I. Upper respiratory tract
1. Structures of normal nose
2. Sinonasal disorders
a. Paranasal sinus cyst
b. Ethmoid hematoma
This slowly growing tissue proliferation arises from the mucosa of the caudal nasal cavity or ethmoid labyrinth. The pathogenesis is not understood because this tissue proliferation acts like a locally invasive neoplasm but morphologically is not. It clinically leads to small volume epistaxis.
c. Nasal polyp
d. Sinonasal inflammation
Viruses, bacteria, and fungi have to be considered as etiologic agents. If bacteria are involved, these usually are Streptococci sp. or Fusobacterium sp. Noninfectious causes may be environmental, irritants, or trauma. The inflammatory response can be classified depending on the nature of the exudate as catarrhal, mucinous, hemorrhagic, purulent, and granulomatous.
Other fungi to affect the nasal cavity are Coccidioides immitis and Cryptococcus neoformans.
The nasal form of glanders is an example of infection with an exotic, zoonotic bacillus. The agent (Burkholderia mallei) causes granulomas and ulcers in the nasal septum.
3. Guttural pouch diseases
Also known as eustachian tube diverticula, the guttural pouches in the horse are large pockets on either side of the caudoventral head, lined by a thin, smooth, shiny mucosa. Each guttural pouch is partially divided by the stylohyoid bone, connected with the pharynx and traversed by the internal carotid artery as well as by cranial nerves and cranial ganglia. Because of this close anatomic relationship of blood vessels, nerves, and pharyngeal opening, the guttural pouches are susceptible to pathogens of the upper respiratory tract with a sometimes serious outcome such as severe fatal epistaxis. Dysphagia and Horner’s syndrome are other clinical signs of guttural pouch diseases resulting from damage of the adjacent cranial nerves and ganglia.
Aspergillus nidulans is a common resident of the guttural pouch. The mycotic infection can spread through the wall of the pouch, with fungi invading the nearby internal carotid artery to induce thrombosis and dissection aneurysm with massive bleeding, sometimes leading to exsanguination. The pathogenesis of the aneurysm may be partially due to fracture of the stylohyoidal apparatus or joint instability due to chronic joint degeneration.
4. Pharynx/Larynx/Epiglottis disorders
Other abnormalities associated with the epiglottis are dorsal abscesses.
5. Sinonasal neoplasia
Nasal neoplasms occur more often than thought and mainly arise from the caudal maxillary sinuses. The common tumor types include squamous cell carcinomas, adenocarcinomas, fibro-chondro-osseous and dental tumors, and hemangiosarcomas. On occasion lymphosarcomas, meningiomas, and carcinoids have been reported. Most of the sinonasal tumors are malignant, destroying nasal structures by local invasion and frequently causing facial deformity, loss of teeth, and breathing difficulty. Spread to regional draining lymph nodes may occur, but distant metastases are very rare. Grossly, the sinonasal tumors are indistinguishable as to histogenesis, and microscopic examination is mandatory for morphologic classification. Figure 4.28, Figure 4.29, Figure 4.30, Figure 4.31, Figure 4.32, and Figure 4.33 are representative of the various tumor lineages.
All of these growths are destructive while invading various tissues of the nose. Their histologic makeup has to be identified by microscopy.