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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • This is a group of disorders that cause a transient obstruction to respiration within the pharynx, larynx, or both
  • Often cyclic and synchronous with inspiration
  • Results from fatigue of the musculature that normally maintains luminal patency of the pharynx and larynx

Pathophysiology!!navigator!!

  • The pathophysiologies of many of the abnormalities resulting in this condition have not yet been characterized. All appear to be different forms of neuromuscular dysfunction or fatigue; however, most occur during the inspiratory phase of respiration, under the pull of high negative inspiratory pressures
  • With increased respiratory effort and muscular fatigue, the condition worsens, the obstruction becomes more severe, and a vicious cycle ensues
  • Airway turbulence may result in abnormal respiratory noise but is not always present

Systems Affected!!navigator!!

  • Upper respiratory tract
  • With severe, more chronic conditions, the cardiopulmonary system may undergo secondary changes from repeated, high negative intrathoracic pressures and hypoxia

Signalment!!navigator!!

  • Any age or breed
  • More commonly diagnosed in racehorses because of the high negative inspiratory pressures created during strenuous exercise
  • Thoroughbreds (2–3 years) are the largest group affected
  • To date, medial deviation of the aryepiglottic folds has been most commonly identified in racehorses
  • Quarter Horses with hyperkalemic periodic paralysis can have paroxysmal spasms of the pharynx and larynx during “paralytic episodes” that result in severe upper respiratory obstruction

Signs!!navigator!!

  • Exercise intolerance or poor performance
  • Abnormal upper respiratory noise during inspiration may occur depending on the degree and type of obstruction
  • Only DDSP results in expiratory noise
  • Coughing
  • Dysphagia

Causes!!navigator!!

  • Laryngeal hemiplegia most commonly affects the left arytenoid cartilage, results from idiopathic degeneration of the left recurrent laryngeal nerve, and causes a paresis of the primary abductor of the left arytenoid, the cricoarytenoideus dorsalis muscle
  • Infrequently, trauma to either the left or right recurrent laryngeal nerve associated with jugular thrombophlebitis can result in laryngeal hemiplegia
  • Epiglottic retroversion is presumed to be a dysfunction of the hypoepiglottic muscle on the basis of experimental reproduction of the disorder after anesthetic blockade of the nerve (i.e. the hypoglossal) that supplies the hypoepiglottic muscle
  • Dorsal pharyngeal collapse can be reproduced experimentally with anesthetically induced dysfunction of the stylopharyngeus muscles
  • Other forms of collapse are thought to represent either specific muscle dysfunction within the pharynx or disproportionate force between the muscle groups

Risk Factors!!navigator!!

  • High-speed exercise
  • Hyperkalemic periodic paralysis in Quarter Horses

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Laryngeal hemiplegia
  • Vocal cord collapse
  • Intermittent DDSP
  • Pharyngeal collapse
  • Epiglottic retroversion
  • Medial deviation of the aryepiglottic folds
  • Combinations of any of the above-mentioned distinct disorders

Diagnostic Procedures!!navigator!!

Resting Endoscopy

  • Helps to evaluate any structural abnormalities predisposing to dynamic obstruction during exercise but rarely is definitive for a dynamic abnormality
  • More difficult to speculate on pharyngeal than on laryngeal forms of dynamic collapse
  • Assesses laryngeal and pharyngeal function during normal breathing, nasal occlusion, and swallowing
  • Horses normally demonstrate some dorsal roof collapse of the pharynx and have air pass between the aryepiglottic folds and pharyngeal ostium during nasal occlusion. This is not an indication of a pharyngeal abnormality during exercise
  • With prolonged nasal occlusion, 60% of horses can be made to displace their palates, but they do not experience dynamic collapse during exercise
  • Horses that very readily displace their soft palate are more likely to displace it during a race
  • Horses that leave their palates displaced for an extended period of time or have difficulty replacing their palates with a swallow are more likely to have a pharyngeal abnormality during high-speed exercise. This may be a crude indication of some pharyngeal weakness
  • Horses commonly demonstrate asynchrony to their arytenoid movement yet achieve full abduction of both arytenoids during nasal occlusion or after swallowing (grade 2 on a scale of 1–4). These horses do not experience dynamic collapse during exercise. Horses that cannot fully and symmetrically abduct both arytenoids after swallowing or nasal occlusion (grade 3) are considered to be impaired
  • The degree of dynamic collapse depends on the degree of paresis and on the intensity of exercise. Racehorses likely undergo significant respiratory compromise with grade 3 laryngeal hemiplegia; show horses are more likely asymptomatic

Ultrasonography

Ultrasonography of the larynx can determine if there is evidence of denervation of the cricoarytenoideus lateralis that would be indicative of recurrent laryngeal neuropathy (laryngeal hemiplegia). This should not be used as the sole criterion for surgical intervention but provides strong evidence of clinically significant dysfunction in racehorses.

Endoscopy During Exercise

  • Often required to determine the cause of upper respiratory collapse
  • The examination can be performed over ground or on a treadmill but, regardless of the surface, the level of strenuous activity must match the performance level
  • Head and neck flexion can have a significant impact on the findings and thus the position during the examination should simulate that which occurs during performance to be valid
  • Other requirements to ensure a valid test:
    • Holter monitor to record heart rate during exercise, to determine heart rate relative to the horse's speed, and to guarantee the horse is maximally exerting itself by achieving a heart rate of ≌220 bpm or greater
    • Videoendoscope linked to a video recorder to visualize the abnormality and play back the video in slow motion if the obstruction occurs too quickly to see in real time
    • A physiologically fit horse exercised in tack (harness for Standardbreds) to minimize spurious results
  • Because these abnormalities are dynamic, and some can be intermittent (e.g. DDSP), other factors that may affect the horse's performance should also be simulated. For example, some racehorses will only demonstrate the abnormalities during an examination if other horses are challenging them during the examination
  • Measurements of upper respiratory pressures and use of flow–volume loops during treadmill exercise can document respiratory obstruction but cannot discriminate between the many different abnormalities

Analysis of Upper Respiratory Sounds

Spectrogram analysis can be used to some degree to determine if an abnormality is present and which abnormality exists.

Pathologic Findings!!navigator!!

N/A

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

To create a stable open airway, resistant to high negative pressures.

Surgical Considerations!!navigator!!

  • Laryngeal hemiplegia is treated with surgical laryngoplasty (i.e. tie-back procedure). The affected arytenoid is held in a fixed, partially abducted position with a nonabsorbable suture simulating the contracted cricoarytenoid muscle. An adjunctive procedure (i.e. ventriculocordectomy) also can be performed to minimize obstruction at the ventral aspect of the glottis after a tie-back. For more on this topic, see chapter Laryngeal hemiparesis/hemiplegia (recurrent laryngeal neuropathy)
  • Medial deviation of the aryepiglottic folds has been treated effectively both with rest and with laser resection of the offending soft tissue; however, surgery affords a quicker return to training
  • There are multiple surgical procedures to consider for intermittent DDSP, which demonstrates our inability to define the exact cause in each patient. For more on this topic, see chapter Dorsal displacement of the soft palate (DDSP)
  • Rest and oral anti-inflammatory treatment has been the only mode of therapy for other forms of dynamic collapse

Medications ⬆ ⬇

MEDICATIONS

Drug(s) of Choice

A 3–4 week course of an anti-inflammatory drug may be indicated during the period of rest to resolve any presumed inflammatory component causing the dysfunction.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Resting endoscopy is necessary several weeks after any surgical intervention, and though it will not be definitive for determining the success of the surgery it will determine the capability of the horse to resume training
  • An increase in performance or diminution of noise often is the criterion used to determine a successful treatment
  • Repeat exercising endoscopy is the best method to determine a successful outcome from any treatment, but only after the horse has regained fitness

Possible Complications!!navigator!!

  • Most common is failure to achieve primary goal
  • Increasing the risk of aspiration or dysphagia is the second most common complication

Expected Course and Prognosis!!navigator!!

Depending on the specific cause of collapse, the prognosis ranges from guarded to good.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Abbreviations!!navigator!!

DDSP = dorsal displacement of the soft palate

Suggested Reading ⬆ ⬇

Davidson EJ, Martin BB, Boston RC, Parente EJ. Exercising upper respiratory videoendoscopic evaluation of 100 nonracing performance horses with abnormal respiratory noise and/or poor performance. Equine Vet J 2011;43(1):3–8.

Garrett KS, Woodie JB, Embertson RM. Association of treadmill upper airway endoscopic evaluation with results of ultrasonography and resting upper airway endoscopic evaluation. Equine Vet J 2011;43(3):365–371.

Leutton JL, Lumsden JM. Dynamic respiratory endoscopic findings pre- and post laryngoplasty in Thoroughbred racehorses. Equine Vet J 2015;47(5):531–536.

Van Erck E. Dynamic respiratory videoendoscopy in ridden sport horses: effect of head flexion, riding and airway inflammation in 129 cases. Equine Vet J Suppl 2011;40:18–24.

Author(s) ⬆

Author: Eric J. Parente

Consulting Editors: Mathilde Leclère and Daniel Jean