The ear, nose and throat

Chapter 6


The ear, nose and throat





Chapter contents



INTRODUCTION


THE NASAL PASSAGES



THE PARANASAL SINUSES



DENTITION



THE NASOPHARYNX



GUTTURAL POUCHES



THE LARYNX



THE EAR





THE NASAL PASSAGES



EXTERNAL NARES


The external nares are the openings to the nasal passages. The comma-shaped alar cartilages support the external nares dorsally, medially and ventrally. Lateral dilatation of the nares is achieved by muscular activity that regulates airflow to the nasal passages, larynx and lower airways. The external nares are the major site of upper respiratory resistance to airflow, therefore in the normal horse the distensible nature of the external nares is vital for athletic function. Narrowing or loss of muscular support to the external nares results in collapse of the nares during the inspiratory part of the respiratory cycle. Upper airway obstruction produced by the external nares can be considered to be either functional or static/luminal.





NASAL DIVERTICULUM (FALSE NOSTRIL)


The false nostril is positioned dorsolaterally to the external nares. Its floor and medial wall are formed by the alar fold, a rostral soft tissue extension of the ventral turbinate. Vibration of the alar fold on expiration during exercise is a normal physiologic occurrence that produces the noise recognized as “high blowing”.



Abnormal alar folds


Partial airway obstruction may occur if either the alar folds are excessively thick or the external nares are relatively small. An inspiratory vibratory noise is produced and airflow to the nasal passages is restricted by dynamic collapse of the vestibule.


The subjective assessment of the alar folds and their contribution to an upper respiratory noise is difficult, particularly in the absence of visible or palpable abnormality. However, some assistance may be obtained in confirmation by placing a temporary mattress suture through each alar fold to the skin of the external nares, thus obliterating the space within the false nostril. If the previous respiratory noise is eliminated or significantly reduced in volume during a subsequent exercise test, then bilateral resection of the alar fold can be recommended to alleviate the horse’s condition.



Atheroma/epidermal inclusion cyst


Atheromas present as spherical, non-painful, unilateral swellings of the caudo-dorsal aspect of the nasal diverticulum. Generally, they are a cosmetic problem seldom obstructing the nasal passages.


Recognized usually in younger horses, the cyst contains a thick, gray sterile fluid of keratinized and non-keratinized cells produced by aberrant location of epithelial tissue. Local drainage, curettage of the cyst lining and daily irrigation with povidone-iodine may be performed under sedation and regional anesthesia. This technique may be advised on cosmetic considerations, however recurrence can be expected if there is inadequate removal of the cyst lining. Complete dissection of the unpunctured cyst and primary skin closure under general anesthesia gives more reliable results.



NASAL CHAMBERS


The nasal passages have a number of physiologic functions apart from acting as the initial conduit for air to the lower respiratory tract. They warm and moisturize the ambient inspired air, they are involved in thermoregulation, act as a crude filtering system of airway contaminants and debris, and provide a location for olfactory receptors. The mucosa of the nasal chambers is closely adherent to the supporting structures of the dorsal and ventral turbinates and midline nasal septum.


The well-developed blood supply that courses through the submucosa of the nasal passages during exercise undergoes vasoconstriction. This causes the mucosal epithelium to adhere more closely to the underlying cartilage, maximizing the airway diameter and resisting potential collapse. The mucosa becomes engorged during general anesthesia and in association with Horner’s syndrome (q.v.), producing significant upper respiratory obstruction. This is due in the latter case to impairment of sympathetic control to the nasal vasculature.





Fungal infection


Mycotic infection (q.v.) is sporadic in occurrence and usually opportunistic, dependent either on impaired local resistance or the presence of devitalized tissue. Clinically, a fetid serosanguineous/mucopurulent and generally unilateral, nasal discharge is observed; however, in some horses this discharge may be minimal. A diagnosis can be made endoscopically by recognizing mycotic plaques and occasionally granulomas on the turbinates and nasal septum. A definitive diagnosis is obtained by biopsy and culture of the lesion.


A wide variety of isolates have been reported including Aspergillus fumigatus, Coccidioides immitis, Cryptococcus neoformans and Rhinosporidium seeberi.


Treatment involves topical application of antifungal agents, e.g. ketoconazole or natamycin, via an external irrigation system that utilizes the paranasal sinuses as a reservoir. This may be supplemented by using systemic antifungal agents and surgical removal of the plaques and granulomas, which is usually best achieved by use of endoscopic or endometrial biopsy forceps, under direct endoscopic visualization. In some cases the lesions may be self-limiting without treatment.



Ethmoidal hematoma


Ethmoidal hematoma is a progressive, expansile, encapsulated lesion of unknown etiology; there is no histologic evidence of neoplasia even though the lesion may act like a tumor. The lesion usually develops from the ethmoidal region, however paranasal sinus origin is also not uncommon.


The lesion is typically encountered in middle-aged to older horses, presenting with an intermittent, unilateral, serosanguinous nasal discharge unrelated to exercise. Depending on the lesion size and position there may be altered nasal airflow and a respiratory noise at exercise may also be detected.


Endoscopic examination reveals a typically yellow-green-brown space-occupying lesion extending rostrally along the nasal meatus. Radiography can be used to delineate the extent of the soft tissue mass and determine the presence of possible secondary lesions. It should be noted that if a lesion is restricted to the paranasal sinus, the only finding may be a trickle of hemorrhage from the sinus ostium in the middle meatus.


Surgical treatment involves resection using a frontomaxillary sinus bone flap approach to the nasal chamber. Access to the ethmoidal region is then through the floor of the frontal sinus and is inevitably a bloody procedure. The lesion may recur following excision depending on the completeness of lesion removal.


The use of a transendoscopic laser (Nd:YAG or diode) to ablate the hematoma has provided a much less invasive procedure, but may be less effective in the confined areas of the ethmoidal region. Injecting lesions with a 10% formalin solution on repeated occasions has also proved effective, although if large volumes are used there is a risk of fatal encephalitis. Such an injecting system can be purchased or made up by attaching a needle to a piece of stiff tubing and carefully inserting it down the biopsy channel of the endoscope. Both the formalin injection and laser techniques can be used for intrasinus lesions, via direct sinus endoscopy.





NASAL SEPTUM


The nasal septum extends from the external nares caudally, dividing the nasal passages as far as the ethmoidal region.







THE PARANASAL SINUSES


The paranasal sinus system consists of the bilaterally paired rostral and caudal maxillary, frontal, ethmoidal and sphenopalatine sinuses. Functionally, the sinus system reduces the weight of the bony covering that supports the skull contents and protects them from external trauma.


The external landmarks of the maxillary and frontal sinuses are readily palpated. A line from the medial canthus of the eye to the nasomaxillary notch marks the dorsal margin of the maxillary sinus and the lateral margin of the frontal sinus. The ventral margin of the maxillary sinus lies parallel and just ventral to the facial crest. The rostral maxillary sinus extends to a line drawn perpendicular to its dorsal margin that intersects the rostral end of the facial crest. The caudal limit of the caudal maxillary sinus is defined by a line bisecting the orbit. The left and right frontal sinuses are divided on the midline by a complete bony septum. The caudal limit of the frontal sinus is represented by a line perpendicular to the midline bisecting the temporomandibular joint. The most rostral extension of the frontal sinus lies within the dorsal turbinate (the concho-frontal sinus) and is delineated by a transverse line halfway between the medial canthus of the eye and the infraorbital foramen.


The paranasal sinus system is lined by mainly pseudostratified, columnar, ciliated epithelium and goblet cells. Drainage of the mucus that is produced by this respiratory lining is assisted by the coordinated beating of cilia toward the caudal maxillary sinus. The frontal, ethmoidal and sphenopalatine sinuses drain and communicate directly with the caudal maxillary sinus. Normally, a complete bony septum overlying cheek teeth four and five divides the rostral and caudal maxillary sinuses. Continuation of this septum divides the nasomaxillary opening through which the whole sinus system drains to the middle nasal meatus.



SINUSITIS


Sinusitis may be divided into primary (infectious) or secondary sinusitis. The predominant clinical sign of sinusitis is a unilateral nasal discharge. Complete obstruction of drainage from the sinuses results in fluid and soft tissue accumulation within the sinuses and turbinates creating internal pressure which may be recognized clinically as facial swelling, reduced airflow at the nostril, respiratory noise at exercise and a dulled resonance and pain on percussion of the sinuses. If the internal sinus pressure causes obstruction of the nasolacrimal duct as it courses through the maxillary sinus, epiphora may also be observed. Neurologic signs produced by caudal extension of a disease process through the cribriform plate are rare complications of sinusitis.


Diagnosis is confirmed endoscopically by recognizing turbinate distension and a discharge emanating from the caudal middle meatus. Radiography is used to determine the extent of sinus involvement and the presence of fluid within the paranasal sinus system, and to evaluate possible dental involvement.



Primary sinusitis


Primary sinusitis is a consequence of an upper respiratory infection extending to involve the paranasal sinus system, altering the normal mucociliary clearance mechanism and reducing normal drainage. The presence of fluid within the sinus system is demonstrated by a horizontal fluid–gas interface on a standing lateral radiographic image of the paranasal sinuses. Confirmation of the diagnosis and isolation of the etiologic agent can be achieved following sinucentesis of the affected sinus. This is most easily achieved via insertion of a Steinmann pin through the facial bone plate into the sinus, and then aspiration via a needle, catheter or swab.


Treatment must be aggressive to prevent secondary complications of inspissation of accumulated pus and bone necrosis. The primary aim of treatment is to re-establish normal drainage to the nasal passages. Systemic broad-spectrum or, ideally, sensitive specific antibiotic therapy should be instituted in association with daily irrigation and lavage of the sinus. Large volumes of physiologic sterile saline should be administered through an ingress balloon catheter (12–16G French) positioned in the affected sinus through a small trephine hole. Therapy should be continued for 7–14 days, during which time exercise is encouraged to assist drainage. (Drainage from the nasomaxillary aperture is temporarily improved during exercise as the nasal mucosa undergoes physiologic vasoconstriction.)


Chronic cases often fail to respond to this conservative management and adequate drainage can then only be established surgically, using a facial flap approach.



Secondary sinusitis



Dental

The last four cheek teeth of the maxillary arcade lie within the maxillary sinus. The development of a periapical abscess affecting any of these teeth is likely to produce a secondary sinusitis. This occurs most commonly in middle-aged horses and most frequently it is the 4th cheek tooth that is affected. The resulting unilateral nasal discharge that is produced is characteristically fetid.


Accurate recognition of dental involvement requires a thorough examination of the oral cavity and oblique radiographs highlighting the roots of the suspected dental arcade. Both procedures can be performed in the standing horse, as can gamma scintigraphy, which is exquisitely sensitive in identifying periapical inflammation. The use of computed axial tomography (CAT) scanning or magnetic resonance imaging (MRI) can also provide high quality detail of dental structures and surrounding alveolar bone. However, both of these techniques normally require the horse to be anesthetized.


Treatment involves removing the affected tooth. Nowadays this is best achieved by oral extraction in the standing horse. However, the affected tooth may be repelled into the oral cavity and sinus contamination with food prevented by packing the alveolar socket with dental wax or methyl methacrylate bone cement, providing postoperative irrigation and ensuring adequate drainage of the affected sinus.


Two surgical approaches are commonly employed. They differ in the degree of surgical exposure provided and the amount of postoperative management of the alveolar socket that may be performed. Trephining directly over the affected tooth root requires accurate localization, using radiographic images and facial structures (facial crest, medial canthus of eye, nasolacrimal duct and other indicators of sinus topography). Postoperatively the alveolar socket may be irrigated daily and the development of granulation tissue monitored. An approach using a sinus bone flap technique provides excellent surgical exposure of the affected tooth roots and more confidence in complete removal of all affected dental tissue. Using either technique, postoperative radiographs are mandatory to ensure complete removal of all dental tissue. Thereafter, management consists of twice-daily lavage of the sinus using a balloon catheter system placed in the frontal sinus ensuring normal drainage to the nares.


Maintaining the alveolar plug in position while the alveolar socket granulates can be difficult and is imperative for successful surgical management. Dietary management has little effect in preventing leakage around the plug and further sinus contamination.



Traumatic sinusitis

Secondary sinusitis may develop following external trauma to the sinus region. Open lacerations and/or depression fractures will produce epistaxis and facial swelling. Radiography should be used to confirm the presence of fracture fragments, fluid or hematomas within the sinus. Specific attention should be paid to the dental arcade to determine any damage to the tooth roots, the consequence of which may affect the prognosis. However, it is amazing how often such lesions resolve with minimal complication.


Fracture fragments may be removed or, if accurate anatomic reduction and stable fixation can be achieved, they may be retained. The sinus should be lavaged to remove organizing hemorrhage and contaminating organisms. Postoperative irrigation is advised. Open wounds should be treated aggressively to prevent sinus fistulae developing. Chronic fistulae can be treated using periosteal flaps, allowing osteogenesis to occur over the defect.



PARANASAL SINUS CYSTS


Paranasal sinus cysts primarily affect the maxillary sinus but they may also occur in the frontal sinus. They have a true cystic lining that contains a characteristic sterile, amber/yellow fluid formed from blood breakdown products. They may be found at any age but there is a predilection for the younger horse (≤1 yr) and older horses (≥9 yr).


The clinical signs are a mucopurulent/serous nasal discharge, which is usually scanty, facial swelling and reduced airflow at the affected nares. Endoscopy reveals turbinate distension. Radiography demonstrates a contained soft tissue density, which may show areas of mineralization, and tooth root distortion but no evidence of periapical abscessation. Rarely there may be additional bizarre dental structures, or anomalous dental eruption. Sinucentesis can be used to confirm the presence of the characteristic amber fluid.


Complete surgical excision is performed by blunt (digital) dissection through an appropriate sinus bone flap, ensuring adequate drainage to the nares and placement of a postoperative irrigation system. The prognosis following surgery is good and recurrence rates are low.


Rarely, more extensive lesions involve multiple sites and can prove extremely difficult or impossible to remove.




SUTURE PERIOSTITIS


Suture (diastasis) periostitis is a non-painful bony swelling over the margins of the paranasal sinuses that is commonly attributed to a traumatic etiology; however, often there is no history of trauma. These swellings are a cosmetic problem only. They are caused by instability at suture lines, usually between the nasal, frontal and maxillary bone plates. Radiography can be used to monitor the periostitis that develops and also rule out the presence of depression fractures, sequestra or neoplasia. The periostitis is self-limiting. It leaves a permanent, residual swelling although this gradually reduces with time.


Similar swellings can be associated with pressure from expansile sinus masses. Such cases always have evidence of soft tissue density within the sinus and treatment should be directed at the primary cause. Sometimes such suture instability may follow facial flap surgery.

< div class='tao-gold-member'>

Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on The ear, nose and throat
Premium Wordpress Themes by UFO Themes