Topographical Anatomy, Computed Tomography, and Surgical Approach of the Guttural Pouches of the Donkey

Topographical Anatomy, Computed Tomography, and Surgical Approach of the Guttural Pouches of the Donkey

REFEREED ORIGINAL RESEARCH Topographical Anatomy, Computed Tomography, and Surgical Approach of the Guttural Pouches of the Donkey M.A.M. Alsafy, Ph...

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REFEREED

ORIGINAL RESEARCH

Topographical Anatomy, Computed Tomography, and Surgical Approach of the Guttural Pouches of the Donkey M.A.M. Alsafy, PhD,a M.H. El-Kammar, PhD,b and S.A.A. El-Gendy, PhDa

ABSTRACT The guttural pouches of the donkey are a pair of pouches located dorsoposteriorly to the posterior pharynx. The pouches have a close contact rostrally with the sphenoid bone, ventrally with the pharynx and esophagus, and caudally with the atlantoaxial joint. The left and right guttural pouches have almost the same capacity. In this study, topographic anatomy, computed tomography (CT) examination, and a surgical approach were carried out in the heads of donkeys. Results of the CT examination, transverse and sagittal sections of the heads, revealed that the donkeys possessed large, symmetrical guttural pouches incompletely divided into medial and lateral compartments by the stylohyoid bone. Moreover, the right and left guttural pouches met each other at the median part to form a thin membranous septum. As CT sections approached the occipital condyle, the lateral compartment disappeared and the medial compartment gradually became smaller toward the base of the skull. Two cadaver heads were examined to assess the anatomic limits of the surgical approach and whether vital structures might be damaged. Direct access to the lateral compartment was performed using a modified Garm’s technique, whereas direct access to the medial compartment was gained through Viborg’s triangle. Keywords: Guttural pouches; Donkey; Topographic anatomy; CT; Surgical approach

INTRODUCTION Domestic horse (Equus caballus) and donkey (Equus asinus) share a common ancestry but have evolved into morphologically distinct species. Donkeys are used mainly as work animals and play an important role in the economies of developing countries.1 From the Departments of Anatomya and Surgery,b Faculty of Veterinary Medicine, Alexandria University, Edfina, Rashed, Behera, Egypt. Reprint requests: Mohamed A.M. Alsafy, PhD, Anatomy Department, Post Box 22785, Faculty of Veterinary Medicine, Edfina, Behera, Egypt. 0737-0806/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jevs.2008.02.005

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Figure 1. The topography of the Viborg’s triangle of donkey. 1, Jugular vein; 2, sternomandibularis muscle; 3, tendon of sternomandibularis muscle; 4, linguofacial vein; 5, caudal angle of the mandible; 6, base of the external ear; 7, masseter muscle; 8, mandibular gland; arrow shows the Viborg’s triangle area.

Guttural pouches (GP) have an anatomic and surgical importance, although their role is not completely understood. They are paired, air-filled, ventral evaginations of the auditory tubes, each ranging in volume from 300 to 600 mL in horse,2 and are unique to the other equines as well as to rodents, bats, and rhinoceros. GP are located in the parotid region under the base of the skull and the atlas bone3-5 and are covered laterally by the pterygoid muscles and parotid and submandibular salivary glands. In the medial plane, the left and right GP are separated by the medial septum, longus capitis, and rectus capitis ventralis muscles. From the lateral aspect, the stylohyoid bone raises a deep ridge that incompletely divides each GP into medial and lateral compartments.2,6 Clinicians need to be aware of the normal appearance of the adjacent anatomic structures when performing endoscopy of the GP.2,7,8 Some advanced studies have demonstrated the functional anatomy and physiologic roles of the guttural pouches.6,9 GP regulate the temperature of arterial blood circulating to the brain.10,11 Observation of the equine guttural pouch has been performed by anatomic approaches, including preparation of mold specimen,4,12

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Figure 2. Sagittal section of the head of the donkey passes at the level of the middle of the nuchal crest, atlas, and axis bones. (A) Right side and (B) left side. 1, right guttural pouch; 2, pharynx; 3, stylohyoid bone; 4, membranous medial wall of the guttural pouch; 5, nasal cavity; 6, brain; 7, frontal sinus; 8, sphenoid bone; 9, laryngeal cartilages; 10, longus capitis muscle; 11, rectus capitis ventralis muscle; 12, atlantoaxial joint; 13, soft palate.

soft x-ray radiography,13,14 and computed tomography.15 Surgical access is required to treat some complicated cases of empyema, tympany, and mycosis and for stylohyoid ostectomy.16 In some cases of empyema, aggressive lavage and endoscopic removal of chondroids has been reported to have a good prognosis.17-19 Currently, the common surgical methods used to approach the GP include hyovertebrotomy, Viborg’s triangle, and the two Whitehouse methods.17 These approaches have been in use for nearly 200 years and mostly are successful. They allow direct access to the medial compartment (MC) of the GP and limited indirect access to the lateral compartment (LC). In a recent report, a modified Whitehouse approach was used in standing horses affected with empyema, allowing chondroids from the LC to be pushed into the MC through the GP ostia using an endoscope.20 Accessing the GP via the contralateral pouch through the medial septum and fistulation of the auditory tube diverticulum using yttrium garnet aluminum laser have been used to treat GP empyema.21,22 However, these techniques do not entirely solve the problem of exudates also in the LC.23 The current study was conducted to examine the heads of donkeys by both topographic anatomy (sagittal and transverse sections) and computed tomography (CT) scans to define the shape and the position of the GP in addition to providing information on best sites for surgical approach to the guttural pouches.

MATERIALS AND METHODS Seven healthy adult donkeys were used in the current study. Topographic Anatomy Four animals were used for topographic anatomy, in a fresh state after the animals were bled. Dissection of the Viborg’s

triangle,24 sagittal section of the donkey’s head passed at the level of the middle of nuchal crest, atlas, and axis bones, whereas transverse section passed cranially from the level of the external ear through the middle of the Viborg’s. All steps were photographed by digital camera. Computed Tomography of the Guttural Pouches The head of one adult donkey was used for CT scans. After being bled, the head was removed from the level of the third cervical vertebra and CT examination was performed as follows. The head was serially sectioned with the CT scanner (CT-W450-10A, HITACHI, Japan) from nasal to nuchal section at 1-cm intervals (scanning conditions: 120 kV, 50 mA). The internal shape of the GP were observed in these sections.15,25 Surgical Approaches to the Guttural Pouch A Viborg’s triangle approach was performed to penetrate the MC of two donkey’s heads,24 whereas Garm’s technique26 as well as a modified Garm’s technique (MGT) were done on both heads. The heads were removed from the level of the second to third cervical vertebrae to avoid disturbing the architecture of the guttural pouches. Garm’s technique was performed on one GP in each dissected head and MGT was performed on the other GP. A metallic, tapered-end trochar with plastic blunt end rigid sheath (20 cm long, 0.7-mm diameter) was used to penetrate into the LC. Care was taken to ensure that the sheath of the trochar stayed close to the stylohyoid bone and the medial side of the pterygoid muscle.27

RESULTS Topographic Anatomy Guttural pouches are paired, air-filled pouches of the auditory tubes. Our topographic anatomy studies showed that

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guttural pouches into medial and lateral compartments (Figs. 1, 2, 3, 4). Computed Tomography The guttural pouches were observed in serial CT sections of the head of a donkey (Fig. 5). Right and left guttural pouches started to appear on both sides of the pharyngeal recess (Fig. 5A, B; #10). In the more nuchal section, each pouch spreads in basal, lateral directions, and they meet at the median part to form a thin septum (Fig. 5C). Moreover, at the head of the mandible in this section, each guttural pouch starts to be divided into medial and lateral compartments by the stylohyoid bone (Fig. 5D, E). In the more nuchal section, the stylohyoid bone more clearly divides the pouch into two compartments (#7). Furthermore, in this section, the thin septum could not be seen because each medial compartment was distant from the center line of the head (Fig. 5F, G). In the same section, the lateral compartment of the pouch is completely separated from the medial compartment by the stylohyoid bone (Fig. 5H). As the CT sections approach the occipital condyle (#32), the lateral compartments disappear (Fig. 5I, J, K) and the medial compartments gradually become smaller toward the base of the skull (Fig. 5L, M; #23).

Figure 3. Transverse section passes from the level of the external ear and through the middle of Viborg’s triangle. L, left side; R, right side. 1, right guttural pouch; 2, left guttural pouch; 3, stylohyoid bone; 4, medial pterygoid muscle; 5, median septum separated between the right and left guttural pouches; 6, ramus of mandible; 7, condylar process of mandible; 8, brain; 9, sphenoid bone; 10, laryngeal cartilages; 11, medial compartment of guttural pouch; 12, lateral compartment of guttural pouch.

they lie between the base of the skull and atlas dorsally, the pharynx and commencement of the esophagus ventrally, and the medial and lateral pterygoid muscles, parotid, and mandibular glands laterally. Medially, the dorsal parts of the two sacs are separated from each other on the midline by the rectus capitis ventralis and longus capitis muscles dorsally, and the median septum ventrally, but below this they meet, forming a thin median septum. The floor lies mainly on the pharynx and covers the stylohyoid bone, which raises a ridge that incompletely divides the

Surgical Approach Direct access to the medial compartment of the guttural pouch was achieved through the Viborg’s triangle in the triangular area formed by the caudal border of the mandible, tendon of the sternocephalicus muscle, and linguofacial vein. The trochar was introduced through the triangle medially to the caudal border of the mandible at a 458 angle toward the base of the external ear (Figs. 1, 4A). Direct access to the lateral compartment was made at a level 2 cm caudal to the vascular incisure within the mandibular space medial and parallel to the facial artery, vein, and parotid duct. The mandibular lymph node was pushed medially to separate the septum between the mylohyoid and digastric muscles. The trochar was introduced carefully at a 458 angle along the medial aspect of stylohyoid bone in a caudodorsal direction for approximately 18 cm to reach the lateral compartment between the stylohyoid bone and pterygoid muscle (Fig. 4B). Direct access to the LC of the GP has been considered unacceptably risky because of the presence of many structures that traverse the LC, such as the external carotid artery and its branches. Facial mandibular nerves, in addition to the glossopharyngeal nerve, lingual artery, and, in between them, the linguofacial artery, pass on the medial side of the stylohyoid bone. The hypoglossal nerve runs ventrally to the rostral end of the stylohyoid bone, and the facial artery turns rostroventrally

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Figure 4. Surgical approach of the guttural pouch. (A) Direct access to medial compartment through Viborg’s triangle; (B) Direct access to lateral compartment. 1, Metallic trochar with plastic sheath passes through vascular incisure of mandible at an angle of 45o, close to the stylohyoid bone and the medial side of the pterygoid muscle until reaching the lateral compartment of the left guttural pouch; 2, metallic trochar with plastic sheath passes through Viborg’s triangle ventral to the caudal border of the mandible, parallel to the jugular vein until reaching the medial compartment of the left guttural pouch; 3, sternomandibularis muscle tendon; 4, linguofacial vein; 5, part of the caudal border of the mandible; 6, lateral compartment of the guttural pouch; 7, part of the medial pterygoid muscle; 8, stylohyoid bone; 9, vascular incisure, facial vein, artery and the duct of parotid gland.

and runs laterally toward the vascular incisure of the mandible.

DISCUSSION The current study demonstrated that donkeys possessed large symmetric guttural pouches, which appeared as paired, air-filled, ventral evaginations of the auditory tubes. They are located in the parotid region under the base of the skull and atlas bone and are covered laterally by the pterygoid muscles and the parotid and mandibular salivary glands. In the medial plane, the left and right GP were separated by the medial septum, longus capitis, and rectus capitis ventralis muscles. From the lateral aspect, the stylohyoid bone raises a deep ridge that incompletely divides each GP into medial and lateral compartments. These results are in agreement with those recorded previously.2-5 In our study, CT scanning was performed to examine the guttural pouch of donkeys. Using CT, the outline of the GP was shown clearly, and the relationship between location of the pouches and the bones was clearly visualized. The nuchal basal section of the medial compartment of the GP gradually became smaller toward the base of the skull in the serial CT sections. This finding was different from those reported previously,5,9,12,28 who found that, in the guttural pouch of the domestic horse, the nuchal basal section of the medial compartment is well developed

in the nuchal direction. Although the donkey has larger ears with large caudal auricular arteries, this made no difference in structure of the GP in the donkey than in the horse. One of the most obvious differences is in the shape and extent of the pharyngeal recess, which in the donkey is remarkable for its large size and has a caudal elongation between the left and right GP. This finding agrees with that reported by Lindsay and Clayton.1 Surgical access to the MC could be performed through Viborg’s triangle, where the boundaries could be palpated in a live donkey after being demonstrated on the cadaver.24 Direct surgical access to the LC of the GP has been considered unacceptably risky.23,27 In conclusion, our examination of the GP by topographic anatomy, serial CT sections, and surgical approach demonstrated that donkeys possess large, symmetric guttural pouches. The GP were framed by the base of the skull at the top, the pharynx and esophagus at the bottom, and the salivary glands and mandible on the sides. A slit-like opening from each GP into the pharynx was noted; this opening was difficult to see and even more difficult to open for examination or drainage purposes. We emphasize that the best site for guttural pouch approach was through Viborg’s triangle, which allows direct access to the MC of the GP. In addition, there was unacceptable predictable risk through direct access to the LC of the GP.

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Figure 5. A-M. CT serial sections of the head of donkey from nasal to nuchal part. All CT scans printed are viewed from cranial to caudal, and the left side appear on the viewer’s left, where soft tissue window images on the left side and bone window images on the right side. 1, sphenopalatine sinus; 2, ramus of the mandible; 3, coronoid process of the mandible; 4, frontal lobe of brain; 5, nasopharynx; 6, frontal bone and frontal sinus; 7, stylohyoid bone; 8, thyrohyoid bone; 9, omohyoideus and sternohyoideus muscles; 10, rostral part of guttural pouch; 11, basihyoid bone; 12, lateral pterygoid muscle; 13, medial pterygoid muscle; 14, zygomatic process of temporal bone; 15, left guttural pouch; 16, right guttural pouch; 17, masseter muscle; 18, laryngeal cavity; 19, nasopharynx; 20, pharyngeal recess; 21, thyroid cartilage; 22, temporomandibular joint; 23, medial compartment of the guttural pouch; 24, lateral compartment of the guttural pouch; 25, corniculate process of arytenoid cartilage; 26, arytenoid cartilage; 27, external ear canal; 28, endotracheal tube; 29, parotid salivary gland; 30, basilar part of occipital bone; 31, longus capitis muscle; 32, occipital condyle; 33, linguofacial vein; 34, nuchal crest; 35, caudal aspect of the right guttural pouch; 36, caudal aspect of the left guttural pouch; 37, mandibular salivary gland; 38, atlantooccipital articulation; 39, jugular process; 40, cricoid cartilage.

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Figure 5. (Continued)

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Figure 5. (Continued)

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