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Anesthetic Management for Dental and Sinus Surgery

Santiago Gutierrez-Nibeyro1 and Jennifer Carter2

1 Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois, 1008 W. Hazelwood Dr., Urbana, IL, 61802, USA

2 Faculty of Veterinary and Agricultural Sciences, Melbourne Veterinary School, University of Melbourne, 250 Princes Highway, Werribee, VIC, 3030, Australia

Introduction

Most adult horses have 36-44 teeth by the time they reach 5 years of age. In general, the dental arcades are composed of 12 incisors, 12 premolars, and 12 molars (some horses will also have additional teeth including canine and wolf teeth). Due to the grinding nature of eating, horse teeth must continue to grow at approximately 1/8? per year until the individual horse reaches old age where teeth can then be completely shed. Throughout the maxillary and frontal areas of the skull, air-filled sinus cavities developed to allow for a large number of premolar and molar teeth without adding significant weight. The linings of the sinuses are rich in vasculature and may play a role in thermoregulation. Significant disease requiring surgical intervention can occur in the teeth or sinus.

Relevant Anatomy

The nasal cavity is a voluminous cavity divided by the nasal septum and vomer bone (Hillmann 1975). The nasal cavity contains the reserve crowns of the maxillary cheek teeth and a portion of the paranasal sinuses of which the major clinically significant sinuses are the frontal and maxillary sinuses (Hillmann 1975). Two major nasal conchae in each nasal cavity divide the nasal passage into the dorsal, middle, ventral, and common meatus.

The frontal sinus has a large communication with the dorsal conchal sinus, and thereby both are known as the conchofrontal sinus (Hillmann 1975). The ventral conchal sinus communicates with the rostral maxillary sinus over the infraorbital canal and is separated from the caudal maxillary sinus by a thin osseous sheet, the caudal bulla of the ventral conchal sinus. The conchae (or turbinates) are delicate scrolls of bone that are attached laterally in the nasal passage and contain the conchal sinuses (Hillmann 1975).

The maxillary sinus is divided by a thin septum into rostral and caudal compartments or rostral and caudal maxillary sinuses, respectively (Hillmann 1975). The rostral maxillary sinus contains the root of the maxillary first molar and the caudal maxillary sinus contains the roots of the second and third molars (Dixon 2005). The caudal maxillary sinus is partially divided by the infraorbital canal, which may be distorted by a disease process within the sinus. The caudal and rostral maxillary sinuses have separate openings into the middle nasal meatus and the caudal maxillary sinus communicates with the frontal sinus through the large frontomaxillary opening (Hillmann 1975).

Diagnostic Techniques

Complete history and physical examination of the horse, including assessment of mental status, cardiopulmonary functions, hydration status, and body temperature are mandatory prior to sedation, anesthesia, and/or local anesthetic techniques for dental and sinus surgery. Frontonasal and maxillary bone flaps are indicated to remove of a wide variety of lesions that may develop in the paranasal sinuses or turbinates, such as paranasal sinus cysts, neoplasia, progressive ethmoid hematomas, and apical infections of maxillary cheek teeth (Nickels 2012). The lesions typically cause unilateral epistaxis or mucopurulent nasal drainage, in contrast with diseases of the pharynx or lungs in which the drainage is typically bilateral. However, appropriate diagnostic techniques are indicated to rule out concurrent diseases of the pharynx and lungs that may affect patient management either under general anesthesia or under standing sedation.

On endoscopy, narrowed nasal meati, purulent material, masses, or blood can be seen in the nasal passage and/or draining from the sinus openings (Nickels 2012). Radiography of the skull may reveal free fluid lines, radiodense masses, paranasal sinus cysts, and lucency and/or proliferation associated with dental disease (Figure 1.1). Sinocentesis can be used to obtain fluid sample for culture and cytological examination. Sinuscopy with the horse standing and sedated is useful for the examination, diagnosis, and treatment of some disorders of the paranasal sinuses (Nickels 2012).

Figure 1.1 Lateral radiograph of a horse skull showing a fluid line (blue arrows) running through the caudal maxillary sinuses. The horse's nose was angled downward resulting in the gravity-dependent fluid line being parallel to the ground. The sinusitis likely resulted from a periapical infection of a cheek tooth (red arrow). Maxillary nerve blockade can be used to desensitize the area for surgical removal of the tooth and drainage and lavage of the sinus.

Local Anesthetic Techniques for the Equine Teeth and Sinuses

Locoregional anesthesia can be performed prior to many dental and surgical procedures for horses under both standing sedation and general anesthesia. It is routinely accomplished using either lidocaine 2% or mepivacaine 2% solutions with mepivacaine providing a longer duration of action compared to lidocaine (two to four?hours versus one to two?hours). It is generally advisable to infuse a small amount (1-2?ml) of local anesthetic into the skin at the site of the nerve block to desensitize the skin prior to attempting the locoregional block. This is especially important under standing sedation conditions. Approximately 5-10?minutes should be allowed to elapse after administration of the local anesthetic to achieve desensitization of the region.

Infraorbital Block

The infraorbital block desensitizes the maxillary teeth to the level of the first molar, the maxillary sinus, the skin from the lip nearly to the medial canthus, and the rostral nose as well as the roof of the nose (Skarda et al. 2010) (Figure 1.2). The infraorbital canal is palpated as the midpoint on a line between the nasoincisive notch and the rostral most aspect of the facial crest (Rice 2017). The levator labii superioris muscle must be manually elevated to facilitate placement of the needle into the canal (Rice 2017). For local anesthesia of the upper lip and nose, a 20 gauge, 2.5?cm needle can be advanced perpendicularly to the skin at the opening of the infraorbital canal using 5?ml of local anesthetic (Skarda et al. 2010). For blockade of the maxillary teeth and sinus, a 25-20 gauge, 3.8-5?cm needle should be advanced into the canal and 3-5?ml of local anesthetic should be injected (Rice 2017; Skarda et al. 2010).

Figure 1.2 The infraorbital block in the horse. The yellow circle indicates the location of the infraorbital canal, and stippling indicates the area of desensitization following administration of local anesthetic.

Figure 1.3 The maxillary nerve block in the horse. The yellow circle indicates the location of the infraorbital canal, and stippling indicates the area of desensitization following administration of local anesthetic.

Maxillary Nerve Block

Blocking the maxillary nerve within the pterygopalatine fossa results in blockade of the maxillary teeth, the paranasal sinus, and the nasal cavity (Woodie 2013) (Figure 1.3). Multiple techniques have been described for performing this block, owing in part to relatively vague surface landmarks for injection. The first involves the injection of local anesthetic into the extraperiorbital fat body (Staszyk et al. 2008). This technique uses a 18 gauge, 3.5? spinal needle to inject approximately 10?ml of local anesthetic into the fat body surrounding the maxillary nerve (Staszyk et al. 2008). The injection site is made perpendicular to the skin at a point located 10?mm ventral to the zygomatic arch transverse to the plane between the middle and caudal 1/3 of the eye and the needle is advanced until it pops through the masseter muscle for a total depth of approximately 4.5-5?cm (Staszyk et al. 2008). The technique was used in horses under standing sedation using a 20 gauge, 3.5? spinal needle and reported generally successful blockade with no reaction of mechanical or thermal stimulus with mild chewing, bleeding, swelling, and turgor at the injection site as the only complications (Rieder et al. 2016a). Another study evaluating the volume of lidocaine necessary to produce anesthesia with the extraperiorbital fat body technique and found that 2?ml/100?kg of body weight should result in sufficient local anesthesia while minimizing side effects (Rieder et al. 2016b).

Another technique involves the use of a 19 gauge, 2.5? spinal needle with an injection site along the ventral border of the zygomatic arch at the narrowest point of the arch (Newton et al. 2000). The needle is inserted into the skin on a rostromedial and ventral angle and is directed along this angle toward the 6th cheek tooth on the contralateral side to a depth of approximately 2? at which 5?ml of local anesthetic is injected...