Pharynx: 4th branchial arch defects in Horses (Equis) | Vetlexicon
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Pharynx: 4th branchial arch defects

ISSN 2398-2977

Synonym(s): RDPA, cricopharyngeal-laryngeal dysplasia, palatopharyngeal arch displacement (rostral


  • Cause: a congenital defect causes failure of development of derivatives of 4th branchial arch which make up components of the larynx and pharynx.
  • Results in unilateral or bilateral hypo- or aplasia of the wings of the thyroid cartilage, cricothyroid articulations, cricothyroideus and cricopharyngeus muscles.
  • Signs: vary depending on the severity of the deformity, ranging from respiratory obstruction and dysphagia to poor performance at exercise.
  • Diagnosis: history, signs, endoscopy, radiography.
  • Treatment: none.
  • Prognosis: depends on severity of condition - generally poor for athletic function.

Geographic incidence

  • Worldwide.

Age predisposition

  • Congenital deformity.
  • Severely affected individuals with marked clinical signs may be diagnosed at an early age.
  • Less severely affected individuals may be diagnosed during routine endoscopic examination at yearling sales or detected once training begins or progresses.

Breed/Species predisposition

Cost considerations

  • Economic loss - generally do not make athletes.
  • Cost due secondary colic and inhalational pneumonia which may necessitate euthansia   Euthanasia  .



  • Congenital.


  • Failure of some or all of the derivatives of 4th branchial arch (unilaterally or bilaterally) to develop normally.
  • These structures include wings of the thyroid cartilage, cricothyroid articulations, cricothyroideus and cricopharyngeus muscles.


  • Derivatives of 4th branchial arch are hypo- or aplastic.
  • No stable structure between wing of thyroid and cricoid cartilages   →   failure to provide intrinsic support for the musculature of the larynx   →   defective arytenoid motility.
  • Absence of cricopharyngeal muscle which is part of the rostral esophageal sphincter   →   inability to close upper esophageal sphincter   →   involuntary aerophagia.
  • There is no means by which the palatal pillars can be anchored into a position caudal to the apices of the corniculate cartilages   →   rostral displacement of the pharyngeal arch (RDPA).


Presenting problems

Client history

  • Variable, depending on the severity of the deformity.
  • Aerophagia and belching.
  • Abnormal respiratory noise at exercise.
  • Nasal discharge   Nose: nasal discharge  .
  • Coughing   Coughing  .
  • Recurrent colic   Abdomen: pain - adult  .
  • Aspiration pneumonia.

Clinical signs

  • Thorough laryngeal palpation required.
  • If the cartilage components are defective, can palpate an abnormally wide gap between the caudal margin of the thyroid and rostral edge of the cricoid (in a normal horse these structures should overlap).

Diagnostic investigation

  • Endoscopy at rest.
  • Treadmill endoscopy   Respiratory: endoscopy  may be the only way to confirm diagnosis     in those with mild deformity which only results in exercise intolerance.
  • RDPA may be evident - the caudal pillars of the palatal arch move rostral to the corniculate processes.  
  • The proximal esophagus may also be open.
  • Laryngeal paralysis   Larynx: hemiplegia  may also be present. This may occur un- or bilaterally and is equally prevalent between left and right sides.
  • This condition should be ruled out in any apparent case of right-sided recurrent laryngeal neuropathy.
  • Lateral radiographs of the laryngeal region   Thorax: radiography  may indicate the presence of a column of air extending from the pharynx to the esophagus (if the upper esophageal sphincter is absent).
  • RDPA may be visualized and a dewdrop shape encroaching into this column of air from a dorsal position.

Gross autopsy findings

  • Absence of one or both cricopharyngeal muscles.
  • Short wing(s) on thyroid cartilage.


Initial symptomatic treatment

  • No treatment available to reconstruct hypo- or aplastic structures.
  • RDPA is a feature of the underlying defects and treatment for this alone will not be effective.
  • Laryngeal surgery may be indicated in mild cases with defective arytenoid motility.
  • Permanent tracheostomy   Trachea: tracheostomy - permanent  .




  • Generally are ineffective as athletes.
  • If condition mild, horse may be suitable for pleasure purposes.
  • Tracheostomy   Trachea: tracheostomy - permanent  and laryngeal surgery may be of some value in some cases.
  • If no upper esophageal sphincter is present, affected individuals ay be prone to recurrent colic.

Reasons for treatment failure

  • Common.
  • Inability to reconstruct hypo- or aplastic structures.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Smith L J & Mair T S (2009) Fourth branchial arch defect in a Welsh section A pony mare. Equine Vet J 41 (6), 366-364 VetMedResource.
  • Blickslager A T, Tate L P & Tudor R (1999) Transendoscopic laser treatment of rostral displacement of the palatopharyngeal arch in four horses. J Clin Laser Med Surg 17 (2), 49-52 PubMed.
  • Gaughan E M & DeBowes R M (1993) Congenital diseases of the equine head. Vet Clin North Am Equine Pract (1), 93-110 PubMed.
  • Dixon P M, McGorum B C & Else R W (1993) Cricopharyngeal-laryngeal dysplasia in a horse with sudden clinical onset idiopathic laryngeal hemiparesis. New Zealand Vet J 41(3), 134-138 PubMed.
  • Goulden B E, Anderson L J, Davies A S & Barnes G R G (1976) Rostral displacement of the palatopharyngeal arch: a case report. Equine Vet J (3), 95-98 PubMed.

Other sources of information

  • Lane J G (1993) Fourth Brachial Arch Defects. In: Proc Bain Fallon Memorial Lecture. Canberra, Australia.