Neuroanatomy of the equine dorsal cricoarytenoid muscle: surgical implications

Equine laryngoplasty: Effects of three anchoring techniques in the muscular process and three positions for suture implantation in the cricoid cartilage

OBJECTIVE

To compare the effects of three anchoring techniques in the muscular process and three positions of laryngoplasty suture implantation in the cricoid cartilage on abduction of the arytenoid cartilage and interaction with the cricoarytenoid dorsalis (CAD) muscle compartments.

STUDY DESIGN

Experimental study.

SAMPLE POPULATION

Twenty-two cadaveric equine larynges.

METHODS

Three implantation techniques were assessed in the left muscular process. They were the use of a titanium corkscrew (CS), a standard caudal passage using a Jamshidi needle (JCa), and a standard cranial passage using a Jamshidi needle (JCr). Each was assessed in combination with three caudal locations in the cricoid cartilage (right, left, and left lateral). Each suture combination was tightened to submaximal abduction (Dixon grade 2). Force on the suture, degree of larynx caudal rotation, and CAD muscle indentation were evaluated.

RESULTS

The force required for optimal arytenoid cartilage abduction was lower (p < .01) for constructs involving a CS (7.45 ± 4 N). The CS also resulted in lower (p < .01) CAD muscle indentation (2.01 ± 1.25 mm) and less larynx rotation (9 ± 3.87°; adjusted p < .05).

CONCLUSION

When inserted into the muscular process at the CAD tendon insertion point, the biomechanical properties of the CS reduced the force required for optimal arytenoid cartilage abduction. The CS also minimized interference with the CAD muscle compartments and reduced caudal displacement of the left arytenoid cartilage when it was under suture tension.

CLINICAL SIGNIFICANCE

The CS implantation avoided larynx deformation and muscle interaction, offering the possibility to combine a nerve graft and laryngoplasty as a treatment for recurrent laryngeal neuropathy.

Development of the hyolaryngeal architecture in horseshoe bats: insights into the evolution of the pulse generation for laryngeal echolocation

BACKGROUND

The hyolaryngeal apparatus generates biosonar pulses in the laryngeally echolocating bats. The cartilage and muscles comprising the hyolarynx of laryngeally echolocating bats are morphologically modified compared to those of non-bat mammals, as represented by the hypertrophied intrinsic laryngeal muscle. Despite its crucial contribution to laryngeal echolocation, how the development of the hyolarynx in bats differs from that of other mammals is poorly documented. The genus Rhinolophus is one of the most sophisticated laryngeal echolocators, with the highest pulse frequency in bats. The present study provides the first detailed description of the three-dimensional anatomy and development of the skeleton, cartilage, muscle, and innervation patterns of the hyolaryngeal apparatus in two species of rhinolophid bats using micro-computed tomography images and serial tissue sections and compares them with those of laboratory mice. Furthermore, we measured the peak frequency of the echolocation pulse in active juvenile and adult individuals to correspond to echolocation pulses with hyolaryngeal morphology at each postnatal stage.

RESULTS

We found that the sagittal crests of the cricoid cartilage separated the dorsal cricoarytenoid muscle in horseshoe bats, indicating that this unique morphology may be required to reinforce the repeated closure movement of the glottis during biosonar pulse emission. We also found that the cricothyroid muscle is ventrally hypertrophied throughout ontogeny, and that the cranial laryngeal nerve has a novel branch supplying the hypertrophied region of this muscle. Our bioacoustic analyses revealed that the peak frequency shows negative allometry against skull growth, and that the volumetric growth of all laryngeal cartilages is correlated with the pulse peak frequency.

CONCLUSIONS

The unique patterns of muscle and innervation revealed in this study appear to have been obtained concomitantly with the acquisition of tracheal chambers in rhinolophids and hipposiderids, improving sound intensity during laryngeal echolocation. In addition, significant protrusion of the sagittal crest of the cricoid cartilage and the separated dorsal cricoarytenoid muscle may contribute to the sophisticated biosonar in this laryngeally echolocating lineage. Furthermore, our bioacoustic data suggested that the mineralization of these cartilages underpins the ontogeny of echolocation pulse generation. The results of the present study provide crucial insights into how the anatomy and development of the hyolaryngeal apparatus shape the acoustic diversity in bats.

A method to identify, dissect and stain equine neuromuscular junctions for morphological analysis

Morphological study of the neuromuscular junction (NMJ), a specialised peripheral synapse formed between a lower motor neuron and skeletal muscle fibre, has significantly contributed to the understanding of synaptic biology and neuromuscular disease pathogenesis. Rodent NMJs are readily accessible, and research into conditions such as amyotrophic lateral sclerosis (ALS), Charcot–Marie–Tooth disease (CMT), and spinal muscular atrophy (SMA) has relied heavily on experimental work in these small mammals. However, given that nerve length dependency is an important feature of many peripheral neuropathies, these rodent models have clear shortcomings; large animal models might be preferable, but their size presents novel anatomical challenges. Overcoming these constraints to study the NMJ morphology of large mammalian distal limb muscles is of prime importance to increase cross‐species translational neuromuscular research potential, particularly in the study of long motor units. In the past, NMJ phenotype analysis of large muscle bodies within the equine distal pelvic limb, such as the tibialis cranialis, or within muscles of high fibrous content, such as the soleus, has posed a distinct experimental hurdle. We optimised a technique for NMJ location and dissection from equine pelvic limb muscles. Using a quantification method validated in smaller species, we demonstrate their morphology and show that equine NMJs can be reliably dissected, stained and analysed. We reveal that the NMJs within the equine soleus have distinctly different morphologies when compared to the extensor digitorum longus and tibialis cranialis muscles. Overall, we demonstrate that equine distal pelvic limb muscles can be regionally dissected, with samples whole‐mounted and their innervation patterns visualised. These methods will allow the localisation and analysis of neuromuscular junctions within the muscle bodies of large mammals to identify neuroanatomical and neuropathological features.

Ex vivo modeling of the airflow dynamics and two-and three-dimensional biomechanical effects of suture placements for prosthetic laryngoplasty in horses

OBJECTIVE

To identify the degree of left arytenoid cartilage (LAC) abduction that allows laryngeal airflow similar to that in galloping horses, assess 2-D and 3-D biomechanical effects of prosthetic laryngoplasty on LAC movement and airflow, and determine the influence of suture position through the muscular process of the arytenoid cartilage (MPA) on these variables.

SAMPLE

7 equine cadaver larynges.

PROCEDURES

With the right arytenoid cartilage maximally abducted and inspiratory airflow simulated by vacuum, laryngeal airflow and translaryngeal pressure and impedance were measured at 12 incremental LAC abduction forces (0% to 100% [maximum abduction]) applied through laryngoplasty sutures passed caudocranially or mediolaterally through the left MPA. Cross-sectional area of the rima glottis and left-to-right angle quotient were determined from photographs at each abduction force; CT images were obtained at alternate forces. Arytenoid and cricoid cartilage markers allowed calculation of LAC roll, pitch, and yaw through use of Euler angles on 3-D reconstructed CT images.

RESULTS

Translaryngeal pressure and impedance decreased, and airflow increased rapidly at low abduction forces, then slowed until a plateau was reached at approximately 50% of maximum abduction force. The greatest LAC motion was rocking (pitch). Suture position through the left MPA did not significantly affect airflow data. Approximately 50% of maximum abduction force, corresponding to a left arytenoid angle of approximately 30° and left-to-right angle quotient of 0.79 to 0.84, allowed airflow of approximately 61 ± 6.5 L/s.

CONCLUSIONS AND CLINICAL RELEVANCE

Ex vivo modeling results suggested little benefit to LAC abduction forces > 50%, which allowed airflow similar to that reported elsewhere for galloping horses.

External transcutaneous ultrasound technique in the equine cricoarytenoideus dorsalis muscle: Assessment of muscle size and echogenicity with resting endoscopy

BACKGROUND

Recent studies have assessed the cricoarytenoideus dorsalis muscle (CAD) using transoesophageal ultrasonography in equine recurrent laryngeal neuropathy (RLN). We assessed the CAD using the external transcutaneous ultrasound technique, which may constitute an easier method in horses.

OBJECTIVES

To evaluate ultrasonographic imaging of the left cricoarytenoideus dorsalis muscle (LCAD) and right cricoarytenoideus dorsalis muscle (RCAD) as a diagnostic tool for RLN using the transcutaneous ultrasound technique.

STUDY DESIGN

Cross-sectional study.

METHODS

The axial plane thickness, cross-sectional area and echogenicity of the LCAD and RCAD were measured using transcutaneous ultrasonography in 164 horses. Assessments of LCAD were compared with those of RCAD. The LCAD:RCAD ratios in thickness and area were compared between control horses (resting grades 1 and 2) and horses with resting laryngeal grades 3 and 4 using the Havemeyer 4-point grading system with subgrades.

RESULTS

The LCAD:RCAD ratios for thickness and area were 0.69 and 0.66 in horses with resting grades 3 and 4 respectively; LCAD was more hyperechogenic than RCAD in resting grades 3 and 4. LCAD:RCAD ratios for thickness and area in grades 3.II, 3.III and 4 were significantly lower than those in control horses. Thickness and area of the LCAD were negatively correlated with resting laryngeal grade MAIN LIMITATIONS: Overground endoscopy was not performed in this study. There were some differences in methodology: measurement of the physical thickness of the LCAD and clipping of hair at the laryngeal region were only performed in horses that underwent laryngoplasty.

CONCLUSIONS

Results of ultrasonographic assessments of the CAD using transcutaneous ultrasonography were similar to those obtained by transoesophageal ultrasonography. This technique enables a simple, noninvasive, direct and easy examination. Assessment of the CAD using transcutaneous ultrasonography may be a useful technique and a potential option for determining whether to perform nerve graft or laryngoplasty.

Functional electrical stimulation following nerve injury in a large animal model

INTRODUCTION

Controversy exists over the effects of functional electrical stimulation (FES) on reinnervation. We hypothesized that intramuscular FES would not delay reinnervation after recurrent laryngeal nerve (RLn) axonotmesis.

METHODS

RLn cryo-injury and electrode implantation in ipsilateral posterior cricoarytenoid muscle (PCA) were performed in horses. PCA was stimulated for 20 weeks in eight animals; seven served as controls. Reinnervation was monitored through muscle response to hypercapnia, electrical stimulation and exercise. Ultimately, muscle fiber type proportions and minimum fiber diameters, and RLn axon number and degree of myelination were determined.

RESULTS

Laryngeal function returned to normal in both groups within 22 weeks. FES improved muscle strength and geometry, and induced increased type I:II fiber proportion (p = 0.038) in the stimulated PCA. FES showed no deleterious effects on reinnervation.

DISCUSSION

Intramuscular electrical stimulation did not delay PCA reinnervation after axonotmesis. FES can represent a supportive treatment to promote laryngeal functional recovery after RLn injury. Muscle Nerve 59:717-725, 2019.

Modified first or second cervical nerve transplantation technique for the treatment of recurrent laryngeal neuropathy in horses

BACKGROUND

In horses, the only established method for reinnervation of the larynx is the nerve-muscle pedicle implantation, whereas in human medicine, direct nerve implantation is a standard surgical technique for selective laryngeal reinnervation in human patients suffering from bilateral vocal fold paralysis.

OBJECTIVES

(1) To describe a modified first or second cervical nerve transplantation technique for the treatment of recurrent laryngeal neuropathy (RLN) in horses and (2) evaluate the outcomes of reinnervation using direct nerve needle-stimulation of the first cervical nerve and exercising endoscopy before and after surgery.

STUDY DESIGN

Case series.

METHODS

Nerve transplantation surgery, in which the first or second cervical nerve is tunnelled through the atrophied left cricoarytenoideus dorsalis muscle, was performed in combination with ipsilateral laser ventriculocordectomy. Ultrasound-guided stimulation of the first cervical nerve at the level of the alar foramen was used to confirm successful reinnervation post-operatively. Exercising endoscopy was performed before and after surgery. The exercising RLN grade of the left arytenoid was blindly determined at the highest stride frequency for each examination.

RESULTS

Surgery was performed in 17 client-owned animals with RLN. Reinnervation was confirmed by nerve stimulation and subsequent arytenoid abduction observed in 11 out of 12 cases between 4 and 12 months post-operatively. Fourteen horses had exercising endoscopy before and after surgery. Nine horses had an improved exercising RLN grade, four horses had the same exercising grade and one horse had a worse exercising grade after surgery.

MAIN LIMITATIONS

A sham-operated control group was not included and follow-up beyond 12 months and objective performance data were not obtained.

CONCLUSIONS

The modified first or second cervical nerve transplantation technique, using tunnelling and direct implantation of the donor nerve into the cricoarytenoideus dorsalis muscle, resulted in reinnervation in 11 out of 12 cases and improved exercising grade in 9 out of 14 horses within 12 months after surgery.

Transoesophageal ultrasound and computer tomographic assessment of the equine cricoarytenoid dorsalis muscle: Relationship between muscle geometry and exercising laryngeal function

REASONS FOR PERFORMING STUDY

Early detection of recurrent laryngeal neuropathy (RLN) is of considerable interest to the equine industry.

OBJECTIVES

To describe two imaging modalities, transoesophageal ultrasound (TEU) and computed tomography (CT) with multiplanar reconstruction to assess laryngeal muscle geometry, and determine the relationship between cricoarytenoid dorsalis (CAD) geometry and function.

STUDY DESIGN

Two-phase study evaluating CAD geometry in experimental horses and horses with naturally occurring RLN.

METHODS

Equine CAD muscle volume was determined from CT scan sets using volumetric reconstruction with LiveWire. The midbody and caudal dorsal-ventral thickness of the CAD muscle was determined using a TEU in the same horses; and in horses with a range of severity of RLN (n = 112).

RESULTS

Transoesophageal ultrasound was able to readily image the CAD muscles and lower left:right CAD thickness ratios were observed with increasing disease severity. Computed tomography based muscle volume correlated very closely with ex vivo muscle volume (R² = 0.77).

CONCLUSIONS

Computed tomography reconstruction can accurately determine intrinsic laryngeal muscle geometry. A relationship between TEU measurements of CAD geometry and laryngeal function was established. These imaging techniques could be used to track the response of the CAD muscle to restorative surgical treatments such as nerve muscle pedicle graft, nerve anastomosis and functional electrical stimulation.

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

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.

Sihler's whole mount nerve staining technique: a review

Sihler's stain is a whole mount nerve staining technique that renders other soft tissue translucent or transparent while staining the nerves. It permits mapping of entire nerve supply patterns of organs, skeletal muscles, mucosa, skin, and other structures after the specimens are fixed in neutralized formalin, macerated in potassium hydroxide, decalcified in acetic acid, stained in Ehrlich's hematoxylin, destained in acetic acid, and cleared in glycerin. The unique advantage of Sihler's stain over other anatomical methods is that all the nerves within the stained specimen can be visualized in their three-dimensional positions. To date, Sihler's stain is the best tool for demonstrating the precise intramuscular branching and distribution patterns of skeletal muscles, which are important not only for anatomists, but also for physiologists and clinicians. Advanced knowledge of the neural structures within mammalian skeletal muscles is critical for understanding muscle functions, performing electrophysiological experiments and developing novel neurosurgical techniques. In this review, Sihler's stain is described in detail and its use in nerve mapping is surveyed. Special emphasis is placed on staining procedures and troubleshooting, strengths and limitations, applications, major contributions to neuroscience, physiological and clinical significance, and areas for further technical improvement that deserve future research.