Brachycephaly in Dogs: What a Flat Face Does to the Body

Brachycephaly is a shortening of the facial skeleton. The soft tissues of the upper airway – nostril wings, the lining of the nasal turbinates, soft palate, tonsils and even the tongue – do not scale down with it. The result is a relative excess of tissue that narrows the nose, pharynx and larynx: brachycephalic obstructive airway syndrome (BOAS).

The primary changes are stenotic nares, where the nostril wing narrows the opening, sometimes to a slit; an elongated, thickened soft palate that extends past the tip of the epiglottis and blocks the entrance to the larynx on inspiration; aberrant turbinates that can grow back into the nasopharynx; and relative macroglossia. Some dogs also have a hypoplastic, abnormally narrow trachea, which worsens signs but is not thought to be a major cause.

Because the dog has to breathe in against high resistance, strong negative pressure builds up. It pulls tissue into the airway: the laryngeal saccules evert, the tonsils are pulled out of their crypts, and the laryngeal cartilages can collapse; laryngeal collapse has been described in puppies under six months old. Chronic negative pressure in the chest is also thought to be a major cause of reflux, regurgitation and vomiting. During sleep, snoring and pauses in breathing (sleep apnoea) are common, and severe obstruction can cause fainting.

The consequences go beyond breathing. Shallow eye sockets make the eyeballs protrude (exophthalmos) and the lids often fail to close fully, which predisposes to corneal ulcers, exposure damage to the cornea and, in extreme cases, the eyeball slipping out of its socket. Hairs from the nasal fold rub on the cornea, and the skin inside the folds becomes inflamed. A normal number of teeth is packed into a shortened jaw, so crowding and malocclusion are common. And because the puppies' heads are too large for the birth canal, difficult births are frequent.

Genetically, brachycephaly is polygenic. A variant in BMP3 and a LINE-1 insertion in SMOC2 are linked to short faces; the SMOC2 insertion explains up to 36% of the variation in face length. BOAS itself is probably polygenic too. That is why breeding programmes test breathing function directly rather than relying on a single gene.

By open nostrils, quiet breathing at rest and at play – and by a breathing test at the vet. Looks alone are not enough: dogs with a slightly longer nose can still have BOAS, and how well a dog breathes only shows under exertion.

The key test is the Respiratory Function Grading (RFG) scheme run by the University of Cambridge and the UK Kennel Club. A trained vet listens to the breathing at rest, has the dog move at a brisk pace for three minutes, then listens again. The result is a grade from 0 (no signs) to 3 (severely affected). The scheme is used in several countries; in Denmark, this kind of BOAS grading has been mandatory before breeding English and French Bulldogs and Pugs since July 2025.

We do not list breeders for Pugs, French Bulldogs or English Bulldogs, nor for King Charles Spaniels, Cavalier King Charles Spaniels or the bully lines Frenchton, Pocket Bully and Exotic Bully. If you already share your life with one, these points are your yardstick at the vet.

It is the right direction, but not a guarantee. Muzzle length and BOAS are clearly linked: in two sub-studies (Packer et al. 2015) of 854 dogs combined, BOAS only occurred when the muzzle was shorter than half the length of the skull. Of dogs whose muzzle measured less than a tenth of the skull length, over 80% were affected.

Within a breed, however, muzzle length is a weak guide. In Pugs, French Bulldogs and Bulldogs, 46% of dogs in the quarter with the longest muzzles still had BOAS. The nostrils told you more: 25% of dogs with open or mildly narrowed nostrils were affected, against 70% of those with moderately or severely narrowed ones.

Breeding projects such as the Retro Pug, which brings in other breeds, or the Continental Bulldog deliberately aim for longer muzzles. The Continental Bulldog's FCI standard asks for a muzzle of about a third of the head length and treats breathing noises as a disqualifying fault. That lowers the risk, but it does not replace the breathing test. For any flat-faced dog, whatever it is called, only the breathing test shows how well it breathes.

Heat is the biggest everyday danger for flat-faced dogs. Dogs lose heat mainly by panting and through the moist lining of the nasal turbinates; when the nose is narrowed, that cooling largely fails, and panting makes the obstruction worse. These dogs overheat quickly, and heatstroke can be fatal.

In practice that means walks in the early morning and late evening, short shady rounds in summer, no hard play in the midday heat, never leaving the dog in a warm car, and keeping excitement in check. A lean dog breathes more easily, because excess weight adds to the BOAS risk. If your dog pants heavily, staggers or vomits, get it into the shade at once and call your vet.

Not across the board, but the authoritative expert opinion calls for a breeding ban on the extreme form. In Germany, Section 11b of the Animal Welfare Act prohibits breeding dogs where the offspring can be expected to suffer inherited pain, suffering or harm. The law does not name specific breeds; whether a case counts as extreme breeding is assessed individually, based on the animal's traits. Owning a flat-faced dog is not prohibited.

The law is interpreted with the help of an expert opinion on Section 11b written for the federal agriculture ministry, usually dated 1999. It names brachycephaly explicitly and recommends a breeding ban for animals with “extreme round-headedness, in particular a disproportionate shortening of the facial bones”, plus a functional test in breeding assessments and exclusion from breeding after a second caesarean. The opinion is an interpretation aid, not law. Since 2022, dogs with inherited extreme-breeding traits may also no longer be shown at exhibitions or other events where dogs are judged (Section 10 of the Dog Welfare Ordinance).

Neighbouring countries are more specific. In Austria, since 2025 it has been illegal to import, acquire, broker or pass on animals with extreme-breeding symptoms such as breathing difficulty or visible extreme-breeding traits, with exceptions, for example for animal shelters, and a transitional rule for animals lawfully bred or imported before that date. In the Netherlands, since 2023 short-nosed dogs may only be bred if they meet six official criteria, including open nostrils and normal breathing sounds at rest; a muzzle shorter than 0.3 of the skull length rules out breeding, as does previous BOAS surgery.