Equine Laryngeal Hemiplegia Study Guide

Overview and Clinical Importance

Laryngeal hemiplegia, also known as recurrent laryngeal neuropathy (RLN) or colloquially as "roaring," is a progressive degenerative neuropathy of the recurrent laryngeal nerve that results in dysfunction of the cricoarytenoideus dorsalis (CAD) muscle. This condition is one of the most common causes of upper airway obstruction and exercise intolerance in performance horses, particularly affecting Thoroughbreds and draft breeds.

The disease predominantly affects the left side (greater than 95% of idiopathic cases) due to the longer course of the left recurrent laryngeal nerve, which loops around the aortic arch before ascending to the larynx. This extended pathway makes the nerve more susceptible to axonal degeneration.

Anatomy and Neuroanatomy

Laryngeal Cartilages

The equine larynx is composed of several cartilages that work together to regulate airflow and protect the airway during swallowing. The arytenoid cartilages are paired, triangular-shaped hyaline cartilages that are the key structures affected in laryngeal hemiplegia. They articulate with the cricoid cartilage via the cricoarytenoid joint, which allows abduction and adduction movements critical for airway control.

Key Laryngeal Structures

Innervation of the Larynx

The intrinsic muscles of the larynx are innervated by the recurrent laryngeal nerve, a branch of the vagus nerve (CN X). The only exception is the cricothyroid muscle, which is innervated by the external branch of the cranial laryngeal nerve.

Critical Anatomical Point: The left recurrent laryngeal nerve has a significantly longer course than the right. After branching from the vagus nerve in the thorax, the left nerve loops around the ligamentum arteriosum and aortic arch before ascending in the neck to the larynx. The right nerve loops around the subclavian artery. This difference in length (the left RLN is the longest nerve in the horse's body) explains the left-sided predominance of idiopathic disease.

Intrinsic Laryngeal Muscles

Etiology and Pathophysiology

Pathogenesis

Recurrent laryngeal neuropathy is characterized by a distal axonopathy predominantly affecting the left recurrent laryngeal nerve. The pathological changes include progressive loss of large myelinated nerve fibers in the distal portion of the nerve, with evidence of Wallerian degeneration. These changes result in denervation atrophy of the intrinsic laryngeal muscles, particularly the CAD muscle.

Key Pathological Features:

• Progressive loss of large myelinated nerve fibers distally
• Neurogenic atrophy of intrinsic laryngeal muscles
• Fiber type grouping indicating denervation and attempted reinnervation
• Bilateral involvement (though left side is clinically more severe)
• Evidence of ongoing damage with concurrent repair attempts

Causes and Risk Factors

Clinical Signs and Presentation

Clinical signs of laryngeal hemiplegia result from dynamic upper airway obstruction during inspiration when the paralyzed arytenoid and vocal fold collapse into the airway, reducing the cross-sectional area of the rima glottidis.

Primary Clinical Signs

• Inspiratory noise ("roaring" or "whistling"): Characteristic sound during exercise, especially at canter or gallop; caused by turbulent airflow through the narrowed glottis
• Exercise intolerance: Reduced performance due to decreased oxygen delivery; may manifest as unwillingness to work or early fatigue
• Dyspnea during exercise: Increased inspiratory effort; extended head and neck position

Physical Examination Findings

• Laryngeal palpation: Prominent muscular process of the left arytenoid due to CAD muscle atrophy; asymmetry compared to right side
• Slap test: Absence or reduction of the adductor reflex on the affected side (slap withers, observe for laryngeal adduction)
• Grunt test: May elicit abnormal inspiratory noise when startling the horse

Diagnostic Approach

Endoscopy

Upper airway endoscopy is the gold standard for diagnosis. The examination should be performed with the horse unsedated (sedation affects arytenoid movement) and should assess symmetry, synchrony, and range of arytenoid movements during quiet breathing, after swallowing, and during temporary nasal occlusion.

Havemeyer Grading System

The Havemeyer grading system is the internationally accepted standard for classifying laryngeal function. It was established in 2003 through consensus of equine clinicians and researchers.

Dynamic (Exercising) Endoscopy

Dynamic endoscopy is performed using a high-speed treadmill or overground endoscopy system. This modality is essential because up to 30% of horses with RLN will have additional dynamic upper airway dysfunction not apparent at rest. It also allows direct visualization of the degree of arytenoid collapse during maximal exercise.

Indications for dynamic endoscopy:

• Equivocal findings on resting endoscopy
• Grade II or III.1 horses where clinical significance is uncertain
• To rule out concurrent dynamic upper airway disorders
• Pre-purchase evaluation of performance horses

Laryngeal Ultrasound

Ultrasonography of the larynx can provide additional diagnostic information. Key findings in RLN include increased echogenicity of the cricoarytenoideus lateralis and dorsalis muscles due to neurogenic atrophy and fibrous replacement. This technique can help predict dynamic collapse and may be useful when endoscopy findings are borderline.

Differential Diagnoses

Treatment Options

Treatment selection depends on the severity of disease (grade), the horse's intended use, the presenting complaint (noise alone vs. exercise intolerance), and owner expectations. Many pleasure horses with mild-moderate disease can continue working without surgical intervention.

Surgical Treatment Options

Prosthetic Laryngoplasty - Detailed Overview

Prosthetic laryngoplasty ("tie-back") is the treatment of choice for racehorses and horses with significant exercise intolerance. The procedure involves placing a non-absorbable suture (typically heavy nylon or stainless steel wire) from the muscular process of the arytenoid cartilage to the caudal border of the cricoid cartilage, permanently abducting the arytenoid to emulate the function of the paralyzed CAD muscle.

Key Surgical Considerations:

• Degree of abduction must be carefully calibrated - too little fails to improve airflow; too much causes aspiration
• Usually combined with ipsilateral ventriculocordectomy
• Can be performed under general anesthesia or standing sedation
• Post-operative rest: typically 30 days stall rest, then 30 days light turnout

Complications of Laryngoplasty

Prognosis

Prognosis depends on the horse's intended use, the grade of disease, and the treatment performed.

• Racing Thoroughbreds with laryngoplasty: 60-70% return to near pre-surgical performance level
• Sport horses and pleasure horses: Generally excellent prognosis; many can work without surgery
• Draft breeds: Good with ventriculocordectomy alone for noise; laryngoplasty for working drafts
• Progression: Idiopathic RLN is progressive; Grade II-III may progress to Grade IV over time

Memory Aids and Board Tips

Memory Aid - "LEFT = LONG":

The LEFT recurrent laryngeal nerve is the LONGEST nerve in the horse (loops around the aorta), which is why the LEFT side is almost always affected in idiopathic cases.

Memory Aid - "ROAR = Racing Obviously Affects Respiration":

Roaring horses can't race at full capacity because the paralyzed arytenoid reduces airflow during high-speed work.

Memory Aid - "CAD = Can't Abduct, Disorder":

The Cricoarytenoideus Dorsalis is the ONLY abductor. When it fails, you Can't Abduct = Disorder (RLN).

Board Tip - Bilateral Paralysis = Toxicosis: If you see BILATERAL laryngeal paralysis on the exam, immediately think LEAD TOXICITY or organophosphates. Idiopathic RLN is almost exclusively left-sided.