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A side view of a French Bulldog showing the facial conformation associated with BOAS.
Diagnostics2026-07-01 · 16 min read

BOAS in Dogs: Brachycephalic Airway Syndrome Components, Grading, and Surgery

BOAS is the upper airway syndrome of flat-faced dogs. Learn the 5 primary anatomical components, the Cambridge grading scale, breed risk data, and surgical solutions.

Ran Chen
Ran Chen
Founder, VetMedGuide. Life-sciences operator and 10× global market-access lead.
Published

The rise of flat-faced (brachycephalic) dogs—especially French Bulldogs, Pugs, and English Bulldogs—has been one of the most significant trends in companion animal ownership over the past two decades. With their large, expressive eyes, round heads, and human-like facial expressions, these breeds are beloved by millions.

However, this visual appeal comes at a severe pathological cost. The flat face is the result of extreme skeletal shortening of the skull, which compresses the soft tissues of the upper airway and leads to Brachycephalic Obstructive Airway Syndrome (BOAS).

To many owners, the snoring, snorting, grunting, and wheezing of their pet is viewed as a normal or "cute" breed trait. To veterinary professionals, however, these noises represent chronic respiratory distress. BOAS is not a single disease, but a progressive, multi-systemic syndrome that causes chronic hypoxia, hyperthermia, gastrointestinal reflux, and, in severe cases, sudden asphyxiation.

This guide provides a comprehensive clinical reference for BOAS. We will detail the five primary anatomical components and secondary airway changes, explain the gold-standard Cambridge Respiratory Function Grading Scheme (RFGS), analyze the breed-specific risk data, explore the complex relationship between respiratory and gastrointestinal pathology, and discuss surgical treatments, including the use of CO2 surgical lasers.

This guide will help you understand the severity of airway obstruction and make informed choices about surgical intervention.

What anatomical components make up BOAS in dogs?

To understand BOAS, we must understand the mismatch between skeletal structure and soft tissue volume. In a brachycephalic dog, the bones of the skull are shortened (specifically the maxilla and nasal bones), but the soft tissues inside—such as the tongue, soft palate, and nasal turbinates—develop to a normal size. These tissues are forced to crowd into a severely reduced space, creating physical blockages at every stage of the respiratory tract.

BOAS is divided into primary components (inherited developmental defects) and secondary changes (acquired complications caused by chronic high-pressure breathing).

[Air Intake: Stenotic Nares]
      │
      ▼
[Nasal Cavity: Aberrant Nasal Turbinates]
      │
      ▼
[Throat: Elongated Soft Palate & Macroglossia]
      │
      ▼
[Larynx: Everted Saccules & Laryngeal Collapse (Secondary)]
      │
      ▼
[Windpipe: Hypoplastic Trachea]

The 5 Primary Components

1. Stenotic Nares

The nostrils are abnormally narrow or collapsed inward. This restriction at the entrance of the respiratory tract forces the dog to exert significant effort to draw air into the nose. Stenotic nares act like a crimped straw, dramatically increasing airway resistance.

Physiologically, the nasal cartilage (ala nasi) lacks structural rigidity and collapses medially during inhalation, shutting down the nasal valve.

2. Elongated and Hyperplastic Soft Palate

The soft palate is the muscular flap separating the oral cavity from the nasal passage. In brachycephalic dogs, the soft palate is too long, extending backward into the larynx. It is also abnormally thick (hyperplastic). During inhalation, this excess tissue is pulled into the glottis, blocking the entrance to the trachea and causing the classic snoring sound (stertor).

The constant mechanical trauma of the palate vibrating against the epiglottis causes chronic inflammation, edema, and further thickening, creating a vicious cycle of obstruction.

3. Aberrant Nasal Turbinates

Nasal turbinates are scroll-like bones inside the nose lined with mucosa that warm, humidify, and filter air. Because the nasal cavity is shortened, these bones grow abnormally. They can extend backward into the nasopharynx (nasopharyngeal turbinates) or crowd the nasal passages, blocking airflow through the nose.

In severe cases, the turbinates are packed so tightly that air cannot contact the mucous membranes, impairing the dog's ability to regulate its body temperature through panting.

4. Relative Macroglossia

The tongue develops to a normal volume, but must fit within a shortened jaw. This creates a tongue that is disproportionately large (macroglossia) relative to the oral cavity.

When the dog relaxes or sleeps, the base of the tongue falls backward into the pharynx, contributing to obstructive sleep apnea.

5. Hypoplastic Trachea

The windpipe (trachea) has an abnormally narrow diameter. The cartilage rings that support the trachea may also overlap or fail to form completely, reducing the lumen to a fraction of its normal size.

A hypoplastic trachea is common in English Bulldogs, increasing airway resistance throughout the entire respiratory system and placing a severe limit on exercise capacity.

Secondary Airway Changes

When a dog has primary airway blockages, they must generate high negative pressure within their chest and throat to pull air past the obstructions. This suction acts like a vacuum on the delicate tissues of the larynx, leading to progressive inflammation, mucosal hyperplasia, and tissue collapse over time:

  • Everted Laryngeal Saccules: The laryngeal saccules are small pockets of mucosa located just in front of the vocal cords. The chronic suction pressure pulls these pockets inside-out into the airway, where they appear as small, fluid-filled white domes that further block air intake.
  • Tonsillar Eversion: The chronic negative pressure pulls the palatine tonsils out of their crypts into the pharynx, adding another layer of physical obstruction.
  • Laryngeal Collapse: The constant high-pressure breathing weakens the cartilage structures supporting the larynx. This is graded in three stages:
    1. Stage I: Eversion of the laryngeal saccules only.
    2. Stage II: Collapse of the cuneiform processes of the arytenoid cartilages, which roll inward and touch.
    3. Stage III: Collapse of the corniculate processes of the arytenoid cartilages, completely closing the airway dorsal to the vocal folds.

Stage III laryngeal collapse is irreversible and carries a very poor prognosis, often requiring a permanent tracheostomy.

Understanding this progression explains why early surgical intervention is critical. Correcting stenotic nares and an elongated soft palate in a young dog removes the negative pressure, preventing the development of everted saccules and laryngeal collapse.

A clinical feature of BOAS that is often overlooked is the high rate of gastrointestinal (GI) disease. Studies show that up to 70% to 80% of dogs with BOAS exhibit clinical GI signs, such as regurgitation, vomiting, gagging, and hypersalivation. This link is directly caused by the abnormal pressure dynamics of the respiratory system.

When a BOAS dog struggles to breathe, the intense inspiratory effort generates massive negative pressure within the thoracic cavity. This suction pressure:

  1. Pulls the stomach forward, leading to a sliding hiatal hernia.
  2. Weakens the lower esophageal sphincter, causing gastroesophageal reflux (GERD).
  3. Leads to chronic inflammation of the esophagus (esophagitis), stomach (gastritis), and duodenum (duodenitis).

This creates a clinical loop: airway obstruction causes gastrointestinal reflux, which causes esophageal pain and swelling, which worsens the dog's panting and respiratory distress.

During surgical workups, managing GI signs with gastroprotectants (like omeprazole) and prokinetics (like metoclopramide) is standard practice to reduce the risk of aspiration pneumonia.

How is BOAS graded using the Cambridge Respiratory Function Grading Scheme?

Historically, diagnosing BOAS was subjective, relying on an owner's report of snoring. In 2012, the Cambridge BOAS Research Group developed the Respiratory Function Grading Scheme (RFGS), which is now the internationally recognized gold standard for assessing airway obstruction.

The RFGS grades dogs from Grade 0 to Grade III based on a physical exam, a detailed stethoscope examination of the throat, and a standardized exercise test.

The Standardized Exercise Test Protocol

A dog's respiratory noise and effort can appear normal at rest, but deteriorate under exercise. The RFGS protocol involves:

  1. Baseline Assessment: The veterinarian evaluates the dog's breathing, nasal flare, and throat sounds at rest in a temperature-controlled room.
  2. Exercise Test: The dog is walked or trotted at a steady pace (typically a 5-minute walk or a 3-minute trot at approximately 4–5 km/h) to simulate mild physical activity.
  3. Post-Exercise Assessment: The veterinarian re-evaluates the dog immediately after exercise, recording recovery time, respiratory noise, and abdominal effort.

Studies by Liu et al. validated the necessity of this exercise test:

  • Clinical exam sensitivity for BOAS is 56.7% before exercise.
  • Sensitivity rises to 70.0% after a 5-minute walk.
  • Sensitivity reaches 93.3% after a 3-minute trot test.

This shows that assessing a brachycephalic dog at rest misses nearly half of all clinically affected animals.

The RFGS Grading Definitions

Grade Clinical Description Respiratory Noise Abdominal Effort Clinical Status
Grade 0 Clinically Unaffected No audible noise at rest or post-exercise. None. Normal chest expansion. No treatment needed.
Grade I Mildly Affected Noise (stertor/stridor) is heard only with a stethoscope. Minimal. Mild nasal flaring. Clinically stable; monitor annually.
Grade II Moderately Affected Noise is audible without a stethoscope, worsening post-exercise. Moderate abdominal effort, prolonged inhalation. Surgical Candidate. Medical/weight management required.
Grade III Severely Affected Loud, constant noise at rest, severe dyspnea, and gasping. Marked abdominal effort, open-mouth breathing, cyanosis. Urgent Surgical Candidate. High risk of life-threatening crisis.

Differentiating Stertor vs. Stridor

During auscultation, veterinarians distinguish between two sounds:

  • Stertor: A low-pitched, snoring or snorting sound produced in the upper pharynx (usually caused by an elongated soft palate or crowded nasal passages).
  • Stridor: A high-pitched, whistling or squeaking sound produced in the larynx or trachea (usually indicating everted saccules, laryngeal collapse, or a severely hypoplastic trachea).

Stridor is a warning sign of laryngeal dysfunction and indicates a higher risk of acute airway crisis.

Breed Predispositions and Conformational Risks

Not all brachycephalic breeds carry the same risk. The Cambridge BOAS Research Group and the Royal Veterinary College (RVC) VetCompass program have quantified these differences across thousands of dogs.

The "Worst-Off" Breeds vs. The Shih Tzu Exception

Research shows that the French Bulldog, Pug, and English Bulldog have the highest rates of BOAS, with 50% to 70%+ of their populations showing clinical signs (RFGS Grade II or III).

By contrast, the Shih Tzu is remarkably healthy:

Conformational Feature French Bulldog Pug English Bulldog Shih Tzu
BOAS Prevalence ~50% – 70% ~50% – 65% ~50% – 60% 0.54%
Stenotic Nares Rate 75.4% 65.3% 44.2% 0.58%
Median Lifespan ~4.5 to 8.0 years 7.7 years 7.4 years 12.7 years

This data shows that while the Shih Tzu has a flat face, they are largely spared from the airway obstruction that affects French Bulldogs and Pugs.

Owners and veterinary teams can read our detailed data profile on Shih Tzu health problems to understand this breed's robust health and long lifespan.

Key Conformational Risk Factors

Studies by Liu et al. (PMC5538678) identified the physical features that predict BOAS:

  1. Nostril Stenosis: The severity of nostril narrowing is the single strongest conformational predictor of BOAS. In the Liu et al. conformational study, dogs with moderate to severe stenotic nares had meaningfully higher BOAS index scores than dogs with open nostrils — and Packer et al. (2015) found that the brachycephalic conformation itself confers an odds ratio of roughly 38 for BOAS.

  2. Body Condition Score (BCS): Excess body fat surrounds the throat and airway, compressing the neck tissues. Overweight dogs have a significantly higher risk of airway obstruction.

    Practices can review our protocol for managing obesity in dogs to structure effective weight-loss programs.

  3. Craniofacial Ratio (CFR): CFR measures muzzle length relative to skull length. The shorter the muzzle (lower CFR), the higher the risk of BOAS. French Bulldogs and Pugs have CFRs close to 0.05, while Shih Tzus retain a slightly longer muzzle (CFR ~0.15), which helps protect their airway.

  4. Neck Girth: A thick neck circumference relative to chest size is a strong predictor of BOAS, particularly in male French Bulldogs.

What does BOAS surgery involve, and how much does it cost?

Corrective airway surgery aims to remove the physical obstructions to airflow. The standard surgical package combines three main procedures.

The Surgical Procedures

1. Staphylectomy (Soft Palate Resection)

The surgeon trims the excess, elongated soft palate back to a normal anatomical boundary (usually the tip of the tonsils). This prevents the palate from being sucked into the larynx during breathing. Techniques include traditional cut-and-suture, thermal ablation, or a folding flap palatoplasty, which thins the palate in addition to shortening it.

2. Rhinoplasty (Stenotic Nares Correction)

The surgeon removes a wedge of cartilage from the outer edge of each nostril (the ala nasi). This opens the airway entrance, reducing resistance at the nostrils. Common techniques include vertical wedge resection, horizontal wedge resection, or a punch resection using a skin biopsy punch.

3. Everted Sacculectomy (Laryngeal Saccule Excision)

If the mucosal pockets in front of the vocal cords have everted, they are trimmed out at the base using cup forceps or surgical scissors to open the entrance to the trachea.

The Role of CO2 Laser Surgery

Airway surgery involves vascular tissues, and swelling (edema) in the throat after surgery can block the airway, requiring a temporary tracheostomy. To minimize this risk, many surgeons perform staphylectomies and rhinoplasties using a CO2 surgical laser.

A CO2 laser cuts tissue while simultaneously cauterizing blood vessels and sealing nerve endings. This provides:

  • Minimal bleeding, giving the surgeon a clear view of the airway.
  • Reduced post-operative swelling, lowering the risk of airway obstruction.
  • Reduced post-operative pain, allowing for a smoother recovery.

For a review of laser surgical technology and its integration into practice, see our guide on CO2 laser surgery in veterinary practice.

Anesthetic Considerations for Brachycephalic Airway Surgery

Anesthetizing a brachycephalic dog is one of the highest-risk procedures in small animal practice. The primary risk occurs during induction and recovery, when the protective airway reflexes are lost.

  • Pre-oxygenation: Mandatory for a minimum of 5 minutes prior to induction to maximize oxygen reserves.
  • Rapid Induction: Using fast-acting induction agents (like propofol or alaxalone) to allow rapid intubation. A smaller endotracheal tube than expected must always be prepared, as tracheal hypoplasia is common.
  • Intensive Intraoperative Monitoring: Because of the high risk of hypoxemia and hypercapnia, capnography, pulse oximetry, and arterial blood pressure are tracked continuously. In addition to a standard lingual pulse oximeter probe, alternative probe locations (such as the vulva, prepuce, or digital webbing) must be established in case the tongue is compromised.
  • Recovery Monitoring: The dog is positioned in sternal recumbency with the head elevated. The endotracheal tube is left in place until the dog is fully awake, swallowing, and actively attempting to spit the tube out.
  • Emergency Preparedness: An emergency intubation kit, suction, and a temporary tracheostomy kit must be immediately available at the recovery table.

Surgical Costs and Financial Planning

Corrective airway surgery is a specialized procedure that requires close monitoring during recovery. Below is an overview of the typical costs involved (estimated for US veterinary clinics in 2026).

Care Phase Item Description Typical Cost Range (US 2026)
Pre-op Assessment Exam, RFGS grading, chest X-rays, bloodwork $400 – $800
Airway Surgery Laser staphylectomy, nares correction, ± sacculectomy $2,500 – $4,500
Anesthesia & Monitoring Specialized monitoring, recovery nurse $600 – $1,200
Post-op ICU Stay 24-hour oxygen monitoring, pain management $800 – $1,800
Total Estimated Cost $4,300 – $8,300

Given these costs, a complete BOAS surgical package is a significant expense. Because BOAS is a congenital condition, some pet insurance providers exclude it or require a waiting period.

Owners should review how policy logic applies to breed-specific conditions in our guide to the best pet insurance for dogs.

Post-Operative Management and Recovery Protocols

The immediate post-operative period is the most critical phase of BOAS management. As the dog wakes from anesthesia, the throat tissue can swell, blocking the airway.

Key Recovery Protocols

  • Delayed Extubation: The endotracheal tube is left in place as long as possible, until the dog is fully awake and swallowing, to protect the airway.
  • Steroid Administration: A fast-acting corticosteroid (such as dexamethasone) is administered pre-operatively or immediately post-operatively to minimize tissue swelling.
  • Stress and Temperature Control: Excitement and stress increase breathing rate, which causes swelling. Recovering dogs are kept in quiet, temperature-controlled ICUs. Sedatives (such as acepromazine or dexmedetomidine) are used to keep them calm.
  • Monitoring for Dyspnea: If a dog struggles to breathe after extubation, they may require re-intubation or an emergency temporary tracheostomy to bypass the swollen throat.

Once past the first 24 to 48 hours, recovery is typically straightforward.

Dogs are sent home with pain medication, anti-inflammatories, gastroprotectants, and strict instructions to use a harness instead of a neck collar for life to prevent pressure on the trachea.

Frequently Asked Questions

What does BOAS stand for in dogs? BOAS stands for Brachycephalic Obstructive Airway Syndrome. It is a set of upper-airway abnormalities caused by the shortened skull structure of flat-faced dog breeds.

Do all French Bulldogs need BOAS surgery? No. While French Bulldogs have a high rate of airway obstruction (50% to 70%+), each dog must be graded individually. Dogs with Grade 0 or Grade I BOAS under the Cambridge Respiratory Function Grading Scheme (RFGS) do not require surgery. Those with Grade II or Grade III BOAS should undergo surgery to improve their quality of life and prevent progressive laryngeal collapse.

Can BOAS be managed without surgery? Mild cases (Grade I) and some moderate cases (Grade II) can be managed conservatively through strict weight control, avoiding heat and humidity, and using a harness instead of a collar. However, medical management does not correct the physical airway blockages. Surgery remains the only treatment that opens the airway.

Is BOAS surgery safe? Airway surgery carries a higher anesthetic risk than routine procedures due to the risk of post-operative airway swelling. However, when performed by an experienced surgeon using appropriate monitoring and swelling controls (such as a CO2 laser), the prognosis is excellent, and the surgery provides a major improvement in quality of life.

Do Shih Tzus suffer from BOAS? Very rarely. Although Shih Tzus are brachycephalic, their facial structure is more moderate than that of Pugs or French Bulldogs. Large-scale VetCompass data shows that BOAS affects only 0.54% of Shih Tzus, making them one of the healthiest flat-faced breeds.

Sources

Topics
Diagnostic ImagingAnesthesia & Analgesia
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