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Chronic upper respiratory disease in cats (Proceedings)

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Nov 01, 2010

Anatomy and Physiology

The nasal cavity is defined as part of the upper respiratory tract that extends from the nares to the choanae. The choanae is the caudal most part of the nasal cavity that communicates with the nasopharynx. The nasal cavity is surrounded by bone and divided in half by a cartilaginous and boney septum. A scroll-like turbinate system, the nasal conchae, is generously supplied by blood vessels and nerves and acts to increase the surface area of the nasal passages. The large surface area is important for filtration, humidification, and warming of inspired air. These actions serve to prevent particulate matter from entering the lower airways and protect the lower airways from cold and dry air.

The nasal conchae/turbinates can be divided into dorsal, ventral and ethmoidal nasal conchae or turbinates. The ethmoidal turbinates fill the caudal nasal cavity and are made up of outgrowths of the ethmoid bone covered with mucosa. A specialized olfactory area is located in the region of the ethmoidal turbinates and ventral nasal conchae. Specialized neuroepithelium covering these turbinates gives rise to the olfactory nerves, which transmit information to the brain through the cribriform plate. The location and proximity of the caudal nasal cavity and cribriform plate to the brain make extreme caution necessary when performing nasal flushes, biopsy, or surgery. Cats have a highly developed sense of olfaction that has a role in protective behavior and appetite. In cats with severe nasal disease anorexia may result from an inability to smell food. The meatuses are air passageways that occupy the space between the nasal turbinates and include the dorsal, middle, ventral, and common. The ventral meatus is the largest and leads into the choanal region. The common meatus unites the dorsal, middle and ventral meatus and is found on either side of the septum. The paranasal sinuses in the cat include the frontal and sphenoid sinuses. They are of questionable function in the normal animal.

Most of the airways including the nasal passages and nasopharynx are covered by a mucociliary transport system. This consists of ciliated pseudo-columnar epithelium, the respiratory epithelium, which contains abundant serous and mucus glands. The epithelium is covered by a mucus layer that is secreted by the serous and mucus glands. The mucus layer is made up of two parts: an outer viscous layer and an inner watery layer. The cilia of the respiratory epithelium are embedded in the more watery layer. When large particles such as dust, pollen, and bacteria are breathed in they are trapped in the viscous mucus layer. The cilia then beat in a direction toward the pharynx to carry the viscous outer mucus layer with the trapped particles to be swallowed. The production of local secretory IgA by epithelial cells also play a role in the defense mechanism of the upper respiratory tract. The ultimate cleaning procedure for the nasal cavity is the sneeze, which starts with a rapid inspiration followed by an involuntary, sudden, violent, and audible expulsion of air through the nose and mouth. This reflex occurs after stimulation of the sensory receptors in the nasal mucosa.

The nasal passages of healthy cats are normally colonized by a variety of gram positive and gram-negative aerobic or facultative commensal bacteria.2 Less is known about the normal flora of the nasopharynx of cats.

The feline nasopharynx has well-defined anatomic boundaries. It is located above the soft palate and bordered rostrally by the nasal choanae. The caudal border of the nasopharynx is defined by the intrapharyngeal ostium with demarcations of the caudal border of the soft palate and the palatopharyngeal arches. It sits at the crossroads to the entrances to the lower respiratory tract and upper gastrointestinal tract and has close anatomic relation to the caudal nasal cavity, nasofrontal openings, and pharyngeal openings of the Eustachian tubes. Since these structures are in such close proximity disease in the upper respiratory tract may be widespread leading to complications in adjoining areas. It has been reported that 34% of cats with CT evidence of nasopharyngeal disease had signs of bulla effusion. 65% of cats in the same previous study that were evaluated for chronic upper respiratory disease had CT evidence of soft-tissue opacity within the paranasal sinuses.

The larynx has three main functions: acts as a conduit for air, protects the lower airway from aspiration during deglutition, vocalization.