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Review of Ultrasonographic Techniques to Evaluate the Equine Skull and Head Structures

AQ: 1 Betsy Vaughan, DVM*; Mary Beth Whitcomb, DVM; and Beth Biscoe, DVM

Ultrasonographic examination is a readily available, inexpensive diagnostic technique for investigat- ing osseous and soft tissue abnormalities of the equine head, is complementary to , and may elucidate bony abnormalities not initially apparent on skull radiographs. Ultrasound may provide enough information to preclude computed tomography examination or may validate recom- mendations for advanced imaging procedures. Authors’ addresses: Department of Surgical and Radiological Sciences (Vaughan and Whitcomb) and William R. Pritchard Veterinary Medical Teach- ing Hospital (Biscoe), School of Veterinary Medicine, University of California, One Shields Avenue, AQ: 2 Davis, CA 95616; e-mail: [email protected]. *Corresponding author. © 2011 AAEP.

1. Introduction raphy (CT) is considered the gold standard to fully Abnormalities of the equine head can cause a myr- characterize osseous abnormalities of the equine head, but it has limited availability both in the field iad of clinical signs including anorexia, dysphagia, 1,2 ptyalism, localized soft tissue swelling, draining and referral setting. The use of MRI to evaluate tracts, evidence of trauma, pain on manipulation of extracranial head structures has also been de- scribed,3,4 but it has similar limitations to CT and the head or cranial cervical region, blepharospasm, requires lengthy anesthesia for image acquisition. or exophthalmus, all of which have their own list of Fortunately, many soft tissue structures of the overlapping differential diagnoses. The anatomy of equine head are superficially located and easily ac- the equine head is complex, and radiographic inter- cessible for ultrasonographic examination. In our pretation is often complicated by summation and 1 experience, ultrasonographic findings frequently superimposition. Many of the soft tissue struc- provide complementary information to skull radiog- tures of the equine head, including the tongue; sub- raphy and help to direct subsequent medical or sur- mandibular and retropharyngeal lymph nodes; the gical treatment. Numerous publications describe parotid, mandibular, and sublingual salivary glands the use of ultrasound for examination of specific and ducts, ocular structures; and the laryngeal and abnormalities or focused regions of the head.5–11 pharyngeal regions are difficult to characterize ra- The purpose of this review is to provide practitioners diographically, and findings may be limited to local- with a concise yet complete guide to ultrasono- ized soft tissue swelling.1,2 Endoscopy is used to graphic examination of the equine head while diagnose abnormalities of the upper airway and gut- summarizing findings from the Large Animal Ultra- tural pouches but provides little information on sound Service at University of California Davis over other head or skull structures. Computed tomog- a 7-year period.

NOTES

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Orig. Op. OPERATOR: Session PROOF: PE’s: AA’s: 4/Color Figure(s) ARTNO: 1st disk, 2nd beb mcquinne 12 1, 3, 4, 5 3125 IMAGING 2. Ultrasonographic Technique A thorough knowledge of the anatomy to be evalu- ated is integral to interpretation of any ultrasono- graphic examination. The skull and head region is no exception. Ultrasound examination of the head can be tailored to the specific region in question (i.e., localized swelling of the ) or may be per- formed broadly in search of a cause for more gener- alized clinical signs. Light sedation is often required because horses frequently object to re- peated manipulation of the head. To maximize im- C age quality, the region to be examined should be O clipped and washed with warm water before the L ultrasound gel is applied. Although not recom- O mended in the head region, if alcohol saturation is R used, care should be taken to protect the eyes from Fig. 1. Transverse ultrasound image of the mandible in a 14- overspray or runoff. year-old Thoroughbred gelding with osteomyelitis that presented Anatomic structures of the head can routinely be for mandibular swelling associated with a draining tract. Ele- imaged with a high-frequency linear transducer (7 vation of the periosteum (arrowheads) is seen in close proximity to 14 MHz), midrange frequency “microconvex” cur- to the draining tract (not seen in this image). This image was vilinear transducer (4.0 to 8.0 MHz) or a rectal obtained with a 13-MHz linear transducer at a depth of 4 cm. transducer at depths ranging from 3 to 8 cm, de- pending on the structure of interest. The microcon- vex transducer is often helpful to image regions that 14 are severely swollen or when the penetration of dividing the joint into dorsal and ventral recesses. the linear transducer is inadequate for visualizing Reports of TMJ disease in horses primarily describe 6,15,16 deeper structures. Furthermore, the sector-shaped cases of septic arthritis or luxation, with a image generated by the curvilinear transducer of- single case report describing neoplastic invasion of 17 fers a wider field of view regardless of scanning the joint. Soft tissue swelling overlying the TMJ depth. A program (application or preset) should be and pain on manipulation or palpation may help selected that maximizes musculoskeletal image localize clinical signs but are not always present. quality. Horses with septic arthritis of the TMJ are often extremely painful and unwilling to masticate.16,18 3. The Skull Radiographic interpretation of the TMJ is challeng- ing, and significant pathology may not be visible due The utility of ultrasound for the examination of bony to superimposition.6,7,16 abnormalities such as fracture, osteomyelitis, and 5 Ultrasonographic techniques for evaluation of the neoplasia has been described. Ultrasound has TMJ are well described in the literature.6,7 The been shown to identify bony pathology consistent TMJ is evaluated along all visible margins, from a with osteomyelitis as early as 1 to 2 days after the rostal, lateral, and caudal window.6,7 Abnormali- onset of symptoms, whereas radiography may take 12,13 ties of the superficial joint margins and intra-artic- up to 2 weeks. The skull is a common location ular disc (meniscus) are readily visible with this for trauma, and underlying bone is frequently con- technique. Roughening of the articular tubercle taminated because of its close proximity to the skin and the mandibular condyle has been described in surface, even with small head wounds. In such cases of septic arthritis.19 Changes in the appear- cases, hypoechoic draining tracts can be followed ance of the intra-articular disc (meniscus) are also from wounds to abnormal bone. Ultrasonographic seen in cases of septic arthritis and include narrow- findings consistent with osteomyelitis include a ing, tearing, obliteration, or extrusion (Fig. 2). F2 thickened, indistinct or proliferative appearance of Thickened or proliferative synovium with or without cortical surfaces, elevation of the overlying perios- excess synovial fluid is also a prominent feature of teum, and/or a thin hypoechoic to anechoic layer 12,13 septic TMJ arthritis. In the author’s experience, F1 overlying the abnormal cortical surface (Fig. 1). ultrasound guidance is advantageous for needle In cases of fracture, step defects can be detected in placement for arthrocentesis and also for through- the otherwise smooth cortical surfaces. Overlying and-through lavage in the standing horse, especially hematoma formation is an inconsistent finding. in cases in which arthroscopy is not an option.

4. 5. The Tongue and Oral Cavity The temporomandibular joint (TMJ) is located be- The inciting cause of nonspecific clinical signs such tween the base of the zygomatic process of the tem- as anorexia, dysphagia, ptyalism, and halitosis can poral bone and the condylar process of the mandible be difficult to identify on physical exam alone, espe- and contains a fibrocartilagenous disc or meniscus cially when pain or lingual swelling precludes a

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Fig. 2. A, Transverse ultrasound image of the lateral aspect of the temporomandibular joint in a 16-year-old Quarter Horse mare with septic arthritis. Note the severe roughening of the bony joint margin and the irregularly shaped heterogeneous appearance of the intra-articular disc (arrowheads). This image was obtained with a 13-MHz transducer at a depth of 5 cm. B, Normal reference image for comparison. Image obtained with a 13-MHz transducer at a depth of 3 cm. CPM indicates condylar process of the mandible; MF, mandibular fossa of the temporal bone.

complete oral examination. Although dental ab- base of the ear to the ramus of the mandible, with its normalities are often readily diagnosed via oral ex- caudal border at the wing of the atlas.22–23 Several amination and skull radiography,20 radiographic smaller ducts join at the rostral and ventral border abnormalities of the tongue or buccal mucosa are of the gland to form the . The duct often limited to identification of radiopaque foreign then courses rostrad alongside the facial vein, bodies, gas accumulation suggestive of anaerobic in- crossing over the ramus of the mandible, entering fection, or soft tissue swelling of the intermandibu- the mouth at the level of the third upper cheek lar space.8,21 tooth.22–23 The tongue is located between the mandibular The is readily visualized ultrasono- rami and is examined ultrasonographically from the graphically and can be examined in its entirety. intermandibular space. Lingual abscesses associ- ated with foreign bodies can be identified from this window in many cases.8 In addition, intraoral ul- trasonographic examination can be performed in amenable horses with the aid of a full-mouth spec- ulum and a rectal transducer placed directly on the tongue.8 Lingual foreign bodies are most com- monly metallic (wires or needles) and demonstrate a linear or slightly curved hyperechoic appearance with variable shadowing. Foreign bodies are often surrounded by hypoechoic to anechoic fluid accumu- lations. Anaerobic infections are common and pro- duce hyperechoic gas echoes that can obscure C F3 visualization of foreign bodies in some cases (Fig. 3). O Lingual abscesses can become very large and painful L if left untreated. Affected horses may require sub- O stantial supportive care due to reduced water and R feed intake. Ultrasonographic guidance is useful for localization, determination of the best approach Fig. 3. Longitudinal ultrasound image of a large, 10 cm ϫ 4cm for removal of lingual foreign bodies, and for pallia- anaerobic abscess at the base of the tongue in a yearling Quarter tive drainage of large abscesses.8 Horse that presented for ptyalism and lingual swelling. The slightly curved linear hyperechoic structure (arrowheads) in the 6. The Parotid, Mandibular, and Sublingual center of the abscess is consistent with the wire foreign body seen Salivary Glands on skull radiographs; however, this image illustrates how hyper- echoic gas echoes can complicate identification of foreign bodies Abnormalities of the salivary glands most commonly when present. This image was taken from the intermandibular 22 involve the parotid gland and duct. It is the larg- space, using an 8.5-MHz curvilinear transducer at a depth of 9 est of the horse and extends from the cm.

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Orig. Op. OPERATOR: Session PROOF: PE’s: AA’s: 4/Color Figure(s) ARTNO: 1st disk, 2nd beb mcquinne 12 1, 3, 4, 5 3125 IMAGING sialoliths can be obscured by gas echoes. Biopsy is required for definitive diagnosis of salivary gland masses; however, melanomas occur with some fre- quency in the parotid gland, especially in gray horses. Melanomas are typically hypoechoic, with smooth to somewhat irregular margins, and may also show hyperechoic areas of mineralization.

7. The Submandibular and Retropharyngeal Lymph Nodes The submandibular lymph nodes are located deep to C the skin surface in the intermandibular space, just O ventral to the tongue, and are commonly visualized L during ultrasound examination of the region. Ret- O ropharyngeal lymph nodes reside lateral to the R guttural pouch, ventral to the parotid salivary gland Fig. 4. Ultrasound image of a markedly enlarged mandibular and dorsolateral to the pharynx, and are un- 5 salivary gland (arrowheads) with anaerobic sialoadenitis in a commonly visualized unless abnormal. Primary 17-year-old Arabian gelding that presented for mandibular swell- abnormalities of the submandibular and retropharyn- ing and ptyalism. The structure visible at the edge of the upper geal lymph nodes include lymphadenitis, abscessation, right-hand corner of the image is the parotid salivary gland and neoplasia.5 Reactive lymphadenitis causing en- (PG). This image was obtained with an 8.5-MHz curvilinear largement of either group of lymph nodes is a frequent transducer at a depth of 7 cm. ultrasonographic finding, especially in horses with re- gional inflammation or infection. In our experience, squamous cell carcinoma is the most common neo- The parotid duct is followed from the parotid gland plasia to affect submandibular or retropharyngeal to its entrance into the oral cavity. The duct is lymph nodes. Affected lymph nodes are enlarged and difficult to visualize in normal horses, but its close may have an irregular surface and a heterogeneous proximity to the facial vein aids in following its appearance. Biopsy is important for definitive course. The parotid duct should be closely evalu- diagnosis. ated in cases of parotid gland dilation and/or absces- Abscessation of submandibular and retropharyn- sation to rule out obstruction. Salivary duct geal lymph nodes can occur with upper respiratory obstruction should be treated promptly because infection and is commonly associated with Strep- chronic obstruction can lead to rupture of the gland tococcus equi subspecies equi infection (stran- or duct.24 Lacerations involving the superficially gles).5,26,27 However, other organisms can cause located parotid duct can lead to salivary fistula for- similar clinical signs, and aspiration is warranted mation or duct obstruction via scar tissue once the for culture and sensitivity in some cases.28 The laceration has healed.22 Salivary duct laceration ultrasonographic appearance of abscessed lymph has been reported to be the most common abnormal- nodes ranges from cavitated, hypoechoic, to anechoic ity of the duct22; however, septic sialoadenitis and areas within the node to complete disruption of the sialolithiasis are found more frequently in the au- node by the abscess.5 thors’ experience. The mandibular salivary gland is located deep to 8. The Eye the parotid gland and mandible, with its medial Ultrasonography provides an excellent noninvasive border intimately associated with the guttural manner to image the globe and retrobulbar space. pouch, larynx, and common carotid artery.22,23 Ultrasound is especially useful when opacification of The mandibular salivary duct courses along the ax- the anterior segment of the eye limits a complete ial border of the mandible in the intermandibular ophthalmoscopic examination or when severe swell- space. The sublingual salivary glands can also be ing of the eyelids prevents examination of the evaluated from the intermandibular space. globe.9–11,29 Ultrasonographic examination pro- Ultrasonographic abnormalities of the salivary vides excellent visualization of the cornea, anterior glands include distention of the glands and/or duct and posterior chambers, anterior and posterior re- with or without hyperechoic gas echoes, consistent flections of the lens capsule, iris and ciliary body, F4 with sialoadenitis (Fig. 4), sialolithiasis leading to corpora nigrans, optic nerve and disc, and retrobul- duct obstruction, and mass lesions involving the bar space.9–11,29 glands. Sialoliths are mineral concretions within Cataracts are the most common ultrasonographic the duct that cause obstruction.22,24,25 They can be abnormality of the equine eye and are characterized seen on ultrasonographic evaluation as small hyper- by increased echogenicity of the lens and increased echoic structures that commonly cast shadows and definition of the lens capsule.9–11,29 Lens luxation cause upstream dilation of the duct. Ascending an- can also be seen in cataractous eyes (Fig. 5). Reti- F5 aerobic infection is not uncommon, in which case nal detachments are seen as curved linear struc-

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Orig. Op. OPERATOR: Session PROOF: PE’s: AA’s: 4/Color Figure(s) ARTNO: 1st disk, 2nd beb mcquinne 12 1, 3, 4, 5 3125 IMAGING 10. Results Ninety-three horses underwent 118 ultrasound ex- aminations of regions of the head performed by the Large Animal Ultrasound Service at the William R. Pritchard Veterinary Medical Teaching Hospital (VMTH) between January 2003 and December 2010. Abnormalities of the salivary glands and neoplasia were the most common diagnoses, seen in 19 horses each, followed by osteomyelitis (nϭ11), temporo- mandibular joint disease (nϭ10), abscesses and/or foreign bodies of the tongue (nϭ9), ocular abnormal- C ities (nϭ9), primary submandibular or retropha- O ryngeal lymph node abnormalities (nϭ8), and L abscessation not involving the tongue or lymph O nodes (nϭ6). Twelve horses were given miscella- R neous diagnoses not categorized above, and some horses were diagnosed with more than one abnor- Fig. 5. Transverse ultrasound image of a luxated cataractous mality. Ultrasonographic findings yielded or cor- lens (LL) in an 18-year-old Appaloosa gelding with chronic equine roborated the final diagnosis in 70 of 97 horses, with recurrent uveitis. LL indicates luxated lens; AC, anterior cham- 25 ultrasound-guided biopsies and/or aspirates per- ber. Arrowhead points to fibrin/vitreal debris. This image was formed in 21 horses. CT examination was ulti- obtained with an 8.5-MHz transducer at a depth of 7 cm. mately performed in 9 horses, primarily to fully characterize invasive neoplastic lesions and septic arthritis of the temporomandibular joint.

tures within the posterior chamber that arise from 11. Summary 9–11,29 the optic disc, creating a “seagull” appearance. Ultrasonographic evaluation of the head has become The retrobulbar space should be carefully eval- an important part of the diagnostic workup at the uated in cases of exophthalmus for evidence of 9–11,29,30 VMTH for horses presenting with facial swelling, neoplasia, hemorrhage, and abscessation. wounds, draining tracts, ptyalism, dysphagia, or fa- Differentiation based solely on ultrasonographic cial trauma or as a complementary imaging modal- findings can be challenging and may require ul- ity to skull radiography. The number and variety trasound-guided sampling or advanced imaging of cases presenting over a 7-year period highlight such as CT or MRI. The retrobulbar space can be the utility of the procedure and its wide acceptance difficult to interpret, and the contralateral orbit at the VMTH. In many cases without radiographic should be examined for comparison in all cases. abnormalities, positive ultrasound findings often 9. Neoplasia provided enough information to formulate a treat- ment plan, whereas in other cases, owners were The head is a common site for neoplasia, and masses prompted to pursue further advanced imaging can be seen involving all structures described 31 procedures. above. The majority of masses affecting the bony The clinical anatomy of the equine head is com- surface or soft tissue structures of the skull are plex, with numerous overlapping structures that amenable to ultrasonographic evaluation, and ultra- can make interpretation of physical exam and radio- sound-guided biopsy or aspiration is possible in graphic findings difficult. The use of ultrasound is many cases. Tumors affecting the equine head in- now widespread in equine practice, and most prac- clude melanoma, squamous cell carcinoma, lympho- titioners have access to a portable ultrasound ma- sarcoma, hemangiosarcoma, sarcoids, odontogenic chine equipped with at least a rectal transducer. tumors, and bony tumors such as , ossify- The superficial nature of the soft tissues overlying ing fibroma, fibrous dysplasia, and osteosarco- 17,20,31–33 the equine skull make it an ideal structure for ul- ma. The ultrasonographic appearance of trasonographic evaluation, and ultrasonographic ex- neoplastic masses varies greatly, depending on the amination should be included in the diagnostic location and tumor type. Masses can be variably arsenal for abnormalities of the equine head. echogenic and may demonstrate anechoic areas con- sistent with necrosis, hyperechoic areas consistent References 5 with dystrophic mineralization, or both. Defini- 1. MacDonald MH. Clinical examination of the equine head. tive diagnosis requires histopathologic classification Vet Clin North Am (Equine Pract) 1993;9:25–48. and cannot be made on the basis of ultrasonographic 2. Morrow KL, Park RD, Spurgeon TL, et al. Computed tomo- appearance alone. However, a presumptive diag- graphic imaging of the equine head. Vet Radiol Ultrasound 2000;6:491–497. nosis of melanoma is often made in gray horses with 3. Rodriquez MJ, Agut A, Soler M, et al. Magnetic resonance multiple masses in the retropharyngeal and/or pa- imaging of the equine temporomandibular joint anat- rotid gland regions. omy. Equine Vet J 2010;42:200–207.

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4. Rodriquez MJ, Agut AA, Lopez-Albors O, et al. Computed 18. Baker GJ. Equine temporomandibular joints (TMJ): mor- tomography and magnetic resonance imaging of the equine phology, function and clinical disease, in Proceedings. Am temporomandibular joint anatomy: technique and normal ap- Assoc Equine Pract 2002;48:442–447. pearance in cadaveric specimens, in Proceedings. Am Assoc 19. Weller R, Cauvin ER, Bowen IM, et al. Comparison of radi- Equine Pract 2009;55:427–428. ography, scintigraphy and ultrasonography in the diagnosis 5. Reef VB. Equine Diagnostic Ultrasound. Philadelphia: of a case of temporomandibular joint arthropathy in a horse. WB Saunders Company, 1998;181–182, 480–547. Vet Record 1999;144:377–379. 6. Weller R, Taylor S, Maierl J, et al. Ultrasonographic anat- 20. Bladon BM, Lane JG. The differential diagnosis and inves- omy of the equine temporomandibular joint. Equine Vet J tigation of facial swellings. Equine Vet Educ 1997;9:230– 1999;31:529–532. 239. 7. Rodriquez MJ, Soler M, Latorre R, et al. Ultrasonographic 21. Engelbert TA, Tate LP. Penetrating lingual foreign bodies anatomy of the temporomandibular joint in healthy pure- in three horses. Cornell Vet 1993;83:31–38. bred Spanish horses. Vet Radiol Ultrasound 2007;48:149– 22. Schumacher J, Schumacher J. Diseases of the salivary 154. glands and ducts of the horse. Equine Vet Educ 1995;7:313– 8. Pusterla N, Latson KM, Wilson WD, et al. Metallic foreign 319. AQ: 3 bodies in the tongues of 16 horses. Vet Record 2006;159: 23. Kainer RA. Clinical anatomy of the equine head. Vet Clin 485–488. North Am (Equine Pract) 1993;9:1–23. 9. Hallowell GD, Bowen IM. Practical ultrasonography of the 24. Bouayad H, Ouragh L, Tnibar DW, et al. Sialoliths in the horse. Equine Pract 1991;13:25–27. equine eye. Equine Vet Educ 2007;19:600–605. 25. Kay G. Sialolithiasis in equids: a report of 21 cases. Equine 10. Scotty NC, Cutler RJ, Brooks DE, et al. Diagnostic ultra- Vet Educ 2006;18:333–336. sonography of equine lens and posterior segment abnormal- 26. Golland LC, Hodgson DR, Davis RE, et al. Retropharyngeal ities. Vet Ophthomology 2004;7:127–139. lymph node infection in horses: 46 cases (1977– 11. Whitcomb MB. How to diagnose ocular abnormalities with 1992). Aust Vet J 1995;72:161–164. ultrasound, in Proceedings. Am Assoc Equine Pract 2002; 27. Whelchel DD, Chaffin MK. Sequelae and complications of 48:272–275. equi subspecies equi infections in the horse. 12. Riebel TW, Nasir R, Nasarenko O. The value of sonography Equine Vet Educ 2009;21:135–141. in the detection of osteomyelitis. Pediatr Radiol 1996;26: 28. Albini S, Korczak BM, Abril C, et al. Mandibular lymph- 291–297. adenopathy caused by Actinomyces denticolens mimick- 13. Chau CLF, Griffith JF. Musculoskeletal infections: ultra- ing strangles in three horses. Vet Record 2008;162:158– sound appearances. Clin Radiol 2005;60:149–159. 159. 14. Rodriguez MJ, Aut A, Gil F, et al. Anatomy of the equine 29. Ramirez S, Tucker RL. Ophthalmic imaging. Vet Clin temporomandibular joint: study gross dissection, vascular Equine 2004;20:441–457. injection and section. Equine Vet J 2006;38:143–147. 30. Plummer CE. Exopthalmus in the horse. Equine Vet Educ 15. Ramzen PHL. The temporomandibular joint: component 2007;19:584–589. of clinical complexity. Equine Vet J 2006;38:102–104. 31. Hance SR, Bertone AL. Neoplasia. Vet Clin North Am 16. Moll HD, May KA. A review of conditions of the equine (Equine Pract) 1993;9:213–234. temporomandibular joint, in Proceedings. Am Assoc Equine 32. MacGillivray KC, Sweeney RW, Del Piero F. Metastatic Pract 2002;48:240–243. melanoma in horses. J Vet Intern Med 2002;16:452–456. 17. Perrier M, Schwarz T, Gonzale O, et al. Squamous cell 33. Pirie RS, Dison PM. Mandibular tumours in the horse: carcinoma invading the right temporomandibular joint in a a review of the literature and 7 case reports. Equine Vet Belgian mare. Can Vet J 2010;51:885–887. Educ 1993;5:287–294.

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