BOAS: What Flat-Faced Breathing Actually Costs

BOAS: What Flat-Faced Breathing Actually Costs

BOAS stands for brachycephalic obstructive airway syndrome. Brachycephalic breaks into brachy, shortened, and cephalic, head. It describes breeds whose skull bones are shortened in length: the Pug, the French Bulldog, the Persian cat.

The syndrome exists because of a simple mismatch. Selective breeding shortened the bone, but the soft tissue inside did not shrink to match. In brachycephalic dogs the scroll-like ridges inside the nose keep growing even though the midface has stopped, producing relatively oversized structures that press against each other and block airflow. The face got smaller. What has to fit inside it did not.

The Components, Defined

BOAS is not one defect. It is a stack of them, and most affected animals have several.

Stenotic nares. Abnormally narrowed nostrils that restrict how much air can flow through to the lungs. In dogs they may also collapse inward on inhalation. This is the one visible from across a room.

Elongated soft palate. The soft flap at the back of the roof of the mouth is too long for a shortened jaw, so the excess length partially blocks the entrance to the windpipe.

Everted laryngeal saccules. Small pouches of tissue near the vocal cords get pulled inward by the suction of struggling to breathe, where they block airflow further. This one is important to understand as a consequence: it develops from the effort of breathing against an obstruction, which is why early surgery may prevent it.

Hypoplastic trachea. A windpipe with a smaller diameter than normal, proportionally too narrow for the animal. Unlike the other three, there is no surgical fix for it.

Nasopharyngeal turbinates. Bony scroll-like ridges that extend from the nasal passages back into the throat and obstruct airflow there.

Laryngeal collapse. Progressive weakening of the voice-box cartilage. Also a consequence rather than a birth defect, developing from chronic upper airway obstruction, and it carries a less favorable prognosis.

How Common It Actually Is

A study of 604 dogs measured prevalence in the three headline breeds directly.

A separate cross-sectional study of 898 dogs across 14 breeds found the problem reaches well beyond those three. Almost every breed studied had some detectable breathing abnormality. Only the Maltese and Pomeranian had no dogs with clinically significant disease. The worst affected were not the famous ones: only 10.9% of Pekingese and 17.4% of Japanese Chin were Grade 0, meaning roughly nine in ten Pekingese showed some grade of BOAS.

At population scale, a UK study of 905,544 dogs found French Bulldogs had adjusted odds of BOAS of 30.89 compared with all other dogs, and adjusted odds of stenotic nares of 42.14. Those were the breed's top two predisposed disorders out of everything studied.

What Predicts It, and What Owners Can Change

Across those 14 breeds, three factors were significant: nostril stenosis, craniofacial ratio, and body condition score.

Craniofacial ratio and nostril shape are fixed at birth. Body condition is not. That single row is the reason weight control is the first line of management in every source, and it is the one thing an owner controls outright. In Bulldogs specifically, neck girth ratio above 0.71 in males predicted BOAS with roughly 70% sensitivity and specificity.

Worth being honest about the limits: this model accounts for only about 20% of the variation, so conformation is a strong signal rather than a complete explanation.

The Grading Scheme, and Where the Line Sits

The Respiratory Function Grading Scheme was developed by the Royal Kennel Club and the University of Cambridge and is licensed to the OFA. It is four steps: a health survey, a calm exam with stethoscope auscultation, a three-minute brisk exercise test at 4 to 5 mph, then post-exercise listening for comparison.

That is the practical test. Noise you can hear from across the room is Grade II, which is clinically affected territory, not a pug being a pug. As of April 2026 the scheme covers not just Pugs, Bulldogs and French Bulldogs but also the Affenpinscher, Boston Terrier, Boxer, Cavalier King Charles Spaniel, Chihuahua, Dogue de Bordeaux, Brussels Griffon, Japanese Chin, English Toy Spaniel, Pekingese, Pomeranian, Shih Tzu and Staffordshire Bull Terrier.

Cats Are Not Small Dogs Here

Feline BOAS is genuinely different, in four ways worth knowing.

The nostrils narrow for a different reason. In dogs, clinically significant stenotic nares involve inward deviation of the alar fold. In cats the most common cause is redundant skin where the floor of the nostril meets the lip. The canine nostril operation is therefore not automatically the right operation for a cat.

The component list is shorter. In cats, stenotic nares and an elongated soft palate are the common abnormalities. Laryngeal collapse, tracheal hypoplasia and bronchial collapse are all rare compared with dogs.

The breathing pattern differs. Brachycephalic cats show reduced minute ventilation, a difference that studies in dogs did not detect.

The presenting sign is mouth breathing. Most affected cats breathe more easily through the mouth than the nose. This deserves emphasis for cat owners: open-mouth breathing in a cat is abnormal in a way it simply is not in a dog.

Reported prevalence in brachycephalic cats: snoring and sneezing more than once or twice a week in 53 to 55%, versus 14 to 18% in other cats. Activity-associated laboured breathing in 31%, versus 9%.

A CT study of 69 Persian cats against 10 domestic shorthairs found all measured airway areas significantly smaller in Persians, and 39.9% of the Persian eye globe sat outside the bony orbit versus 9.90% in domestic shorthairs, a measure of how shallow the skull has become. Affected cat breeds include the Persian, Himalayan, Burmese and Exotic Shorthair. See our Persian care guide, which covers the doll-face versus extreme-type distinction that matters most here.

Heat, Which Is the Part That Kills

Dogs shed heat mainly by evaporation from the moist surfaces of the nose, mouth and tongue during panting. A compressed skull leaves greatly reduced surface area to evaporate from, while the narrowed airway means more muscular work to move air, generating more heat.

A UK study of 905,543 dogs recorded 395 heat-related illness events, with an overall fatality rate of 14.18%.

Brachycephalic dogs overall had 2.10 times the odds of heat-related illness compared with normal-muzzled dogs, rising to 3.07 times for heat illness in vehicles.

Encouragingly, surgery measurably improves this: mean rectal temperature rise after a standard exercise test was 0.60°C before BOAS surgery and 0.30°C after.

What Surgery Achieves

Stenotic nares are corrected by removing a wedge of tissue from the nostril. An elongated soft palate is shortened. Everted laryngeal saccules are removed.

In dogs reassessed a median of 1,645 days after surgery, the BOAS index fell from 71.6% before surgery to 46.7% shortly after and 49.1% long-term, so the benefit largely held across roughly four and a half years. 17 of 31 owners, 54.8%, felt their dog no longer had breathing issues.

Timing matters: dogs operated on under two years of age have a better prognosis, and early correction may prevent everted saccules developing at all.

For the two breeds where this comes up most on this site, see our French Bulldog guide and the Pug entry in our small breed guide.

Sources & Further Reading