Bilateral asymmetry of equine laryngeal muscles

One problem in the diagnosis of subclinical roarers is that abnormal auditory signals occur only during a fast gait, and these fade quickly. This paper offers a novel technique to assist this problem. Sound recordings were obtained from five horses following exercise; two roarers, two non-roarers, and one suspected roarer. These signals were converted into spectrograms by Rayspan processing. All frequencies in the 0 to 2.25 kHz range during both inspiration and expiration, as well as occasional noises in the 2–4 kHz band, characterised roarers. Signals from non-roarers consisted of frequencies in the 0 to 2 kHz range with long quiet periods, especially during expiration. An explanation for these phenomena is offered, and supported by diagrammatic cross-sections of equine upper-respiratory tracts. It is suggested that the present technique could assist in the diagnosis of sub- clinical roarers, estimated to be between 20 and 40% of the competitive horse population.     

Morphologic and morphometric studies of the intrinsic laryngeal muscles in clinically normal adult dogs

To establish normal histologic and histochemical data and to determine reference values for fiber type proportions (percentages, mean fiber diameters, atrophy and hypertrophy factors, and variability coefficients), a histochemical study was carried out on intrinsic muscles of the larynx (cricothyroid, cricoarytenoid lateralis, cricoarytenoid dorsalis, and thyroarytenoid muscles) of clinically normal dogs. Using myosin adenosine triphosphatase stain under acidic preincubation (pH 4.3) conditions, 3 histochemical fiber types--1, 2A, and 2C--were recognized. The percentage of type-2C fibers varied from 1 to 2% in thyroarytenoid muscles to approximately 10% in cricoarytenoid lateralis muscles. There was no significant difference in mean diameter between left- and right-side specimens of each muscle for type 1 vs type 2. The largest fibers (mean +/- SD) of both types were observed in the cricothyroid muscles (type 1, 38.19 +/- 7.76 microns; type 2, 43.25 +/- 8.66 microns), and the smallest fibers were found in the thyroarytenoid muscles (type 1, 29.38 +/- 5.12 microns; type 2, 33.84 +/- 6.20 microns). Respective mean diameters of fiber types from cricoarytenoid dorsalis (type 1, 32.05 +/- 5.69 microns; type 2, 38.95 +/- 7.75 microns) and cricoarytenoid lateralis (type 1, 33.75 +/- 5.98 microns; type 2, 37.09 +/- 7.01 microns) muscles were similar. The histographic distribution of fiber type diameters was unimodal in all muscles. In each muscle, the mean fiber diameter of type-2 fibers was greater than that of type-1 fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

A study of the weights of some intrinsic laryngeal and palatine muscles in the thoroughbred horse

The weights of some intrinsic laryngeal and palatine muscles from 46 thoroughbred horses of varying ages are presented. Thesehorses had no clinical history of wind affliction. The muscles studied were the dorsal cricoarytenoid, the lateral cricoarytenoid, the cricothyroid, the palatine levator and the palatine tensor.

The muscles supplied by the left recurrent laryngeal nerve tended to be lighter in older horses than the muscles supplied by the right recurrent laryngeal nerve. However there was no significant difference in weight between the left and right side for all muscles studied except the lateral cricoarytenoid. In this muscle the left side was significantly lighter than the right (P<0.001).     

Biomechanical characterisation of equine laryngeal cartilage

Reasons for performing the study: Upper airway obstruction is a common problem in the performance horse as the soft tissues of the larynx collapse into the airway, yet there is a paucity of information on biomechanical properties for the structural cartilage components. Objective: To measure the geometry and compressive mechanical properties of the hyaline cartilage to improve understanding of laryngeal function and morphology. Methods: A total of 11 larynges were harvested from Thoroughbred and Standardbred racehorses. During gross dissection, linear dimensions of the cricoid were obtained. From both the cricoid and arytenoid, specimens were cored to obtain 6 mm disc samples from 3 sites within the dorsal cricoid (caudal, middle and rostral) and 2 central sites in the arytenoids (inner, outer). The specimens were mechanically tested using radial confined compression to calculate the aggregate modulus and permeability of the tissue. The biomechanical data were analysed using a nested mixed effects model. Results: Geometrically, the cricoid has relatively straight walls compared to the morphology of human, ovine and canine larynges. There were significant observations of higher modulus with increasing age (0.13 MPa per year; P = 0.007) and stiffer cricoid cartilage (2.29 MPa) than the arytenoid cartilage (0.42 MPa; P<0.001), but no difference was observed between the left and right sides. Linear contrasts showed that the rostral aspect (2.51 MPa) of the cricoid was 20% stiffer than the caudal aspect (2.09 MPa; P = 0.025), with no difference between the arytenoid sites. Conclusions: The equine larynx is a well supported structure due to both the geometry and material properties of the cricoid cartilage. The hyaline structure is an order of magnitude higher in compressive modulus compared to the arytenoids and other hyaline‐composed tissues. Potential relevance: These characterisations are important to understand the biomechanics of laryngeal function and the mechanisms involved with surgical interventions.     

Pathological classification of equine recurrent laryngeal neuropathy

Recurrent Laryngeal Neuropathy (RLN) is a highly prevalent and predominantly left‐sided, degenerative disorder of the recurrent laryngeal nerves (RLn) of tall horses, that causes inspiratory stridor at exercise because of intrinsic laryngeal muscle paresis. The associated laryngeal dysfunction and exercise intolerance in athletic horses commonly leads to surgical intervention, retirement or euthanasia with associated financial and welfare implications. Despite speculation, there is a lack of consensus and conflicting evidence supporting the primary classification of RLN, as either a distal (“dying back”) axonopathy or as a primary myelinopathy and as either a (bilateral) mononeuropathy or a polyneuropathy; this uncertainty hinders etiological and pathophysiological research. In this review, we discuss the neuropathological changes and electrophysiological deficits reported in the RLn of affected horses, and the evidence for correct classification of the disorder. In so doing, we summarize and reveal the limitations of much historical research on RLN and propose future directions that might best help identify the etiology and pathophysiology of this enigmatic disorder.     

The pathogenesis of equine laryngeal hemiplegia--a review

Recent research on the muscular and nervous changes which occur in idiopathic equine laryngeal hemiplegia has indicated that many of the traditional concepts of the aetiology of this disease are erroneous. In light of the new knowledge gained, the various predispositions and possible causes of laryngeal hemiplegia are discussed, and it is suggested that the underlying mechanism of axonal damage in this neuropathy of horses may be related to abnormal energy metabolism in the axon.     

Neurogenic atrophy of the laryngeal muscles of the dog

Neurogenic atrophy is described in biopsy specimens of laryngeal muscles in seven large breed dogs. Signs shown by the dogs were similar but varied in degree of severity; most had a rough bark and stridorous respiration of several months' duration which progressed to obstructive dyspnoea and, in some cases, to glottic obstruction with collaspe on exertion. At laryngoscopy, there were signs of both chronic and acute laryngitis; vocal cords were oedematous and almost midline; they did not abduct with vigorous inspiratory efforts. A tracheotomy was performed in six dogs. Laryngeal cup forceps were used for unilateral excision of the vocal cord and ventricular band, the obstructing portion of the arytenoid cartilage and the aryepiglottic fold. Neurogenic atrophy of laryngeal muscles was a consistent finding upon histological examination of the resected tissues. Résumé. On décrit, chez sept chiens de grosse race, une atrophie neurogène dans des spécimens de biopsie de muscles laryngiens. Les signes que montraient les chiens étaient semblables, mais leur degré de sévérité variait. La plupart avaient un aboiement rauque et une respiration striduleuse d'une durée de plusieurs mois progressant jusqu'à une dyspnée obstructive et dans certains cas jusqu'à une obstruction glottique avec collapsus sous l'effort. à la laryngoscopie, il y avait des signes et de laryngite chronique et de laryngite aiguë; les cordes vocales étaient oedémateuses et presque à ligne médiane; elles ne s'écartaient pas sous de vigoureux efforts inspiratoires. Six chiens ont subi une trachéotomie. On a utilisé des forceps laryngiens à pinces pour excision unilatérale de la corde vocale et de la bande ventriculaire, de la partie obstruante du cartilage aryténoïd et du repli aryténoépiglottique. Sur examen histologique des tissus réséqués, on a trouvé, de façon consistante, une atrophie neurogénique des muscles laryngiens. Zusammenfassung. Nervurspriingliche Atrophie wurde in Biopsie Proben von Kehlkopfmuskeln in sieben grossen Zuchthunden beschrieben. Symptǒme die die Hunde zeigten waren ǎhnlich aber verschieden in der Schwere des Leidens; die meisten hatten ein rauhes Bellen und kreischende Respiration iïber mehrere Monate hin, welche sich zur obstruensen Dyspnoe entwickelte und in einigen Fàllen zu Stimmritzen Verschluss mit Kollaps bei Anstrengung. Bei Laryngoscopie fand man Anzeichen von beiden chronischer wie akuter Laryngitis; Stimmbǎnder waren ǒdematos und fast Mittellinie, sie abduzierten nicht bei heftigen inspiratorischen Anstrengungen. Eine Trachéotomie wurde in sechs Hunden ausgefùhrt. Kehlkopfschalenzangen gebrauchte man zur einseitigen Exzision des Stimmbandes und des ventrikularen Bandes, der obstruensen Menge des Giessbeckenknorpels und der Giessbecken-Kehldeckelfalte. Nervursprǔnglich Atrophie war ein ubereinstimmender Befund bei histologischer Untersuchung des resezierten Gewebes.     

A comparison of techniques to enhance the evaluation of equine laryngeal function

This study was designed to define a simple, unequivocal test for the evaluation of laryngeal function and the diagnosis of idiopathic laryngeal hemiplegia (ILH). ILH is a disorder that results from left recurrent laryngeal neuropathy and in which there is no movement of the left arytenoid cartilage and vocal fold. Laryngeal function was evaluated in seven horses using four techniques designed to stimulate laryngeal movements:-nasal occlusion, exercise, swallowing and administration of a respiratory stimulant. In addition, the effects of sedation and twitching on the endoscopic examination were also examined. The cross-sectional area of the rima glottidis was measured in each horse at rest and after each technique was performed. There was no statistically significant difference in the increase in area seen after nasal occlusion or exercise. Doxapram hydrochloride increased the cross-sectional area of the rima glottidis, whereas xylazine caused a decrease. Neither of these pharmacological agents exaggerated or decreased the amount of asynchronous movement or tremoring of the arytenoid cartilages. Manual occlusion of the external nares during endoscopy is a simple, yet effective method of stimulating arytenoid function and hence diagnosing ILH.     

Evaluation of partial arytenoidectomy as a treatment for equine laryngeal hemiplegia

The efficacy of partial arytenoidectomy was assessed in 6 Standardbred horses, with surgically induced laryngeal hemiplegia, at rest (Period A) and during exercise at speeds corresponding to maximum heart rate (Period C) and 75% of maximum heart rate (Period B). Peak expiratory and inspiratory airflow rate (PEF and PIF), and expiratory and inspiratory transupper airway pressure (P_UE and P_UI) were measured and expiratory and inspiratory impedance (Z_E and Z_I) were calculated. Simultaneously, tidal breathing flow-volume loops (TBFVL) were acquired using a respiratory function computer. Indices derived from TBFVL included airflow rates at 50 and 25% of tidal volume (EF₅₀, IF₅₀, EF₂₅. and IF₂₅) and the ratios of expiratory to inspiratory flows. Measurements were made before left recurrent laryngeal neurectomy (baseline), 2 weeks after left recurrent laryngeal neurectomy (LRLN) and 16 weeks after left partial arytenoidectomy coupled with bilateral ventriculectomy (ARYT). After LRLN, during exercise Periods B and C, Z₁ and the ratio of EF₅₀/IF₅₀ significantly increased and PIF, IF₅₀ and IF₂₅ significantly decreased from baseline values. At 16 weeks after ARYT, Z₁ returned to baseline values during Periods B and C. Although PIF, IF₅₀, IF₂₅, PEF/PIF, and EF₅₀/IF₅₀ returned to baseline values during Period B, these indices remained significantly different from baseline measurements during Period C. After ARYT, TBFVL shapes from horses during Period C approached that seen at the baseline evaluation. Partial arytenoidectomy improved upper airway function in exercising horses with surgically induced left laryngeal hemiplegia, although qualitative and quantitative evaluation of TBFVLs suggested that some flow limitation remains at near maximal airflow rates. These results indicate that, although the procedure does not completely restore the upper airway to normal, partial arytenoidectomy is a viable treatment option for failed laryngoplasty and arytenoid chondropathy in the horse.     

Quantitative effect of tenorrhaphy on intrinsic vasculature of the equine superficial digital flexor tendon

OBJECTIVE: To compare effects of the locking-loop suture pattern (LLP) and 3-loop pulley (3LP) suture pattern for tenorrhaphy on the intrinsic vasculature of the superficial digital flexor tendon (SDFT) of horses in vitro after surgery. SAMPLE POPULATION: 16 forelimbs obtained from 8 mature horses. PROCEDURE: Tenotomy and subsequent tenorrhaphy was performed in anesthetized horses. Following systemic administration of heparin, horses were euthanatized and the limbs were removed and placed under tension to load the flexor tendons. The intrinsic vasculature was then perfused with a mixture of barium sulfate and water. Four-millimeter sections of the SDFT were prepared for microangiographic analysis. Mean vessel density was calculated for each section by use of a grid consisting of 1.5-mm2 vascular assessment squares (VAS). Comparisons were made among the control, LLP and 3LP groups. RESULTS: Mean +/- SD vessel density was 3.11 +/- 0.38, 1.47 +/- 0.47, and 2.01 +/- 0.63 perfused vessels/1.5 mm2 for control, LLP and 3LP groups, respectively. Significant differences in vascular density were detected between the control and 3LP groups, control and LLP groups, and LLP and 3LP groups. CONCLUSIONS AND CLINICAL RELEVANCE: Use of the LLP and 3LP pattern has deleterious effects in vitro on the intrinsic vasculature of the SDFT. However, the 3LP pattern was less disruptive to the intrinsic vasculature, compared with the effects for the LLP. Use of the 3LP tenorrhaphy suture pattern in clinical situations may result in less damage to the intrinsic vasculature of the SDFT of horses during convalescence.     

A study of an electrodiagnostic technique for the evaluation of equine recurrent laryngeal neuropathy

The electrodiagnostic measurement of the thoracolaryngeal reflex (TLR) ('slap test') latency was compared to 5 other diagnostic techniques used for evaluation of laryngeal function, namely laryngeal muscle palpation, resting and immediately postexercise endoscopic examinations, and palpable and endoscopic responses to the TLR. Compared to resting endoscopy, the electrodiagnostic measurement of TLR latency was not found to be an accurate test for the evaluation of recurrent laryngeal neuropathy (RLN), nor was laryngeal muscle palpation or the endoscopic response to the TLR. Twenty-five (71%) of 35 Clydesdale horses examined were affected by RLN; 16 (46%) had mild, 5 (14%) moderate, 3 (9%) severe hemiparesis and 1 (3%) had total laryngeal paralysis. Within these 35 horses, physical traits such as height or neck length did not correlate with the incidence or severity of RLN. Ten control ponies showed no evidence of abnormal laryngeal function.     

Myosin heavy chain composition in normal and atrophic equine laryngeal muscle

The myosin heavy chain (MHC) composition of a given muscle determines the contractile properties and, therefore, the fiber type distribution of the muscle. MHC isoform expression in the laryngeal muscle is modulated by neural input and function, and it represents the cellular level changes that occur with denervation and reinnervation of skeletal muscle. The objective of this study was to evaluate the pattern of MHC isoform expression in laryngeal muscle harvested from normal cadavers and cadavers with naturally occurring left laryngeal hemiplegia secondary to recurrent laryngeal neuropathy. Left and right thyroarytenoideus (TA) and cricoarytenoideus dorsalis (CAD) were obtained from 7 horses affected with left-sided intrinsic laryngeal muscle atrophy and from 2 normal horses. Frozen sections were evaluated histologically for degree of atrophy and fiber type composition. MHC isoform expression was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of muscle protein. Histologic atrophy was seen in all atrophic muscles and some right-sided muscles of 3 affected horses, as well as the left TA of 1 normal horse. Fiber type grouping or loss of type I muscle fibers was observed in the left-sided laryngeal muscles in all but 1 affected horse, as well as in the right muscles of 2 affected horses, and the left TA of 1 normal horse. SDS-PAGE showed 2 bands corresponding to the type I and type IIB myosin isoforms in the CAD and TA of the 2 normal horses. Affected horses demonstrated a trend toward increased expression of the type IIB isoform and decreased expression of the type I isoform in atrophic muscles. This study confirmed the presence of histologic abnormalities in grossly normal equine laryngeal muscle, and it demonstrated an increased expression of type IIB MHC with a concurrent decreased expression of type I MHC in affected muscles. Evaluation of muscle fiber changes at the cellular level under denervated and reinnervated conditions may aid in assessing future strategies for reinnervation or regeneration of atrophic laryngeal muscle.     

Preferential denervation of the adductor muscles of the equine larynx. II: Nerve pathology

The terminal branches of the recurrent laryngeal nerve (RLN) of three normal ponies and six horses with sub-clinical laryngeal disease were examined qualitatively and quantitatively in an attempt to explain the preferential denervation of the laryngeal adductor muscles in the neuropathy of idiopathic laryngeal hemiplegia (ILH). The myelinated fibre spectra of all the motor nerve fibres in the left and right abductor and adductor branches of the RLN in three normal ponies were measured. The density of myelinated fibres was also calculated. There was no significant difference between the larger group of myelinated fibres in the adductor or abductor branches. In the six horses with laryngeal hemiparesis, however, there was a marked preferential loss of the medium/large size myelinated fibres in the left adductor branch, although nerve fibre densities were not significantly different. While no simple morphometrical feature was found to explain the selective muscle denervation, the greater loss of large diameter myelinated fibres in the adductor branches confirms the earlier observation of adductor muscle susceptibility in the neuropathy of ILH.