How to Ultrasound the Equine Larynx - AAEP · How to Ultrasound the Equine Larynx Katherine S. Garrett, DVM Author’s address: Rood and Riddle Equine Hospital, PO Box 12070, Lexington,

How to Ultrasound the Equine Larynx

Katherine S. Garrett, DVM

Authors address: Rood and Riddle Equine Hospital, PO Box 12070, Lexington, Kentucky 40580-2070; e-mail: [email protected]. 2010 AAEP.

1. Introduction

Abnormalities of the upper airway (UA) are fre-quently implicated as a cause of poor performance,exercise intolerance, or abnormal respiratory noise inthe equine.13 Common abnormalities involving thefunction of the arytenoid cartilages include recurrentlaryngeal neuropathy, arytenoid chondritis, and laryn-geal dysplasia. Dorsal displacement of the soft palate(DDSP) and pharyngeal collapse affect the pharyngealregion. Congenital abnormalities and dynamic lar-ynx collapse associated with poll flexion can affectmultiple regions of the larynx and pharynx.

Resting UA endoscopy is widely available and isoften the first diagnostic tool used in evaluation ofUA abnormalities. In many cases, resting endos-copy is sufficient for diagnosis. However, dynamicevaluation, either using a treadmill or a portableover ground videoendoscopy system, is considered toprovide a more accurate diagnosis than resting en-doscopy.46 Unfortunately, dynamic UA examina-tion requires specialized equipment that may not beavailable locally, may be cost prohibitive for someclients, and may not be appropriate for some horses.Additionally, some UA abnormalities cannot be fullyassessed using UA endoscopy.

Laryngeal ultrasonography, first described byChalmers et al. in 2006,7 is a relatively new additionto the diagnostic repertoire for UA disorders. Ithas proven to be useful clinically in the diagnosis of

many conditions of the UA, especially disorders re-sulting in abnormal arytenoid movement includingrecurrent laryngeal neuropathy, arytenoid chondri-tis, and laryngeal dysplasia, a congenital malforma-tion of the larynx.710 Laryngeal ultrasonographycan be especially helpful in cases where a dynamicexamination cannot be performed.

An appreciation of the pathologic changes that occurin diseases of the UA is helpful in understanding theultrasonographic changes that accompany each dis-ease. Recurrent laryngeal neuropathy results in de-nervation atrophy and subsequent loss of function ofthe intrinsic laryngeal muscles innervated by the re-current laryngeal nerve, including the cricoarytenoi-deus dorsalis (CADM), cricoarytenoideus lateralis(CALM), transversus arytenoideus, vocalis, and ven-tricularis muscles.11,12 Clinically, this condition man-ifests itself as decreased or absent arytenoid abductionand affects the left arytenoid more commonly than theright arytenoid.4 Denervated muscle has a charac-teristic ultrasonographic appearance; the muscle be-comes hyperechoic to normal muscle and has a morehomogeneous appearance with a loss of the normalstriated pattern.1315 These changes have been ob-served ultrasonographically in the CALM and CADMof horses with treadmill UA endoscopy-confirmed re-current laryngeal neuropathy.8,10

The precise etiology of arytenoid chondritis re-mains unclear but likely results from local trauma

AAEP PROCEEDINGS Vol. 56 2010 249

IMAGING

NOTES

to and subsequent bacterial infection of the aryte-noid cartilage.16 Some horses may have granulo-mas only on the axial surface of the arytenoidcartilage, whereas the pathology may progress toinvolve the entire arytenoid cartilage in otherhorses. In more severe cases, the arytenoid be-comes thickened with irregular margins and hasimpaired movement, which may be appreciated en-doscopically.17 The presence of arytenoid cartilagethickening, irregular margination, and abnormalechogenicity typical of this disease has been imagedusing ultrasonography.7

A congenital defect known as laryngeal dysplasia(also known as fourth branchial arch defect or4-BAD) may cause abnormal arytenoid movement ofeither the left or right arytenoid cartilage.18,19

Horses with this condition may also have constantor intermittent rostral displacement of the palato-pharyngeal arch during UA endoscopy or dorsal dis-placement of the soft palate.9,20 Horses withlaryngeal dysplasia have characteristic anatomicabnormalities, including lack of a cricothyroid artic-ulation and extension of the thyroid lamina dorsalto the muscular process of the arytenoid cartilage,and they may have pharyngeal muscle abnormali-ties as well.1921 The ultrasonographic appearanceof these anatomic features has been described.9

The pathophysiology of DDSP has not been fullyelucidated but may involve an inability to maintaina rostral position of the larynx.22 Interestingly, achange in the position of the basihyoid bone after sur-gical treatment (laryngeal tie-forward) is associatedwith an increased likelihood of racing post-operative-ly.23 This finding prompted initial investigation into

an ultrasonographic marker for DDSP, which hasshown that horses with DDSP had a smaller averagedistance between the skin and basihyoid bone thanhorses without DDSP.24 However, the difference inthis measurement between horses with and withoutDDSP was less than 2 mm, limiting its use in clinicalsituations.

The etiology of other UA disorders, including pha-ryngeal collapse, billowing of the soft palate, epiglot-tic retroversion, laryngeal collapse associated withflexed head position, and axial deviation of thearyepiglottic folds, remains unclear.16,25 No char-acteristic ultrasonographic findings of these disor-ders have been described.

The purpose of this paper is to provide the prac-titioner with instruction on how to perform a laryn-geal ultrasonographic examination and incorporatethis tool it into his or her evaluation of the UA.

2. Materials and Methods

Horses are prepared for the examination by sedationwith xylazine hydrochloride (0.4 mg/kg, IV). Mosthorses do not need to have the laryngeal regionclipped before examination, but if the coat is coarseor thick, clipping will improve image quality. Thehead is held in an extended position by a handleror stand to move the laryngeal region caudally inrelation to the mandible. A linear or curvilineartransducer can be used for the examination. A fre-quency of 810 mHz typically provides adequatepenetration while preserving resolution.

Examination of the lateral portion of the larynxcan be performed in the dorsal and transverseplanes. In the dorsal plane, an initial image of the

Fig. 1. Dorsal plane ultrasound image of the lateral aspect of a normal larynx (A). Transducer position is shown in B. Note theposition of the cricoarytenoideus lateralis muscle (small arrowheads) between the thyroid cartilage (small arrows) and the arytenoidcartilage (large arrowhead). The cricoid cartilage (large arrow) is caudal to the thyroid cartilage. Rostral is to the left of the image andcaudal is to the right of the image.

250 2010 Vol. 56 AAEP PROCEEDINGS

IMAGING

superficially positioned thyroid cartilage, the cricoidcartilage caudal to the thyroid cartilage, and thearytenoid cartilage deep to the thyroid cartilage canbe obtained (Fig. 1). The CALM is imaged betweenthe thyroid and arytenoid cartilages, and the crico-thyroideus muscle is imaged between the thyroidand cricoid cartilages. By moving the transducerslightly dorsally, one can image the cricothyroid ar-ticulation, which is formed by the caudal cornu ofthe thyroid cartilage and the articular process ofthe cricoid cartilage (Fig. 2). Mineralization of thecaudal cornu of the thyroid cartilage is common, butthis does not seem to be clinically significant. Fromthe cricothyroid articulation, the transducer is moveddorsally and angled slightly ventrally. In this loca-tion, one can image the cricoarytenoid articulationbetween the muscular process of the arytenoid car-tilage and the dorsal cricoid cartilage. The lateralportion of the CADM may be evaluated as well(Fig. 3).

The lateral aspect of the larynx is also evaluatedin the transverse plane. The initial image obtainedis the superficially positioned thyroid lamina withthe arytenoid cartilage deep to the thyroid laminaand the CALM and vocalis muscle between the thy-roid and cricoid cartilages (Fig. 4). In some horses,the vocalis muscle is imaged distinctly from theCALM, but in others, the distinction between thetwo muscles cannot be defined. If the transducer ismoved caudally, the caudal cornu of the thyroid

cartilage, the cricothyroid articulation, and the cri-coid cartilage are imaged.

The ventral portion of the larynx can be examinedin transverse and median planes. In a transverseplane, the tracheal rings can serve as a referencepoint. From the tracheal rings, as the transducer ismoved rostrally, the ventral aspect of the cricoidcartilage is identified (Fig. 5), followed by the thy-roid cartilage. Deep to the thyroid cartilage, thevocal folds may be imaged, and their movements canbe observed (Fig. 6). The mineralized rostral as-pect of the ventral thyroid cartilage is encounterednext. Between the thyroid cartilage and the basi-hyoid bone, the thyrohyoid bones may be imagedlaterally. The basihyoid bone appears as a horizon-tal line (Fig. 7), and if the transducer is angledrostrally from this position, the ceratohyoid bonesare imaged. By moving the transducer rostrally,the lingual process of the basihyoid bone is imaged.The depth of the basihyoid bone can be measured atthe junction between the lingual process and body ofthe basihyoid bone. In the median plane, the rela-tionship between the lingual process and the miner-alized rostral aspect of the thyroid cartilage can beevaluated. The ventral aspect of the cricoid andtracheal rings can also be imaged.

Fig. 2. Dorsal plane ultrasound image of the lateral aspect of anormal larynx. This image is slightly dorsal and caudal to Figure1. The cricothyroid articulation (small arrowhead) is formed bythe caudal cornu of the thyroid cartilage (small arrows) and thearticular process of the cricoid cartilage (large arrow). The mus-cular process of the arytenoid cartilage is also imaged (largearrowhead). Rostral is to the left of the image and caudal is to theright of the image.

Fig. 3. Dorsal plane ultrasound image of the dorsolateral aspectof a normal larynx. This image is dorsal to Figure 1. The crico-arytenoid articulation (small arrowhead) is formed by the mus-cular process of the arytenoid (large arrowhead) and thedorsolateral cricoid cartilage (large arrow). The lateral portion ofthe cricoarytenoideus dorsalis muscle is imaged (small arrows).Rostral is to the left of the image and caudal is to the right of theimage.


IMAGING

In general, the overall symmetry of the laryngealcartilages and associated musculature should beevaluated. The cartilages should be smoothly mar-ginated with homogeneous echogenicity, althoughmineralization of the thyroid and arytenoid carti-lages is fairly common (Fig. 8), especially in olderhorses. The muscles normally have heterogeneousechogenicity and have a striated pattern in a longi-tudinal view.

Comparison of the relative echogenicity of the leftand right CALM and CADM enables the practitio-ner to assess whether one structure has the hyper-echogenicity characteristic of denervation atrophy

of recurrent laryngeal neuropathy. Ultrasoundmachine settings should be kept constant betweenleft and right sides of the larynx, and images shouldbe evaluated in dorsal and transverse planes. Sideby side comparison is often useful (Fig. 9). Itshould be borne in mind that many factors will con-tribute to the ultrasonographic appearance of themuscles, and therefore, comparison within a horse ispreferable to comparison between horses.

The contour of the arytenoid cartilages should beassessed critically. The arytenoid cartilages shouldhave a trumpet bell shape with smooth margins.Horses with chondritis have irregularity of the axialand abaxial margins with thickening of the cartilageand abnormal echogenicity within the arytenoid car-tilage (Fig. 10). Horses may have only a granulomaor chondroma on the axial surface of the arytenoidcartilage without diffuse arytenoid chondritis. Ul-trasonographically, this manifests as focal irregular-ity of the axial margin, smooth abaxial margin, andnormal arytenoid cartilage width.

The anatomic abnormalities characteristic oflaryngeal dysplasia can also be observed. The ex-tension of the thyroid cartilage dorsal to the muscu-lar process of the arytenoid cartilage can best beimaged in the transverse plane (Fig. 11), whereasthe lack of a cricothyroid articulation is best imagedin the dorsal plane (Fig. 12). In these horses, therelationship between the thyroid cartilage, cricoary-tenoideus lateralis muscle, and arytenoid cartilageis abnormal; the CALM is often positioned caudal tothe thyroid cartilage in the gap between the thyroidcartilage and cricoid cartilage.

Fig. 5. Transverse plane ultrasound image of the ventral aspectof the cricoid cartilage (arrows) of a normal larynx. Left is to theleft of the image and right is to the right of the image.

Fig. 4. Transverse plane ultrasound image of the lateral aspect of a normal larynx (A). Transducer position is shown in B. Note theposition of the cricoarytenoideus lateralis muscle (CAL) between the thyroid cartilage (arrows) and the arytenoid cartilage (arrow-heads). The arytenoid cartilage has a trumpet bell shape and the cricoarytenoideus lateralis muscle has a striated appearance withheterogeneous echogenicity. Dorsal is to the left of the image and ventral is to the right of the image.


IMAGING

Medical records of Rood and Riddle Equine Hos-pital were examined to identify horses that had un-dergone UA endoscopy (resting and/or dynamic) aswell as laryngeal ultrasonography. Results of UAendoscopy and laryngeal ultrasonography as well asthe final diagnosis were recorded.

3. Results

Between 2008 and 2009, 330 horses that presentedto Rood and Riddle Equine Hospital for evaluationof the UA underwent laryngeal ultrasonography.A variety of breeds and uses were represented inthis group. All horses underwent resting UA en-doscopy, and a subset (148 horses) underwent tread-mill UA endoscopy as well.

One hundred thirty-one horses were diagnosedwith left recurrent laryngeal neuropathy based onresults of resting and/or treadmill UA endoscopy or

previous prosthetic laryngoplasty. One hundredtwenty of these horses (92%) had hyperechogenicityof the left CALM and/or CADM characteristic ofmuscle denervation. The remaining 11 horses hadnormal echogenicity of the left CALM and CADM; 4of these 11 horses were also diagnosed with DDSPduring dynamic UA examination. In the opinion ofthe attending clinician, DDSP was felt to be theprimary cause of poor performance. Three of theeleven horses did not undergo dynamic examination,and diagnosis was made during resting UA endos-copy only.

Fig. 6. Transverse plane ultrasound image (A) of the ventral aspect of the thyroid cartilage (arrows) of a normal larynx at the levelof the vocal folds (arrowheads). Transducer position is shown in B. The movement of the vocal folds can be observed during respiration.Left is to the left of the image and right is to the right of the image.

Fig. 7. Transverse plane ultrasound image of the basihyoid bone(arrowheads) and the ceratohyoid bones (arrows) of a normallarynx, obtained with the transducer positioned ventrally. Left isto the left of the image and right is to the right of the image.

Fig. 8. Transverse plane ultrasound image of the lateral aspectof a normal larynx. Note the mineralization of the muscularprocess of the arytenoid cartilage (arrow). Dorsal is to the left ofthe image and ventral is to the right of the image.


IMAGING

Conversely, there were 127 horses with hyper-echogenicity of the CALM and/or CADM. One hun-dred twenty-two (96%) of these horses werediagnosed with recurrent laryngeal neuropathy us-ing resting and/or dynamic UA endoscopy. The re-maining five horses were diagnosed with DDSP(three horses), axial deviation of the aryepiglotticfolds (one horse), and axial deviation of the vocalcords and aryepiglottic folds (one horse) during dy-namic UA endoscopy.

Twenty-nine horses were diagnosed with arytenoidchondritis using resting UA endoscopy. In three ofthese cases, arytenoid chondritis was initially diag-nosed during UA endoscopy, but ultrasonographic ex-amination showed a normal arytenoid contour. Oneof these three horses was subsequently diagnosed withDDSP during a dynamic treadmill examination. An-

other horse had hyperechogenicity of the CALM andwas diagnosed with recurrent laryngeal neuropathyduring dynamic treadmill examination. The thirdhorse did not undergo dynamic examination but wasdiagnosed with DDSP based on results of resting UAendoscopy.

Fig. 9. Comparison of echogenicity of the cricoarytenoideus late-ralis (arrows) and cricoarytenoideus dorsalis (arrowheads) mus-culature. Horses with recurrent laryngeal neuropathy haveincreased echogenicity and more homogeneous echogenicity of thecricoarytenoideus lateralis and cricoarytenoideus dorsalis mus-cles. Dorsal plane ultrasound images of the cricoarytenoideuslateralis muscle of a horse with recurrent laryngeal neuropathy(A) and a normal horse (B). Transverse plane ultrasound imagesof the cricoarytenoideus dorsalis muscle of a horse with recurrentlaryngeal neuropathy (C) and a normal horse (D). Dorsal planeultrasound images of the cricoarytenoideus lateralis muscle of ahorse with recurrent laryngeal neuropathy (E) and a normalhorse (F). In the dorsal plane images, rostral is to the left andcaudal is to the right and in the transverse plane images, dorsalis to the left of the image and ventral is to the right of the image.

Fig. 10. Transverse plane ultrasound image of the lateral aspectof the larynx of a horse with arytenoid chondritis. The arytenoidcartilage (arrows) is severely thickened with irregular marginsand increased echogenicity in its interior. Dorsal is to the left ofthe image and ventral is to the right of the image.

Fig. 11. Transverse plane ultrasound image of the lateral aspectof the larynx of a horse with laryngeal dysplasia. The thyroidlamina (arrowhead) extends dorsal to the muscular process of thearytenoid cartilage (arrow). Dorsal is to the left of the image andventral is to the right of the image.


IMAGING

Five horses were diagnosed with arytenoid gran-ulomas or chondromas during resting UA endos-copy. In all five horses, ultrasonography revealedthe mass on the axial surface of the arytenoid car-tilage, and there was no evidence of arytenoid chon-dritis affecting the entire cartilage.

Seven horses were diagnosed with laryngeal dys-plasia. In all cases, ultrasonography showed a lackof a cricothyroid articulation and extension of thethyroid cartilage dorsal to the muscular process ofthe arytenoid cartilage unilaterally or bilaterally.Abnormalities seen during UA endoscopy of thesehorses included abnormal left or right arytenoid car-tilage movement, rostral displacement of the pala-topharyngeal arch, and dorsal displacement of thesoft palate.

Other more unusual anatomic malformationshave been diagnosed in individual cases. One foalwith abnormal UA noise and exercise intolerancehad cyst-like structures in the thyroid, arytenoid,and cricoid cartilages as well as the first trachealring. An adult horse with dorsal displacement ofthe soft palate was found to have a basihyoid bonemalformation. A Paint horse heterozygous (N/H)for hyperkalemic periodic paralysis (HYPP) had hy-pertrophy of the left vocalis muscle.

Seventy-five horses were diagnosed with DDSP.These horses did not have any characteristic ultra-sonographic abnormalities.

All horses with normal UA endoscopic exami-nations had normal ultrasonographic examinations.Additionally, horses with pharyngeal collapse, bil-lowing of the soft palate, epiglottic retroversion,laryngeal collapse associated with flexed head po-

sition, abnormal arytenoid movement secondaryto thrombophlebitis, and axial deviation of thearyepiglottic folds had normal ultrasonographicexaminations.

4. Discussion

Ultrasonography of the laryngeal region can be per-formed easily in an ambulatory or hospital setting,is a non-invasive and safe procedure, and requiresno specialized equipment beyond an ultrasound ma-chine with a linear or curvilinear transducer. Clip-ping of the hair is not required, and light sedation isgenerally sufficient to ensure patient compliance;therefore, client and patient acceptance of the pro-cedure has been excellent.

Currently, ultrasonography of the larynx is anexcellent tool for investigation of the reason for ab-normal arytenoid movement, because the cause maybe difficult to determine with resting UA endoscopyalone. Evidence of hyperechogenicity of the leftCALM and CADM would support a diagnosis ofleft recurrent laryngeal neuropathy as a cause ofpoor performance or abnormal UA noise. In casesof arytenoid chondritis, ultrasonography permitsimaging of nearly the entire arytenoid cartilage.This has allowed us to assess the extent of disease inthe cartilage, diagnose any associated abscesses orperilaryngeal inflammation, and monitor responseto treatment. In some of the cases in this report,the ultrasonographic findings did not support adiagnosis of arytenoid chondritis, leading to addi-tional investigation and revision of the diagnosis.Congenital abnormalities involving the laryngealcartilages can be imaged directly instead of beinginferred from UA endoscopic abnormalities.

Although previous work24 has shown a small(2 mm) difference in the depth of the basihyoidbone between horses with and without DDSP, nosignificant difference was identified between thesegroups in our population. No abnormal ultraso-nographic findings were observed for a group ofconditions of the UA (pharyngeal collapse, billowingof the soft palate, epiglottic retroversion, and axialdeviation of the aryepiglottic folds), but furtherwork may reveal characteristic findings in thesediseases.

In our practice, ultrasonography of the larynx hasbeen incorporated as a routine procedure for inves-tigation of UA disorders, because it contributes to amore thorough evaluation of the UA. Endoscopy isan excellent tool for assessment of laryngeal func-tion and abnormalities visible at the luminal aspectof the larynx, but ultrasonography allows more com-plete examination of the laryngeal cartilages andassociated musculature, structures that were pre-viously difficult to evaluate. Additionally, ultra-sonography can be useful in evaluation of poorperformance or abnormal UA noise in cases where adynamic UA examination is not feasible (because ofavailability, financial or liability concerns, concur-

Fig. 12. Dorsal plane ultrasound image of the lateral aspect ofthe larynx of a horse with laryngeal dysplasia. The thyroid car-tilage (arrow) and the cricoid cartilage (small arrowhead) do notarticulate. The cricoarytenoideus lateralis muscle (large arrow-heads) is positioned between the thryoid cartilage and cricoidcartilage in the gap between the two cartilages. Rostral is to theleft of the image and caudal is to the right of the image.


IMAGING

rent lameness, or other factors). Hopefully, futurework will expand the uses of laryngeal ultrasonog-raphy for a variety of UA conditions.

References1. Martin BB, Davidson EJ, Durando MM, et al. Clinical ex-

ercise testing: overview of causes of poor performance. In:Hinchcliff KW, Kaneps AJ, Geor RJ, et al., eds. Equine sportsmedicine and surgery. Edinburgh: Saunders, 2004;3241.

2. Martin BB Jr., Reef VB, Parente EJ, et al. Causes ofpoor performance of horses during training, racing, or showing:348 cases (19921996). J Am Vet Med Assoc 2000;216:554558.

3. Morris EA, Seeherman HJ. Clinical evaluation of poor per-formance in the racehorse: the results of 275 evaluations.Equine Vet J 1991;23:169174.

4. Anonymous. Workshop summary consensus statements onequine recurrent laryngeal neuropathy, in Proceedings.Havemeyer Foundation Workshop on Equine Recurrent La-ryngeal Neuropathy 2003;9397.

5. Desmaizieres LM, Serraud N, Plainfosse B, et al. Dynamicrespiratory endoscopy without treadmill in 68 performanceStandardbred, Thoroughbred and saddle horses under natu-ral training conditions. Equine Vet J 2009;41:347352.

6. Rakestraw PC, Hackett RP, Ducharme NG, et al. Arytenoidcartilage movement in resting and exercising horses. VetSurg 1991;20:122127.

7. Chalmers HJ, Cheetham J, Yeager AE, et al. Ultrasonographyof the equine larynx. Vet Radiol Ultrasound 2006;47:476481.

8. Chalmers HJ, Cheetham J, Mohammed HO, et al. Ultra-sonography as an aid in the diagnosis of recurrent laryngealneuropathy in horses, in Proceedings. 16th American Col-lege of Veterinary Surgeons Symposium 2006;34.

9. Garrett KS, Woodie JB, Embertson RM, et al. Diagnosisof laryngeal dysplasia in five horses using magnetic resonanceimaging and ultrasonography. Equine Vet J 2009;41:766771.

10. Garrett KS, Woodie JB, Embertson RM. Association oftreadmill upper airway endoscopic evaluation of arytenoidmovement with results of laryngeal ultrasonography andresting upper airway endoscopic evaluation for diagnosis ofrecurrent laryngeal neuropathy in Thoroughbred racehorses.Equine Vet J, submitted.

11. Duncan ID, Amundson J, Cuddon PA, et al. Preferentialdenervation of the adductor muscles of the equine larynx.I. Muscle pathology. Equine Vet J 1991;23:9498.

12. Duncan ID, Reifenrath P, Jackson KF, et al. Preferentialdenervation of the adductor muscles of the equine larynx.II. Nerve pathology. Equine Vet J 1991;23:99103.

13. Gunreben G, Bogdahn U. Real-time sonography of acuteand chronic muscle denervation. Muscle Nerve 1991;14:654664.

14. Kullmer K, Sievers KW, Reimers CD, et al. Changes ofsonographic, magnetic resonance tomographic, electromyo-graphic, and histopathologic findings within a 2-month pe-riod of examinations after experimental muscle denervation.Arch Orthop Trauma Surg 1998;117:228234.

15. Walker FO. Neuromuscular ultrasound. Neurol Clin 2004;22:563590.

16. Holcombe SJ, Ducharme NG. Abnormalities of the upperairway. In: Hinchcliff KW, Kaneps AJ, Geor RJ, et al.,eds. Equine sports medicine and surgery. Endinburgh:Saunders, 2004;559598.

17. Haynes PF, Snider TG, McClure JR, et al. Chronic chondri-tis of the equine arytenoid cartilage. J Am Vet Med Assoc1980;177:11351142.

18. Kannegieter NJ, Goulden BE, Anderson LJ. Right-sidedlaryngeal dysfunction in a horse. New Zeal Vet J 1986;34:6668.

19. Lane JG. Fourth branchial arch defects in Thoroughbredhorses: a review of 60 cases, in Proceedings. 2nd WorldEquine Airways Symposium 2001. Available at http://www.ivis.org/proceedings/weas/2001/Lane.pdf. Accessed on Janu-ary 21, 2010.

20. Goulden BE, Anderson LJ, Davies AS, et al. Rostral dis-placement of the palatopharyngeal arch: a case report.Equine Vet J 1976;8:9598.

21. Dixon PM, McGorum BC, Else RW. Cricopharyngeal-laryngeal dysplasia in a horse with sudden clinical onsetidiopathic laryngeal hemiparesis. New Zeal Vet J 1993;41:134 138.

22. Ducharme NG, Hackett RP, Woodie JB, et al. Investigationsinto the role of the thyrohyoid muscles in the pathogenesis ofdorsal displacement of the soft palate in horses. Equine Vet J2003;35:258263.

23. Cheetham J, Pigott JH, Thorson LM, et al. Racing perfor-mance following the laryngeal tie-forward procedure: a case-controlled study. Equine Vet J 2008;40:501507.

24. Chalmers HJ, Yeager AE, Ducharme N. Ultrasonographicassessment of laryngohyoid position as a predictor of dorsaldisplacement of the soft palate in horses. Vet Radiol Ultra-sound 2009;50:9196.

25. Strand E, Fjordbakk CT, Holcombe SJ, et al. Effect of pollflexion and dynamic laryngeal collapse on tracheal pressurein Norwegian Coldblooded Trotter racehorses. Equine Vet J2009;41:5964.


IMAGING

Table of Contents

Documents

How to Ultrasound the Equine Larynx - AAEP · How to Ultrasound the Equine Larynx Katherine S. Garrett, DVM Author’s address: Rood and Riddle Equine Hospital, PO Box 12070, Lexington,