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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • This disease is a result of neuropathy of the recurrent laryngeal nerve
  • Failure of an anatomically normal left (rarely right) arytenoid cartilage to abduct fully during inspiration results in inspiratory respiratory obstruction
  • Genomic analysis establishes a correlation between growth and laryngeal neuropathy in Thoroughbreds

Pathophysiology!!navigator!!

  • Although trauma to the left recurrent laryngeal nerve can produce the clinical condition, in most cases the underlying pathologic basis is a bilateral mononeuropathy characterized by distal loss of large myelinated fibers (i.e. distal axonopathy) predominantly in the left recurrent laryngeal nerves
  • The right recurrent laryngeal nerve is also affected histologically. Lesions in the right nerves are far less severe, however, and clinical signs rarely, if ever, are associated with these abnormalities
  • The peripheral neuropathy is progressive and accompanied by attempts at axonal regeneration such that both axonal degeneration and regeneration are observed histologically with their respective lesions in the laryngeal muscles
  • The loss of abductory function associated with neurogenic atrophy of the CAD muscle causes the clinical signs. Impaired CAD function leads to inability of the left arytenoid cartilage to abduct and to (in a sustained way) resist pressure swings in the upper airway during exercise. As a result, negative pressure during inspiration leads to adduction of the left arytenoid cartilage, which obstructs the airway, leading to inspiratory stridor and diminished airflow during inspiration

Systems Affected!!navigator!!

Respiratory—upper respiratory tract.

Genetics!!navigator!!

Evidence beyond the tendency to have tall offspring suggests this is an inheritable defect. Genetic analysis is mixed—there is an association with height and RLN in Thoroughbreds and a protective haplotype in Warmbloods.

Incidence/Prevalence!!navigator!!

  • Worldwide in horses
  • Rarely affects ponies

Geographic Distribution!!navigator!!

Worldwide

Signalment!!navigator!!

  • Tall male horses, particularly Thoroughbreds, Warmbloods, and draft horses, are more commonly affected
  • Approximately 5% (range 1.8–9.5%) of Thoroughbreds and up to 42% of draft breeds are affected; far less common in Standardbreds and almost nonexistent in ponies
  • In Thoroughbreds, the incidence is reported to increase from 6.5% in 2-year-old horses to 9.5% in 6-year-olds

Signs!!navigator!!

  • Horses are presented for upper respiratory noise, exercise intolerance, or both
  • Laryngeal collapse significantly interferes with ventilation in horses that perform at high speed.
  • In show horses, abnormal upper respiratory noise is the main owner concern

Causes!!navigator!!

  • Most commonly RLN of unknown cause
  • Genetic predisposition correlated with height
  • Any peripheral or central neural damage/neuropathy affecting the left (or right) recurrent laryngeal nerve can result in laryngeal hemiparesis/hemiplegia
  • Viral neuritis
  • Intoxications (e.g. organophosphate, lead) may cause bilateral paresis of the laryngeal nerves

Risk Factors!!navigator!!

  • Perivenous injection—look for damage to vagosympathetic trunk (e.g. Horner syndrome)
  • Cervical trauma
  • Surgical procedures near the left recurrent nerve

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Arytenoid chondritis
  • Laryngeal collapse not associated with RLN
  • Arytenoid subluxation
  • Fourth branchial arch defect—be alert for congenital malformation of the muscular process of the arytenoid cartilage and/or of the thyroid lamina and/or cricoid cartilage in right-sided paresis/paralysis

CBC/Biochemistry/Urinalysis!!navigator!!

Of no value.

Other Laboratory Tests!!navigator!!

Arterial blood gases during exercise, with which hypoventilation can be evaluated—in affected horses at maximal exercise, PaCO2 can be >55 mmHg and PaO2 can be <65 mmHg.

Imaging!!navigator!!

  • External laryngeal US allows identification of the increase in echogenicity and change in muscle fiber pattern in the ipsilateral CAL and vocalis muscle compared with the contralateral side
  • Esophageal US, CT, and MRI yield various measurements of the geometry of the CAD muscle which have been shown to correlate with presence or absence of laryngeal collapse at exercise

Other Diagnostic Procedures!!navigator!!

Videoendoscopy at Rest (see the Havemeyer Laryngeal Grade)

  • The larynx is evaluated for morphologic abnormality and is “graded” based on its resting endoscopy in unsedated horses
  • Statistically, horses with laryngeal grade I (complete and symmetrical abduction) and II (complete but asymmetrical abduction) are generally normal at exercise while most (but not all) horses with laryngeal grade III (incomplete or unstained abduction) have some form of collapse at exercise. Virtually all horses with grade IV (no significant abduction) have dynamic arytenoid collapse at exercise
  • On external palpation, the left CAD muscle may be atrophied compared with the right

Examination at Exercise (Gold Standard)

  • Endoscopy at exercise is the most accurate and precise test to identify and determine the degree of laryngeal collapse as well as recognizing additional structural collapse such as right aryepiglottic fold and right focal fold
  • Arterial blood gases during exercise, with which hypoventilation can be evaluated

Pathologic Findings!!navigator!!

Gross

  • Atrophy of the left CAL usually is most severe and is best detected by laryngeal US
  • Atrophy of the CAD is most obvious clinically and can be estimated by manual palpation of the dorsal aspect of the cricoid and muscular process of the arytenoid cartilage

Histopathologic

  • Distal loss of large, myelinated fibers in the left (and to a much less degree in the right) recurrent laryngeal nerves
  • Left intrinsic laryngeal muscles exhibit angular fiber atrophy and fiber-type grouping (far milder lesion on the right)

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

  • Reduce or eliminate abnormal upper respiratory sound associated with the collapsed vocal cord and resulting dilated ventricle
  • Improve airway patency by preventing collapse (adduction) of the arytenoid cartilage during inhalation and by dynamically (i.e. nerve transplant) or fixed (i.e. laryngoplasty) increasing the cross-sectional diameter of the larynx by abduction of the arytenoid cartilage

Appropriate Health Care!!navigator!!

N/A

Nursing Care!!navigator!!

N/A

Activity!!navigator!!

N/A

Client Education!!navigator!!

Unless a traumatic or iatrogenic cause is identified, clients should be informed of the possible genetic basis for this disease and that breeding to affected horses may not be indicated.

Surgical Considerations!!navigator!!

  • Treatment is not necessary if exercise intolerance is not present and owners are willing to tolerate the upper respiratory noise
  • Placement of a laryngeal prosthesis that fixes the left arytenoid cartilage in near-maximal abduction coupled with cordectomy or ventriculocordectomy is the treatment of choice in horses used for strenuous athletic activities. Chronic coughing during eating is seen in as many as 10–20% of horses after this surgery
  • Unilateral or bilateral ventriculocordectomy is superior to laryngoplasty in reducing or eliminating the abnormal upper respiratory noise associated with this condition
  • Unilateral or bilateral ventriculocordectomy can improve exercise tolerance in selected horses with laryngeal grade III, in which arytenoid abduction is adequate yet vocal cord collapse is present, and in sport horses where the exercise is less intense
  • Laryngeal reinnervation of the CAD muscle by nerve implantation

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

None, other than routine, perioperative antimicrobial and anti-inflammatory agents.

Contraindications!!navigator!!

N/A

Precautions!!navigator!!

N/A

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Upper airway endoscopy is required 4–6 weeks after surgery to monitor response to fixed abduction surgery (laryngoplasty), and by dynamic endoscopy for laryngeal reinnervation at ~6 months
  • Determining the final response to treatment or monitoring of affected horses is made on the basis of evaluating exercise tolerance and upper respiratory noise

Prevention/Avoidance!!navigator!!

N/A

Possible Complications!!navigator!!

Horses undergoing laryngeal prosthesis may experience chronic coughing and, rarely, aspiration pneumonia.

Expected Course and Prognosis!!navigator!!

  • Laryngeal hemiplegia—horses with grade IV will not exhibit any further deterioration of athletic activity or upper respiratory noise
  • Laryngeal hemiparesis—horses with a varying degree of collapse may exhibit further deterioration of athletic activity or upper respiratory noise

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

Untreated horses may be predisposed to exercise-induced pulmonary hemorrhage if submitted to strenuous exercise. In addition, it is likely that untreated horses are distressed or are at risk of orthopedic injury because of the marked hypoxia associated with this condition during intense exercise.

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

N/A

Abbreviations!!navigator!!

  • CAD = dorsal cricoarytenoid muscle
  • CAL = lateral cricoarytenoid muscle
  • CT = computed tomography
  • MRI = magnetic resonance imaging
  • PaCO2 = partial pressure of carbon dioxide in arterial blood
  • PaO2 = partial pressure of oxygen in arterial blood
  • RLN = recurrent laryngeal neuropathy
  • US = ultrasonography, ultrasound

Suggested Reading ⬆ ⬇

Barakzai SZ, Dixon PM. Correlation of resting and exercising endoscopic findings for horses with dynamic laryngeal collapse and palatal dysfunction. Equine Vet J 2011;43:18–23.

Boyko AR, Brooks SA, Behan-Braman A, et al. Genomic analysis establishes correlation between growth and laryngeal neuropathy in Thoroughbreds. BMC Genomics 2014;15:259–267.

Brown DL, Derksen FJ, Stick JA, et al. Ventriculocordectomy reduces respiratory noise in horses with laryngeal hemiplegia. Equine Vet J 2003;35:570–574.

Chalmers HJ, Cheetham J, Yeager AE, Ducharme NG. Ultrasonography of the equine larynx. Vet Radiol Ultrasound 2006;47:476–481.

Duncan ID, Griffith IR, Madrid RE. A light and electron microscopic study of the neuropathy of equine idiopathic laryngeal hemiplegia. Neuropathol Appl Neurobiol 1978;4:483–501.

Robinson NE. Consensus statements on equine recurrent laryngeal neuropathy: conclusions of the Havemeyer Workshop. Equine Vet Educ 2004;16:333–336.

Shappel KK, Derksen FJ, Stick JA, Robinson NE. Effects of ventriculectomy, prosthetic laryngoplasty, and exercise on upper airway function in horses with induced left laryngeal hemiplegia. Am J Vet Res 1988;49:1760–1766.

Taylor SE, Barakzai SZ, Dixon P. Ventriculocordectomy as the sole treatment for recurrent laryngeal neuropathy: long-term results from ninety-two horses. Vet Surg 2006;35:653–657.

Author(s) ⬆

Author: Norm G. Ducharme

Consulting Editors: Daniel Jean and Mathilde Leclère

Acknowledgment: The author and editors acknowledge the prior contribution of Richard P. Hackett.