invasion by a squamous cell carcinoma. The tumor originated in the caudal nares. B, Destruction of the frontal, nasal, and maxillary bones as a result of osteosarcoma. C, The right fourth premolar and the first molar tooth of the upper jaw are displaced as a result of invasion of the maxilla by a carcinoma originating in the mouth. D, This 8-year-old cross-bred Terrier had a hard, painless swelling on the dorsal aspect of the cranium. It was slow growing and had been present for 1 year. The lateral radiograph shows a well-circumscribed radiopaque mass on the dorsal aspect of the cranium. There is no periosteal reaction on adjacent bones. Diagnosis: osteoma. E, This old male Labrador Retriever had a swelling on the left mandible. A ventrodorsal occlusal study demonstrates a discrete, osteolytic, expansile lesion (arrows) enveloping the roots of the fourth premolar and first molar teeth in the middle third of the horizontal ramus. A proliferative periosteal reaction is seen medially. Diagnosis: soft tissue tumor invading bone. This was a melanoma. F, This 16-year-old cat had a mass over the left maxilla. The dorsoventral study shows there is a generalized decrease in opacity of the left rostral zygomatic arch with small pinpoint radiolucencies scattered throughout. A soft tissue mass extends laterally. Diagnosis: soft tissue tumor invading bone—a squamous cell carcinoma. G and H, An 11-year-old Labrador Retriever with epistaxis and a swelling over the right eye. G, A dorsoventral view of the skull shows an increased opacity in the right nasal chamber. The lateral margin of the frontal sinus is not visible, and faint irregular mineralized opacities mark its remnant (curved arrow). Calcified lenses are seen as an incidental finding (straight arrows). H, A lesion-oriented oblique view profiles the right frontal sinus. The bone margin is irregular and stippled. A soft tissue swelling overlies the region. This was a soft tissue neoplasm invading bone.

(F, Courtesy Dr. B. O’Malley.)

Caudal Occipital Malformation Syndrome: Congenital Malformation of the Foramen Magnum (Occipital Dysplasia, Chiari Malformation)

Caudal occipital malformation syndrome is a congenital anomaly of the occipital bone. The foramen magnum is formed in the occipital bone. Congenital malformation of the foramen magnum occurs in small and toy dog breeds. The foramen magnum is enlarged dorsally and is abnormal in shape. It has been described as having a “keyhole” appearance. In the normal animal the shape of the foramen magnum varies, making it difficult to evaluate the exact shape of the foramen without the use of CT or MRI. The thin plate of bone at the dorsal aspect of the foramen is virtually radiolucent. Radiographically, this may give the appearance of an enlarged foramen. The condition is believed to occur as a result of mismatch of the cerebellar and caudal fossa volumes, which leads to increased intracranial pressure. This causes part of the cerebellum to be herniated through the foramen magnum. The cerebellar herniation obstructs flow of cerebrospinal fluid (CSF) to the subarachnoid space of the spinal cord, resulting in reduced pressure in this space. High-velocity pulsatile flow of CSF into the central canal of the spinal cord leads to hydromyelia or syringomyelia. Hydromyelia is dilation of the central canal of the spinal cord. Syrinxes are fluid-filled cavities that may be continuous or segmental, tubular or saccular, and are seen in the cervical and sometimes in the cranial part of the thoracic spinal cord. There may also be dilation of the lateral and third ventricles.

Clinical signs are variable. Some dogs are apparently normal; others may show pain, obsessive scratching, ataxia, and quadriparesis. Ataxia has been reported in approximately 25% of cases with enlarged foramen magnum. The brainstem and cerebellum can herniate through the enlarged foramen, giving rise to neurologic signs. There may be an associated hydrocephalus with open suture lines. Cisternal puncture for CSF collection or myelography is contraindicated in dogs in which cerebellar herniation may occur. MRI is the imaging method of choice. Sagittal plane T1- and T2-weighted images are most helpful. The caudal border of the cerebellum appears flattened, and part of the cerebellum is herniated through the foramen. Syringomyelia and hydromyelia appear as tubular hypointense structures within the spinal cord on T1-weighted images and hyperintense structures on T2-weighted images (see Figure 5-32 and p. 510).

The foramen magnum is best shown on a rostrocaudal view of the skull. The animal is placed in dorsal recumbency, and the occipitoatlantal articulation is flexed so that the hard palate is at an angle of approximately 70 degrees to the tabletop. The x-ray beam is directed perpendicular to the tabletop (R30°V-CdDO). It may also be demonstrated on an open-mouth rostrocaudal view. Care should be exercised in positioning animals for radiography of this area if anomalies are suspected. A seizure can be induced in an apparently normal animal by extremes of manipulation of the occipitoatlantal articulation if an abnormality is present.

A number of other abnormalities are encountered in this area. The first cervical vertebra may be shortened, and there may be hypoplasia and nonfusion of the odontoid process (dens). The significance of anatomic changes seen in this region is often uncertain. A lateral view of the cervical spine is required to demonstrate these changes (Figure 5-7).

Figure 5-7 A, Normal foramen magnum. Suture lines can be seen in this young dog’s skull. B, Malformation of the foramen magnum. The horizontal arrows indicate the dorsal limits of the foramen magnum. The area of radiolucency above the black arrows, the dorsal limit of which is indicated by the open arrow, is the central sulcus of the caudal fossa. The bone of the central sulcus, called the vellum, may be very thin and almost radiolucent. As a result, the extent of the deformity of the foramen magnum may be overestimated. C, Shortening of the first cervical vertebra (arrow). The space between the axis and the atlas, dorsally, is increased.

There may be an accompanying hydrocephalus and syringomyelia or hydromyelia.

Craniomandibular Osteopathy (Craniomandibular Osteoarthropathy, Mandibular Periostitis, Craniomandibular Osteodystrophy, “Lion Jaw”)

Craniomandibular osteopathy is a disease seen in young West Highland White Terriers. Also affected are the Boston, Scottish, and Cairn Terriers and, more rarely, other small breeds. There have been isolated reports of this disease in Labrador Retrievers and Doberman Pinschers. The condition is of uncertain etiology except in the West Highland White Terrier, where it is known to be hereditary. The condition is first noticed between 3 and 10 months of age. Affected animals present with difficulty and pain when opening the mouth or chewing food. The pain may be intense. Palpation of the skull reveals bilateral or sometimes unilateral swelling on the horizontal rami and perhaps on the vertical rami. These areas are tender on palpation. There may be atrophy of the temporal and masseter muscles. Some animals recover with supportive treatment after a period of months, at least to the stage at which nutrition can be maintained. More severely affected patients may require placement of a feeding tube. Some have to be euthanized because of the inability to eat.

Craniomandibular osteopathy produces proliferative changes, particularly on the mandible and in the areas of the osseous bullae. The calvarium may be thickened. New bone formation at the distal metaphyses and along the diaphyses of the radius and ulna has been reported, although this is rare.

Clinically, the condition has to be distinguished from eosinophilic myositis of the head muscles, although that condition is more common in larger dogs.

Radiologic Signs

1. Periosteal new bone formation and sclerosis are seen, usually affecting both mandibles, which are thickened and irregular in appearance.

2. Masses of new bone are seen in the area of the tympanic bullae and on the petrosal part of the temporal bone.

3. The occipital bones may be similarly affected, with thickening of the calvarium.

4. In rare cases, periosteal changes appear on the limbs similar to those seen in hypertrophic osteodystrophy (metaphyseal osteopathy) (see Chapter 4, p. 437).

The temporomandibular joints are rarely directly involved in the new bone growth, but their movement may be inhibited because of the bony masses around them (Figure 5-8, A to E).

Figure 5-8 A to C, Craniomandibular osteopathy. A and B, Thickening of the calvarium associated with craniomandibular osteopathy. There is extensive new bone formation on the mandibles and around the osseous bullae. C, Masses of periosteal new bone are seen along the horizontal rami. The calvarium is thickened. D and E, Craniomandibular osteopathy. New bone formation on the temporal bones. E, A 6-month-old West Highland White Terrier presented with recurrent pain around the mouth. A lateral study of the skull shows extensive new bone formation in the region of the temporal bones, tympanic bullae, and mandibles. The calvarium is thickened. F and G, Rubber jaw. Gross demineralization of the skull caused by renal secondary hyperparathyroidism. The zygomatic arches have virtually disappeared, and the mandibles are abnormally radiolucent. The teeth stand out prominently because of lack of bone opacity in the skull.

Calvarial hyperostosis has been described in Bullmastiff puppies and affects the frontal and parietal bones. It is of unknown etiology and regresses with age.

Renal Secondary Hyperparathyroidism (“Rubber Jaw,” Renal Osteodystrophy, Renal Rickets, Renal Osteitis Fibrosa)

A secondary hyperparathyroidism in older animals may result from renal insufficiency caused by chronic renal disease. It is occasionally seen in young animals with congenital nephropathies. In short, if the glomerular filtration rate is reduced, there is retention of phosphorus and resulting progressive hyperphosphatemia. The hyperphosphatemia causes a lowering of the blood calcium level, which provokes hyperparathyroidism. The resulting increase in parathyroid hormone levels causes a resorption of bone and a release of calcium. The skull is primarily involved with demineralization of the mandible and the maxilla. Demineralization elsewhere is slower. Clinically, the mandible becomes softened (rubbery), teeth become loose, and breathing may be impaired because of collapse of bones around the nasal cavity. The face may appear swollen as a result of proliferation of fibrous tissue. Salivation is common.

Nutritional Secondary Hyperparathyroidism (Juvenile Osteodystrophy)

Nutritional secondary hyperparathyroidism may cause changes in the skull similar to those seen in the skeleton. However, changes are usually seen concurrently in the appendicular skeleton and spine (see Chapter 4, p. 420).

Infection

Unilateral infection in the conchae causes an increased radiographic opacity on the affected side compared with the normal side. This is caused by elimination of air around the conchae as a result of the presence of inflammatory exudate. The nasal septum provides a sharp line of demarcation between affected and unaffected sides. If the condition is bilateral, there will be increased radiographic opacity on both sides. Destruction of the septal cartilages is not visible radiographically. Animals with acute rhinitis may be normal radiographically. The frontal sinus may be affected secondarily. A fluid or soft tissue opacity will be seen within it. Clinically there is nasal discharge, which may be serous, mucopurulent, or bloodstained.

Chronic hyperplastic rhinitis is a condition of increased radiographic opacity without evidence of destruction of the conchal pattern. In early cases of infection, it is often possible to distinguish the underlying scroll pattern through the increased opacity. As the mucous membrane swells and exudates accumulate within the affected chamber, the scroll pattern is masked. Severe infections may erode the conchae, disrupting their normal linear pattern. Chronic hyperplastic rhinitis is usually bilateral.

Cryptococcosis

This fungal infection occurs most commonly in cats, although other species may be affected. The nasal cavities, the paranasal sinuses, and occasionally the lungs are involved. The pigeon is a common vector. Clinical signs include sniffling and sneezing, with a unilateral or bilateral nasal discharge. Oral ulceration is occasionally present. Skin nodules with or without ulceration are common. The central nervous system may be involved through spread from the nose through the cribriform plate of the ethmoid bone. Radiographically, there is an increased opacity within the nasal chambers with occasional bone erosion and involvement of the frontal sinuses. Cryptococcosis usually results in a hyperplastic reaction rather than a destructive one.

Linguatula spp. infestation in cats and dogs may cause rhinitis with erosion of the vomer bone. It can mimic neoplasia.

Rhinitis may be caused by dental disorders. Rhinitis and sinusitis in the cat may occur from viral infection. Otitis media has been reported as a sequela (Figure 5-10).

Figure 5-10 Nasal infiltration. A, There is extensive soft tissue infiltration with ill-defined margins affecting the left nasal turbinate bone with focal areas of lysis. The nasal septum is apparently intact. The differential diagnosis should include infection (bacterial or fungal) and neoplasia. B, Diffuse soft tissue opacities are seen in both nasal chambers. The nasal septum cannot be clearly identified rostrally. The turbinate pattern is obscured. The frontal sinuses have lost their normal air opacity. This 7-year-old German Shepherd had a fight with a cat 2 years previously. This was a mixed type of infection. C, A unilateral nasal infection. An obvious loss of normal air opacity is seen on the left side compared with the right (normal) side. This is the result of an undifferentiated soft tissue infiltration completely obliterating the fine trabeculae of the turbinate bones. D, A dorsoventral, intraoral (occlusal) view of the maxilla in a 2-year-old German Shepherd with a mucopurulent discharge from the left nostril. The nasal conchae are indistinct because of a subtle soft tissue opacity infiltration with focal pinpoint radiolucencies. This was a destructive rhinitis caused by aspergillosis. E, An 11-year-old Poodle with a nasal discharge on the left side. This occlusal view shows subtle erosion of the left rostral conchae extending from the palatine fossa caudally. A discrete semicircular area of bone lysis with surrounding sclerosis is seen around the rostral roots of the upper fourth premolar tooth. Several incisor teeth are absent. The blurring on the right side is caused by poor film-screen contact. This is destructive rhinitis with a periapical abscess. F to I, Nasal aspergillosis in a dog. This patient had unilateral disease. This series of CT images are through the nasal cavities from rostral to caudal. F, There is complete destruction of the nasal turbinates within the left nasal chamber rostrally. There is thickening of the mucosa along the lateral border of the septum. The nasal turbinates in the right nasal chamber are intact. G, In the middle part of the nasal chambers, there is incomplete destruction of the nasal turbinates on the left side. The nasal mucosa is moderately and unevenly thickened (arrows), best seen in the middle part of the nose. H, At the caudal aspect of the nasal chambers, a granuloma (arrows) is present within the left nasal chamber, dorsal to the ethmoid turbinates. This contains multiple, small, irregularly shaped pockets of gas. There is mild thickening of the overlying maxillary bone. I, The left frontal sinus is filled with soft tissue, with multiple small interspersed gas pockets. There is moderate, nonuniform thickening of the frontal bone (arrows). The findings of extensive turbinate lysis, mucosal thickening, granuloma formation, and hyperplasia of the facial bones is characteristic of nasal aspergillosis. J and K, Feline nasal Cryptococcus infection. J, Rostrocaudal projection of the skull. There is an increased soft tissue opacity throughout the left frontal sinus, while the right sinus has been spared. This can also be seen on the ventrodorsal open-mouth view (K), where the air-filled right frontal sinus (asterisk) contrasts with the soft tissue opacity within the left frontal sinus (black arrowheads). There is also uniform increased soft tissue opacity throughout the left nasal chamber (arrows). The nasal turbinates are partly destroyed. These changes are similar to those seen with primary nasal neoplasia, such as lymphoma, in cats.

Foreign Body

Radiopaque foreign bodies are readily recognized on radiographs. The skull should be examined as far caudally as the pharynx. Clinically, there may be sneezing and a nasal discharge. Radiolucent foreign bodies are not seen. However, there may be increased opacity within the affected chamber as a result of inflammatory exudate. The introduction of contrast medium into the nasal chamber (contrast rhinography) is seldom informative, except perhaps in the case of a large radiolucent foreign body (Figure 5-11).

Figure 5-11 A and B, Foreign body. A mineral opacity (arrow) is seen in the caudal nares both on the lateral and ventrodorsal views. This was a stone. The dog had a bilateral nasal discharge for several weeks.

Neoplasia

The most common nasal neoplasms are adenocarcinoma, squamous cell carcinoma, undifferentiated carcinoma, fibrosarcoma, chondrosarcoma, and undifferentiated sarcoma. Lymphoma may occur but is more common in cats. Neoplasia is more common in older animals. In general, neoplasms are more destructive and aggressive than infection and may invade the conchae, nasal septum, and walls of the nasal chambers.

Clinically there is a nasal discharge, which is often blood stained. There may be swelling of the face, which is usually unilateral.

Radiologic Signs

1. There is increased radiographic opacity in the affected chamber as a result of the tumor mass and associated exudation.

2. The nasal septum is frequently displaced or eroded by the growing tumor.

3. The nasal scroll pattern is destroyed.

4. The tumor frequently invades and surrounds adjacent bones, the vomer bone, and bones of the nasal cavity.

5. The facial bones may show a periosteal reaction, and there may be an associated soft tissue mass or facial deformities.

6. The tumor mass may invade the frontal sinus and cause erosion or deformity.

Because the radiologic signs of infection and neoplasia are similar in many respects, it is usually impossible to be sure on radiographic evidence alone whether the cause is infection or a neoplasm. This is particularly true in cats. Some infections can be very destructive, such as aspergillosis. If there is an associated soft tissue mass, there is a strong presumption that the lesion is neoplastic. CT and MRI are useful to evaluate fully the precise area and extent of the lesion (Figure 5-12).

Figure 5-12 Neoplasia. A and B, Nasal adenocarcinoma in a dog. A, The rostrocaudal ventrodorsal open-mouth view of the maxilla shows increased soft tissue opacity within the left nasal chamber. The fine linear opacities of the nasal turbinates have been destroyed. The right nasal chamber is normal. B, The rostrocaudal view of the frontal sinuses shows uniform increased soft tissue opacity within the left frontal sinus. There is no evidence of bone destruction. The increased opacity in the frontal sinus may be the result of a tumor mass within the sinus or fluid accumulation caused by obstruction of the sinus drainage by a tumor within the nasal cavity. C and D, Chondrosarcoma. C, There is obliteration of the nasal turbinates within the right nasal chamber. The right first premolar tooth is absent. D, The caudorostral view shows an increased soft tissue opacity in the right frontal sinus. E to H, This Terrier had a protruding right eye and a blocked right nostril. Dorsal (E) and transverse (F) MRI studies show a large mass (arrows) in the right nasal cavity. G and H, Dorsal plane slices located more dorsally than E show the tumor (arrows) extending into the retrobulbar area. This was a carcinoma. Rt, Right side. The arrowhead in F points to a marker capsule on the right side of the face. I to L, Nasal adenocarcinoma in a dog. I, A mass within the rostral part of the left nasal chamber has completely destroyed the nasal turbinates. The mass has also destroyed part of the septum and extended into the right nasal chamber (arrows). J, In the mid part of the nasal chamber, there is lysis of the hard palate and maxilla (arrows). Complete destruction of the nasal turbinates is again present at this level. K, In the caudal nasal chamber, there is partial destruction of the ethmoid turbinates. There is also moth-eaten lysis of the ventral aspect of the medial wall of the orbit on the left side (black arrow). The tumor extends into the internal choanae, occluding the left side (white arrow). The frontal sinus is filled with tissue or fluid. L, The image at the level of the frontal sinuses shows almost complete filling of the left frontal sinus. A small gas pocket is seen at the dorsal aspect of the sinus. This may represent accumulation of nasal secretions (obstructive sinusitis) or tumor extension into the frontal sinus.

Hemorrhage

Hemorrhage within a nasal chamber produces an increase in opacity that may cause blurring or obliteration of the nasal turbinate pattern. The cause may be trauma, infection, or neoplasia. Fracture lines can be difficult to demonstrate radiographically unless there is displacement.

Infection

With infection in a frontal sinus, the normal air opacity within it is replaced by a fluid or soft tissue opacity (Figure 5-13, D). Unless the condition is of long standing, there is usually no associated bone abnormality. Thickened mucous membranes may be visible within the sinus. Infection may reach the sinus from the nasal chambers. Fluid may occasionally be seen on a horizontal beam caudorostral view. A horizontal line is evident at the air/fluid interface. Infection in a maxillary sinus, such as that associated with malar abscess, is not readily demonstrable radiographically unless there are associated bone changes. Expansion of the frontal sinus may be caused by a cyst in the sinus (Figure 5-13, E and F). Obstructive sinusitis is a common sequela to primary nasal neoplasia in dogs.

Neoplasia

Neoplasia affecting the frontal sinus causes loss of the normal air opacity within the sinus. The tumor mass replaces the air. Invasion of the surrounding bone, with destruction and periosteal reaction, becomes evident as the condition progresses. The frontal bone may be a site of osteosarcoma. Squamous cell carcinoma is the most common primary soft tissue tumor. Adenocarcinoma also occurs. These tumors are highly destructive, and considerable erosion of bone may have occurred before they are detected. Invasion of the sinuses by tumors arising in the nasal cavity is more common than primary sinus tumors (Figure 5-14, A and B).

Figure 5-14 A and B, An osteosarcoma affecting the right frontal sinus. There is destruction of bone and invasion of the overlying soft tissues. The ventrodorsal view shows invasion of the cranium on the right side. C, Longitudinal sonogram of a swelling on the dorsal aspect of the rostral cranium. Rostral (R) lies to the left; caudal (C) lies to the right. The bright hyperechoic line lying under the R is the frontal bone. The anechoic area (arrow) caudal to the rostral frontal bone represents disruption of the bone. This is the result of an expanding soft tissue tumor that has eroded the bone and is extending dorsally. The irregular hyperechoic area seen caudal to the defect is an irregular periosteal reaction. This was a fibrosarcoma. D, Otitis media and bulla osteitis in a dog. The right tympanic bulla is normal. There is increased soft tissue opacity within the left tympanic bulla. Moderate, uniform thickening of the bony wall of the left bulla is also present. No evidence of destruction/lysis of the bulla is present. The endotracheal tube is visible between the two bullae. R, Right. E, A 10-year-old cat with recurrent otitis externa. This rostrocaudal projection shows the tympanic bullae lying ventral to the mandibles. The right bulla has an increased opacity and a thickened irregular ventral wall. Diagnosis: otitis media. F, This young cat had a chronic stridor and nasal discharge. The lateral radiograph illustrates a large pharyngeal mass in the caudal pharynx, occluding the normal airway (arrow). Diagnosis: nasopharyngeal polyp. There is an endotracheal tube in place. G to I, Otitis media. Lateral (G), dorsoventral (H), and right lateral oblique (I) views of the tympanic bullae. This cat had a history of nasal discharge and dyspnea. The wall of the right tympanic bulla is thickened and expanded. The external ear canal on the right side is occluded by a nasopharyngeal polyp. J, An 11-year-old cat presented with a history of head tilt to the right and inability to swallow food. The open-mouth rostrocaudal view shows an increased opacity within the left tympanic bulla. Diagnosis: squamous cell carcinoma. K, A 10-month-old Cocker Spaniel. The left auditory canal is not visible. There is a soft tissue swelling extending caudally from the zygoma (curved arrow). The air outlining the pinna (straight arrow) is displaced caudally and laterally compared with the opposite ear. This was an avulsion of the left auditory canal with a subsequent paraaural abscess. L, Nasopharyngeal polyp and otitis media in a cat. Note the large pulp cavities within the canine teeth, indicating this patient is young with recently erupted permanent dentition. A soft tissue mass is present in the nasopharynx, causing ventral displacement of the soft palate. The caudal border of the mass is well defined and convex, outlined by air. The rostral border is obscured by the superimposed mandibles. There is increased soft tissue opacity within both bullae, which are normally filled with air. Otitis media and nasopharyngeal polyps are a common sequela to viral upper respiratory tract infections in cats. M, Bulla osteitis/otitis media in a cat. A transverse CT image of the skull displayed in a bone window shows uniform soft tissue filling the right bulla. There is mild thickening of the wall of the bulla. The semicircular canals can be seen within the petrous temporal bone of the left ear.

Hemorrhage

The frontal sinus may be the site of hemorrhage. Blood within the sinus causes loss of the normal air opacity. Hemorrhage may be the result of fracture of the frontal bone. The hemorrhage may make identification of the fracture line more difficult.

Positive Contrast Ear Canalography

This technique may occasionally be useful when it is difficult or impossible to visualize the tympanic membrane directly. In many cases this procedure is unrewarding and of limited value. The dog is placed in lateral recumbency with the affected ear uppermost. The ear canal should be gently cleansed. Iopamidol or iohexol is diluted with an equal volume of normal saline. The ear canal is filled with the mixture. The contrast material is massaged along the length of the vertical and horizontal canals. The ear is plugged with cotton wool or a cotton ball, and radiographs of the area are made. If the tympanic membrane is ruptured, contrast material will leak into the tympanic bulla. If the horizontal canal is occluded with debris, the procedure may prove nondiagnostic. The ear should be flushed out after the procedure to remove the contrast agent. Ultrasonography of the tympanic bullae has been reported as useful in assessing the presence of fluid or air within the bullae.

Otitis Externa

Otitis externa is inflammation of the external ear canal. Clinically, there is scratching of the ear, head shaking, a bad smell, and pain. The ear is tender to the touch. Acute cases show no radiologic signs.

Otitis Media

Inflammation of the middle ear, otitis media, is frequently a sequela to inflammation of the external auditory canal. The inflammation is usually accompanied by exudate. It may be present in the absence of radiographic evidence. Clinically there is head tilt and possibly circling. There may be signs of otitis externa if the condition has arisen externally. Otitis media occurs in cats as a sequela to viral upper respiratory infections because the auditory tubes are lined with respiratory epithelium. Nasopharyngeal polyps should be ruled out. Otitis interna sometimes ensues. Radiographs are insensitive in the diagnosis of otitis media. CT is significantly more sensitive but does not detect all cases.

Avulsion of the Auditory Canal

This is an uncommon condition in which the horizontal ear canal has been avulsed from the skull. It follows trauma, often a road traffic accident. Clinically, there is swelling around the base of the pinna. Over time a paraaural abscess develops. This is not an acute condition.

Nasopharyngeal Polyp

In cats a polyp sometimes extends on a stalk from the eustachian tube into the pharynx and nasal cavity, which it may obstruct. The presenting signs are nasal discharge, sneezing, and stridor.