The Canine Larynx: Anatomy, Pathology, and Tie-Back Surgery

Introduction

The larynx is an essential structure in dogs, playing a vital role in breathing, vocalization, and protecting the lower respiratory tract from aspiration. Laryngeal paralysis is a common condition affecting the larynx, especially in older, large breed dogs. This condition can lead to significant respiratory distress and necessitates surgical intervention. Arytenoid lateralization (tie-back surgery) is the most common surgical procedure performed to alleviate the symptoms of laryngeal paralysis. This lesson aims to provide a comprehensive overview of the canine larynx, the pathophysiology of laryngeal paralysis, and the surgical approach and considerations for tie-back surgery.

Anatomy of the Canine Larynx

The larynx is located in the ventral neck, caudal to the pharynx and cranial to the trachea. It is composed of several cartilages, muscles, and ligaments that work together to facilitate its functions.

Cartilages

The larynx is composed of four main cartilages:

1. Epiglottis: A leaf-shaped cartilage that acts as a lid to close the laryngeal opening during swallowing, preventing aspiration.

2. Thyroid cartilage: The largest cartilage, forming the majority of the laryngeal structure. It provides attachment points for muscles and ligaments.

3. Cricoid cartilage: A ring-shaped cartilage that forms the caudal portion of the larynx, connecting it to the trachea.

4. Arytenoid cartilages: Paired cartilages that play a crucial role in vocal fold movement and airway patency.

Muscles

The intrinsic muscles of the larynx control the position and tension of the vocal folds:

1. Cricoarytenoideus dorsalis: This muscle is responsible for abducting the arytenoid cartilages, opening the glottis.

2. Cricoarytenoideus lateralis: This muscle adducts the arytenoid cartilages, closing the glottis.

3. Thyroarytenoideus: This muscle adjusts the tension of the vocal folds.

Innervation

The recurrent laryngeal nerves, branches of the vagus nerve, innervate the intrinsic muscles of the larynx, except for the cricothyroid muscle, which is innervated by the external branch of the superior laryngeal nerve.

Blood Supply

The larynx receives blood supply from the cranial and caudal laryngeal arteries, which are branches of the thyroid arteries.

Pathology: Laryngeal Paralysis

Etiology

Laryngeal paralysis can be congenital or acquired. Acquired laryngeal paralysis is more common and can result from:

- Idiopathic causes

- Trauma or injury to the recurrent laryngeal nerves

- Systemic neuromuscular diseases

- Hypothyroidism

Pathophysiology

Laryngeal paralysis occurs when the recurrent laryngeal nerves lose function, leading to the inability of the cricoarytenoideus dorsalis muscles to abduct the arytenoid cartilages. This results in the collapse of the arytenoids and vocal folds into the airway, causing partial or complete airway obstruction.

Clinical Signs

Common clinical signs include:

- Inspiratory stridor

- Exercise intolerance

- Voice change (dysphonia)

- Gagging or coughing, especially during eating or drinking

- Severe respiratory distress in advanced cases

Diagnosis

Clinical Examination

A thorough physical and neurological examination is essential. Observation of respiratory effort and auscultation of the larynx and trachea can provide initial clues.

Laryngoscopy

Direct visualization of the larynx using a laryngoscope is the definitive diagnostic method. It allows assessment of the movement (or lack thereof) of the arytenoid cartilages during respiration.

Additional Diagnostic Tests

- Radiography: To rule out concurrent thoracic or cervical abnormalities.

- Electromyography: To assess the function of the laryngeal muscles.

- Thyroid function tests: To rule out hypothyroidism as a contributing factor.

Arytenoid Lateralization (Tie-Back Surgery)

Indications

Tie-back surgery is indicated in dogs with moderate to severe laryngeal paralysis causing significant clinical signs and respiratory compromise.

Surgical Technique

1. Preoperative Preparation: The patient is anesthetized and positioned in lateral recumbency. The surgical site is clipped and aseptically prepared.

2. Incision and Exposure: A skin incision is made over the larynx. The subcutaneous tissues and muscles are carefully dissected to expose the thyroid and cricoid cartilages.

3. Arytenoid Lateralization: A suture is placed between the muscular process of the arytenoid cartilage and the cricoid cartilage or caudal edge of the thyroid cartilage. This suture is tightened to abduct the arytenoid cartilage, opening the glottis.

4. Closure: The incision is closed in layers. A temporary tracheostomy may be performed in severe cases to ensure a patent airway during the immediate postoperative period.

Postoperative Care

- Monitoring: Close monitoring for respiratory distress and aspiration pneumonia is essential.

- Antibiotics and Analgesics: Administered as needed.

- Dietary Management: Feeding soft foods or using a feeding tube to minimize the risk of aspiration.

Complications

Potential complications include:

- Aspiration pneumonia

- Failure of the suture or recurrence of clinical signs

- Over-abduction leading to aspiration

Conclusion

The canine larynx plays a critical role in respiratory and vocal functions. Laryngeal paralysis is a common and serious condition that can significantly impact a dog's quality of life. Arytenoid lateralization (tie-back surgery) is a well-established surgical procedure to alleviate the symptoms of laryngeal paralysis. Understanding the anatomy of the larynx, the pathophysiology of laryngeal paralysis, and the surgical technique and postoperative care for tie-back surgery is essential for veterinarians managing this condition. With appropriate intervention, many dogs can achieve a significant improvement in their respiratory function and overall quality of life.

References

1. Fossum, T. W. (2019). Small Animal Surgery Textbook. Elsevier Health Sciences.

2. Tobias, K. M., & Johnston, S. A. (2018). Veterinary Surgery: Small Animal. Elsevier Health Sciences.

3. Monnet, E. (2003). Arytenoid lateralization technique for treatment of laryngeal paralysis in dogs. Journal of the American Veterinary Medical Association, 223(9), 1318-1320.

4. Harvey, C. E., & O'Brien, J. A. (1982). Laryngeal paralysis in dogs. Journal of the American Veterinary Medical Association, 180(5), 512-516.

5. Beasley, M. J., & Packer, R. A. (1990). Laryngeal paralysis in dogs: a review of 58 cases. Journal of Small Animal Practice, 31(10), 505-510.