Lisa M. Mitchell

Investigations into the role of the thyrohyoid muscles in the pathogenesis of dorsal displacement of the soft palate in horses

REASONS FOR PERFORMING STUDY Contributes to the understanding of the pathogenesis of dorsal displacement of the soft palate during exercise so that management of this condition could be enhanced. HYPOTHESIS That the thyrohyoid muscles play an important role in the stability of the laryngo-palatal relationship and that dysfunction of these muscles leads to dorsal displacement of the soft palate (DDSP) during exercise. METHODS Ten horses were exercised on a high-speed treadmill under 4 different treatment conditions: control conditions (n = 10), after resection of thyrohyoid muscles (TH, n = 10), after sham-treatment (n = 5), or after restoration of function of the thyrohyoid muscles with surgical sutures (prosthesis-treatment, n = 6). During trials, the following determinations were made: videoendoscopy of the upper airway, gait frequency and pharyngeal and tracheal static pressures. RESULTS None of the 10 horses developed DDSP during 2 separate treadmill-exercise trials under the control conditions. Seven of the 10 horses developed DDSP after resection of the TH muscles, 4 of 5 of these horses still experienced DDSP after sham-treatment, but 5 of 6 horses no longer experienced DDSP at exercise after the prosthesis-treatment. There were significant anomalies in airway pressures, respiratory frequency, and occurrence of DDSP in both the TH resection and sham-treatment conditions compared to control conditions. In contrast, no statistical differences were noted in any of the parameters measured between the prosthesis-treatment and control conditions. CONCLUSIONS That the function of the TH muscles is important to the stability of the laryngo-palatal relationship and plays a role in the pathophysiology of exercise-induced DDSP. POTENTIAL RELEVANCE Management of horses with DDSP could be enhanced by restoring the function of the TH muscles.

Measurement of plasma cardiac troponin I concentration by use of a point-of-care analyzer in clinically normal horses and horses with experimentally induced cardiac disease

OBJECTIVE To compare cardiac troponin I (cTnI) concentrations determined by use of a point-of-care analyzer with values determined by use of a bench-top immunoassay in plasma samples obtained from clinically normal horses with and without experimentally induced cardiac disease, and to establish a reference range for plasma equine cTnI concentration determined by use of the point-of-care analyzer. ANIMALS 83 clinically normal horses, 6 of which were administered monensin to induce cardiac disease. PROCEDURES A blood sample was collected from each of the 83 clinically normal horses to provide plasma for analysis by use of the point-of-care analyzer; some of the same samples were also analyzed by use of the immunoassay. All 83 samples were used to establish an analyzer-specific reference range for plasma cTnI concentration in clinically normal horses. In 6 horses, blood samples were also collected at various time points after administration of a single dose of monensin (1.0 to 1.5 mg/kg) via nasogastric intubation; plasma cTnI concentration in those samples was assessed by use of both methods. RESULTS The analyzer-specific reference range for plasma cTnI concentration in clinically normal horses was 0.0 to 0.06 ng/mL. Following monensin treatment in 5 horses, increases in plasma cTnI concentration determined by use of the 2 methods were highly correlated (Pearson correlation, 0.83). Peak analyzer-determined plasma cTnI concentrations in monensin-treated horses ranged from 0.08 to 3.68 ng/mL. CONCLUSIONS AND CLINICAL RELEVANCE In horses with and without experimentally induced cardiac disease, the point-of-care analyzer and bench-top immunoassay provided similar values of plasma cTnI concentration.

Clinical Findings and Serum Cardiac Troponin I Concentrations in Horses after Intragastric Administration of Sodium Monensin

Six adult horses were administered sodium monensin, 1.0–1.5 mg/kg, via gastric gavage. Anorexia and/or diarrhea occurred within 24 hr after monensin administration in all 6 horses. Cardiac disease and dysfunction were evaluated by both elevations in heart rate, echocardiography, and an increase in serum concentrations of cardiac troponin I (cTnI), occurred in 4 horses. The development and severity of cardiac disease was likely affected by the monensin dose, vehicle (water or corn oil) mixed with monensin, and/or whether the monensin was administered to fed or fasted horses. Initial increases in cTnI concentrations occurred between 24 and 72 hr after monensin administration. The 2 horses with the highest cTnI concentrations died or were euthanized within 5 days after monensin administration and had severe cardiac disease. One horse had increased cTnI concentrations from day 2 to day 16, but no apparent change in ventricular contractile function was evident on echocardiography. The fourth diseased horse did not return to cTnI reference intervals until day 27 after monensin administration, and the ventricular function was still abnormal just before euthanasia 9 months later. Cardiac troponin I measurements could be useful in managing farm outbreaks of accidental monensin feeding by the early identification of horses with cardiac disease.

Contractile behavior of the forelimb digital flexors during steady-state locomotion in horses (Equus caballus): An initial test of muscle architectural hypotheses about in vivo function

The forelimb digital flexors of the horse display remarkable diversity in muscle architecture despite each muscle-tendon unit having a similar mechanical advantage across the fetlock joint. We focus on two distinct muscles of the digital flexor system: short compartment deep digital flexor (DDF(sc)) and the superficial digital flexor (SDF). The objectives were to investigate force-length behavior and work performance of these two muscles in vivo during locomotion, and to determine how muscle architecture contributes to in vivo function in this system. We directly recorded muscle force (via tendon strain gauges) and muscle fascicle length (via sonomicrometry crystals) as horses walked (1.7 m s(-1)), trotted (4.1 m s(-1)) and cantered (7.0 m s(-1)) on a motorized treadmill. Over the range of gaits and speeds, DDF(sc) fascicles shortened while producing relatively low force, generating modest positive net work. In contrast, SDF fascicles initially shortened, then lengthened while producing high force, resulting in substantial negative net work. These findings suggest the long fibered, unipennate DDF(sc) supplements mechanical work during running, whereas the short fibered, multipennate SDF is specialized for economical high force and enhanced elastic energy storage. Apparent in vivo functions match well with the distinct architectural features of each muscle.

Effect of a tart cherry juice blend on exercise-induced muscle damage in horses.

OBJECTIVE To evaluate whether administering a tart cherry juice blend (TCJB) prior to exercise would reduce skeletal and cardiac muscle damage by decreasing the inflammatory and oxidative stress response to exercise in horses. ANIMALS 6 horses. PROCEDURES Horses were randomly allocated into 2 groups in a crossover study with a 2-week washout period and orally administered either TCJB or a placebo solution (1.42 L, twice daily) in a double-masked protocol for 2 weeks prior to a stepwise incremental exercise protocol. Horses were tested for serum activities of creatine kinase and aspartate aminotransferase (AST) and concentrations of cardiac troponin I (cTnI), thiobarbituric acid reactive substances (TBARS; an indicator of oxidative stress), and serum amyloid A (SAA; an indicator of inflammation). To ensure that treatment would not result in positive results of an equine drug-screening protocol, serum samples obtained from each horse prior to and after 2 weeks of administration of TCJB or the placebo solution were tested. RESULTS All horses had negative results of drug screening at both sample times. The exercise protocol resulted in a significant increase in TBARS concentration, SAA concentration, and serum AST activity in all horses. Administration of TCJB or placebo solution was not associated with an effect on malondialdehyde or SAA concentrations. However, administration of TCJB was associated with less serum activity of AST, compared with administration of placebo solution. CONCLUSIONS AND CLINICAL RELEVANCE Administration of TCJB may diminish muscle damage induced by exercise.

Superficial digital flexor tendon lesions in racehorses as a sequela to muscle fatigue : A preliminary study

REASON FOR PERFORMING STUDY Racing and training related lesions of the forelimb superficial digital flexor tendon are a common career ending injury to racehorses but aetiology and/or predisposing causes of the injury are not completely understood. OBJECTIVES Although the injury takes place within the tendon, the lesion must be considered within the context of the function of the complete suspensory system of the distal limb, including the associated muscles. METHODS Both muscle and tendon function were investigated in vivo using implanted strain gauges in 3 Thoroughbred horses walking, trotting and cantering on a motorised treadmill. These data were combined with assessments of muscle architecture and fibre composition to arrive at an overview of the contribution of each muscle-tendon unit during locomotion. RESULTS The superficial digital flexor muscle has fatigue-resistant and high force production properties that allow its tendon to store and return elastic energy, predominantly at the trot. As running speed increases, deep digital flexor tendon force increases and it stabilises hyperextension of the fetlock, thus reinforcing the superficial digital flexor in limb load support. The deep digital flexor muscle has fast contracting properties that render it susceptible to fatigue. CONCLUSION Based on these measurements and supporting evidence from the literature, it is proposed that overloading of the superficial digital flexor tendon results from fatigue of the synergistic, faster contracting deep digital flexor muscle. POTENTIAL RELEVANCE Future research investigating distal limb system function as a whole should help refine clinical diagnostic procedures and exercise training approaches that will lead to more effective prevention and treatment of digital flexor tendon injuries in equine athletes.

Equine Laryngoplasty Sutures Undergo Increased Loading During Coughing and Swallowing

OBJECTIVES To report (1) the force required on a single laryngoplasty suture to achieve optimal abduction of the left arytenoid cartilage, (2) peak forces experienced by the suture during induced swallowing and coughing, and during 24-hour resting activity in a stall, and (3) peak forces during induced swallowing and coughing after left recurrent laryngeal nerve blockade. STUDY DESIGN Experimental study. ANIMALS Horses (n=8). METHODS Each laryngoplasty suture was instrumented with an E-type buckle force transducer to measure the force required for optimal intraoperative left arytenoid cartilage abduction. This was correlated with abduction observed postoperatively. Change in suture force from baseline was measured during induced coughing and swallowing, and during normal stall activity. RESULTS Optimal intraoperative arytenoid abduction was achieved with a mean (±SD) force of 27.6±7.5 N. During saline-induced swallowing and coughing mean force on the suture increased by 19.0±5.6 N (n=233 measurements; 7 horses) and 12.1±3.6 N (n=31; 4 horses), respectively. Sutures underwent increased loading a mean of 1152 times in 24 hours. No change in suture force was observed with respiratory rhythm. CONCLUSION Swallowing increases laryngoplasty suture force to a greater extent than coughing.

Successful treatment of persistent dorsal displacement of the soft palate and evaluation of laryngohyoid position in 15 racehorses

REASONS FOR PERFORMING STUDY Caudal descent of the larynx has been proposed to be associated with intermittent dorsal displacement of the soft palate (DDSP) as it leads to a loss of the seal between the epiglottis and soft palate but further investigation of this theory is required. OBJECTIVES To evaluate laryngohyoid position of horses with persistent DDSP in comparison to horses with intermittent DDSP and evaluate the outcome of treatment. HYPOTHESES Horses with persistent DDSP have a different laryngohyoid position compared to those with intermittent DDSP. Horses with persistent DDSP can be returned successfully to racing with a laryngeal tie-forward procedure. METHODS Medical records of 15 racehorses presented for persistent DDSP between 2002 and 2007 were reviewed. Age, sex and breed matched horses diagnosed with intermittent DDSP were used as a comparison group. Treatment of all horses was performed by laryngeal tie-forward, 8/15 horses with persistent DDSP had a subsequent laser staphylectomy. Preoperative laryngohyoid position was compared between the 2 groups using a radiographic reference system. Surgical effect on position was assessed by comparing pre- and post operative radiographic measurements. Outcome was assessed by return to racing and comparison of pre- and post operative race earnings (

Functional electrical stimulation of intrinsic laryngeal muscles under varying loads in exercising horses.

). RESULTS Thirteen of 15 horses with persistent DDSP returned to racing. Seven of 15 horses were treated with laryngeal tie-forward alone while 8/15 horses were also treated with a laser staphylectomy. Horses with persistent DDSP had a more caudal larynx (ossification of the thyroid cartilage) (13 mm, P = 0.014), a more caudal (10 mm, P = 0.044) and dorsal (7 mm, P = 0.01) basihyoid bone, and a more dorsal thyrohyoid-thyroid articulation (10 mm, P = 0.002) than horses with intermittent DDSP. CONCLUSIONS AND POTENTIAL RELEVANCE Racehorses with persistent DDSP can be treated successfully by laryngeal tie-forward or by laryngeal tie-forward followed by laser staphylectomy. Horses with persistent DDSP have a more caudal larynx and more caudal and dorsal basihyoid bone than horses with intermittent DDSP.

Considerations for pacing of the cricoarytenoid dorsalis muscle by neuroprosthesis in horses.

Bilateral vocal fold paralysis (BVCP) is a life threatening condition and appears to be a good candidate for therapy using functional electrical stimulation (FES). Developing a working FES system has been technically difficult due to the inaccessible location and small size of the sole arytenoid abductor, the posterior cricoarytenoid (PCA) muscle. A naturally-occurring disease in horses shares many functional and etiological features with BVCP. In this study, the feasibility of FES for equine vocal fold paralysis was explored by testing arytenoid abduction evoked by electrical stimulation of the PCA muscle. Rheobase and chronaxie were determined for innervated PCA muscle. We then tested the hypothesis that direct muscle stimulation can maintain airway patency during strenuous exercise in horses with induced transient conduction block of the laryngeal motor nerve. Six adult horses were instrumented with a single bipolar intra-muscular electrode in the left PCA muscle. Rheobase and chronaxie were within the normal range for innervated muscle at 0.55±0.38 v and 0.38±0.19 ms respectively. Intramuscular stimulation of the PCA muscle significantly improved arytenoid abduction at all levels of exercise intensity and there was no significant difference between the level of abduction achieved with stimulation and control values under moderate loads. The equine larynx may provide a useful model for the study of bilateral fold paralysis.