Dorsal Displacement of the Soft Palate (DDSP) is a performance-limiting upper respiratory tract condition that occurs when the caudal free margin of the soft palate displaces dorsal to the epiglottis.
Overview and Clinical Importance
Dorsal Displacement of the Soft Palate (DDSP) is a performance-limiting upper respiratory tract condition that occurs when the caudal free margin of the soft palate displaces dorsal to the epiglottis. This creates a functional airway obstruction during expiration, resulting in reduced airflow to the lungs and a characteristic gurgling respiratory noise. DDSP is one of the most common causes of poor performance in athletic horses, particularly Thoroughbred and Standardbred racehorses.
Horses are obligate nasal breathers, meaning they breathe exclusively through their nostrils under normal conditions. The soft palate forms an airtight seal with the epiglottis, separating the nasopharynx from the oropharynx. When DDSP occurs, this seal is disrupted, causing the soft palate to act like a parachute during expiration, dramatically reducing airflow and oxygen delivery to working muscles.
High-YieldDDSP creates EXPIRATORY obstruction (not inspiratory). The gurgling noise heard during exercise is caused by vibration of the displaced soft palate during exhalation. This is a key distinction from laryngeal hemiplegia, which causes primarily inspiratory noise.
| Muscle |
Function |
| Palatinus |
Controls caudal soft palate position; innervated by pharyngeal branch of vagus nerve |
| Palatopharyngeus |
Maintains caudal soft palate stability; innervated by pharyngeal branch of vagus nerve |
| Tensor veli palatini |
Tenses rostral soft palate; contributes to palatal stability |
| Levator veli palatini |
Elevates soft palate during swallowing |
Relevant Anatomy
The Soft Palate
The soft palate is a musculomucosal sheet approximately 15 cm (6 inches) long that separates the nasopharynx (dorsal compartment) from the oropharynx (ventral compartment). The caudal free border of the soft palate normally fits snugly around the base of the epiglottis, creating a tight seal that allows the horse to breathe exclusively through its nose.
Key Palatal Muscles
NAVLE TipThe pharyngeal branch of the vagus nerve (CN X) provides motor innervation to the palatinus and palatopharyngeus muscles. Experimental bilateral blockade of this nerve in the guttural pouch consistently produces DDSP, strongly implicating neuromuscular dysfunction in the pathogenesis.
The Laryngohyoid Apparatus
The position of the larynx relative to the soft palate is maintained by the thyrohyoid muscles, which connect the thyroid cartilage to the basihyoid bone. These muscles oppose the caudal pull of the sternothyroideus and sternohyoideus muscles, maintaining the larynx in a rostral and dorsal position that allows the epiglottis to sit firmly on top of the soft palate.
The basihyoid bone position can be assessed ultrasonographically, and horses with a more ventral basihyoid position may be predisposed to DDSP. This anatomical finding forms the basis for the laryngeal tie-forward surgical procedure.
| Category |
Factors and Mechanism |
| Neuromuscular Dysfunction |
Dysfunction of palatinus and palatopharyngeus muscles leading to soft palate instability. Biopsies show chronic denervation, mild atrophy, and moth-eaten fibers. Associated with vagal nerve dysfunction. |
| Inflammatory |
Upper respiratory tract inflammation and retropharyngeal lymphadenopathy may cause neurapraxia of the pharyngeal branch of the vagus nerve due to anatomical proximity. |
| Anatomical |
Epiglottic hypoplasia or flaccidity, caudal laryngeal position, ventral basihyoid position, subepiglottic cysts or masses, granulomas along caudal soft palate border. |
| Secondary |
Post-laryngoplasty (tie-back surgery) horses are at increased risk. Horses with equine protozoal myelitis (EPM) may develop DDSP due to nerve damage. |
Etiology and Pathophysiology
The pathophysiology of DDSP is multifactorial and incompletely understood. Current evidence points to neuromuscular dysfunction as the primary mechanism, though anatomical and inflammatory factors may also contribute.
Proposed Etiologic Factors
Exam Focus: Research by Holcombe et al. demonstrated that bilateral blockade of the pharyngeal branch of the vagus nerve in the guttural pouch caused DDSP in ALL experimental horses within 2-15 minutes. This strongly supports the neuromuscular hypothesis and explains why conditions affecting the guttural pouch or retropharyngeal lymph nodes can predispose to DDSP.
| Finding |
Description |
| Respiratory Noise |
Characteristic GURGLING or vibrating sound, primarily during expiration. Often described as "growling" or "snoring." Distinct from the whistling/roaring of laryngeal hemiplegia. |
| Exercise Intolerance |
Horse "stops" or "chokes down" during intense exercise. Dramatic drop in performance once displacement occurs. May be sudden onset during high-speed work. |
| Swallowing |
Horse attempts to swallow repeatedly during exercise in an attempt to replace the soft palate. Swallowing typically returns palate to normal position temporarily. |
| Open Mouth Breathing |
Some horses exhibit open mouth breathing when displaced as air is directed through the mouth during exhalation. |
| Intermittent Nature |
Condition is almost always INTERMITTENT. Episodes vary in frequency and severity. May only occur during peak exercise. Persistent DDSP at rest is rare and suggests severe dysfunction. |
Clinical Signs and History
Classic Presentation
The hallmark clinical presentation is a horse with poor performance and expiratory respiratory noise during intense exercise. The condition is typically intermittent, occurring during high-speed work when respiratory demands are greatest.
Key Clinical Features
High-YieldThe GURGLING noise of DDSP is primarily EXPIRATORY and low-pitched, while the WHISTLING/ROARING of laryngeal hemiplegia is primarily INSPIRATORY and high-pitched. Both conditions cause exercise intolerance, but the character and timing of the noise helps differentiate them.
| Modality |
Description |
Clinical Utility |
| Static Endoscopy |
Performed at rest with standing sedation or without sedation |
Rules out other pathology; may show persistent displacement or easy displacement with swallowing/nasal occlusion; most horses appear NORMAL at rest |
| Dynamic Endoscopy (Treadmill) |
High-speed treadmill with videoendoscopy |
Visualizes DDSP during exercise; allows controlled conditions; may not replicate natural exercise conditions |
| Overground Endoscopy |
Portable endoscope attached to bridle during ridden exercise |
GOLD STANDARD; evaluates in natural training environment; allows normal head carriage; most accurate for diagnosis |
| Laryngeal Ultrasound |
Measures depth of basihyoid bone |
Increased basihyoid depth associated with DDSP; useful for pre-surgical assessment; helps predict response to tie-forward |
| Lateral Radiographs |
Evaluates laryngohyoid position |
Ventral/caudal larynx position associated with DDSP; useful for pre- and post-operative assessment |
Diagnosis
Diagnosis of DDSP requires visualization of the soft palate displacing dorsal to the epiglottis. Because the condition is typically intermittent and occurs during exercise, dynamic endoscopy is the gold standard for definitive diagnosis.
Diagnostic Modalities
Endoscopic Findings
Normal endoscopy: The triangular epiglottis sits firmly dorsal to the soft palate, visible above the smooth palatal surface. The arytenoid cartilages and vocal folds are visible through the laryngeal opening.
DDSP endoscopy: The epiglottis is obscured by the soft palate, which has displaced dorsally and now sits on top of the epiglottis. The soft palate billows with respiration and vibrates during expiration, creating the characteristic noise.
Palatal instability: Before full DDSP occurs, dynamic endoscopy often shows palatal billowing and increased swallowing as the horse attempts to stabilize the palate. This instability is an early warning sign.
NAVLE TipThere is NO reliable correlation between DDSP observed at rest (during nasal occlusion or induced swallowing) and DDSP occurring during exercise. Many horses displace easily at rest but never displace during exercise, and vice versa. Dynamic endoscopy during actual exercise is ESSENTIAL for definitive diagnosis.
Differential Diagnosis
| Condition |
Noise Character |
Phase |
Key Features |
| DDSP |
Gurgling, vibrating |
Expiratory |
Intermittent; sudden stopping; swallowing helps |
| Laryngeal Hemiplegia |
Whistling, roaring |
Inspiratory |
Left side more common; progressive; arytenoid asymmetry at rest |
| Epiglottic Entrapment |
Variable |
Both |
Visible at rest; epiglottis trapped in aryepiglottic fold |
| Pharyngeal Collapse |
Variable |
Both |
Poor prognosis; nasopharyngeal wall collapse |
| Aryepiglottic Fold Collapse |
Flutter |
Inspiratory |
Only seen on dynamic endoscopy |
Treatment Options
Conservative Management
Conservative measures are typically attempted first, especially when inflammation is suspected or the horse is young and still developing fitness.
NAVLE TipIf any evidence of upper respiratory tract inflammation is present on endoscopy, conservative treatment with rest and anti-inflammatory therapy should be attempted BEFORE surgical intervention. Resolution of inflammation may resolve the DDSP.
Surgical Treatment Options
High-YieldThe LARYNGEAL TIE-FORWARD is currently the GOLD STANDARD surgical treatment for DDSP with the highest success rate (80-85%). It works by repositioning the larynx rostrally and dorsally, reducing the distance between the epiglottis and soft palate edge, making displacement less likely.
Prognosis
Overall prognosis for complete resolution is fair, with success rates ranging from 60-85% depending on the surgical procedure performed. Factors affecting prognosis include:
- Definitive diagnosis by dynamic endoscopy (better outcomes than presumptive diagnosis)
- Absence of concurrent upper airway abnormalities
- Intermittent DDSP has better prognosis than persistent DDSP
- Post-operative laryngeal position (more dorsal = better outcome)
- Concurrent epiglottic entrapment worsens prognosis
DDSP = "D.D.S.P." Mnemonic: "Dorsal Displacement Stops Performance" - Remember that DDSP causes dramatic exercise intolerance and the horse "stops" or "chokes down" when displacement occurs.
Noise Character Mnemonic: "DDSP = Deep, Dull Sound on Pushing air out (Expiration)" versus "LH = Loud High whistle on Inhaling". The GURGLING of DDSP is EXPIRATORY; the ROARING of laryngeal hemiplegia is INSPIRATORY.
Tie-Forward Mnemonic: "TF = Tying Forward = Top Fix" - The tie-forward procedure has the TOP success rate (80-85%) for DDSP because it FIXes the larynx in a more FORWARD (rostral) and dorsal position.
Vagus Nerve Connection: "VaGus = Very Guilty for DDSP" - The pharyngeal branch of the VaGus nerve innervates the palatinus and palatopharyngeus muscles. Dysfunction = DDSP. Think of guttural pouch pathology affecting the vagus.
Diagnosis Rule: "Dynamic Diagnosis for Dynamic Disease" - DDSP is DYNAMIC (intermittent, exercise-induced), so you need DYNAMIC endoscopy (during exercise) to diagnose it. Static endoscopy at rest is often NORMAL.
| Intervention |
Rationale and Application |
| Fitness Improvement |
First-line approach; improved cardiovascular fitness reduces respiratory effort required at submaximal speeds |
| Figure-8 Noseband |
Prevents mouth opening during exercise; maintains negative oropharyngeal pressure |
| Tongue Tie |
Prevents caudal tongue retraction; maintains laryngeal position; common in racing |
| Specialized Bits |
Spoon bit or Z-bit prevents tongue playing and caudal tongue displacement |
| Anti-inflammatory Therapy |
Topical throat sprays, systemic NSAIDs; addresses underlying inflammation; rest if URI present |
| Cornell Collar |
Laryngohyoid support device designed to elevate and stabilize larynx position |
| Procedure |
Description |
Success Rate |
Anesthesia |
| Laryngeal Tie-Forward |
Prosthetic sutures advance larynx rostrally/dorsally by fixing thyroid cartilage to basihyoid bone; replaces thyrohyoid muscle function |
80-85% |
General (can be standing) |
| Sternothyrohyoideus Myectomy |
Removes 4-inch portion of muscle on ventral neck; prevents caudal laryngeal retraction |
60-70% |
Standing sedation |
| Staphylectomy |
Partial resection of caudal free margin of soft palate; reduces tissue bulk; induces fibrosis for stability |
60% |
General |
| Soft Palate Thermoplasty |
Laser or cautery creates fibrosis in soft palate to stiffen; may be oral or nasal approach |
36-53% |
Standing or General |
| Combined Procedures |
Myectomy + staphylectomy + thermoplasty; addresses multiple proposed mechanisms |
63-67% |
General |