With their bulging eyes and cute looks, flat-faced dogs have become fashionable.
But breeds such as English and French bulldogs, pugs and boxers are going blind, with many not able to close their eyes, vets have warned.
They said over-breeding is leading to changes in skull shapes, making eyes protrude and leaving the surface too exposed.
Researchers said this is causing ulcers and erosion of the eyeball, leading to vision loss. Many dogs have a condition called macroblepharon, which is an abnormally large opening of the eyelids. This, combined with the way their eyes stick out, often makes them impossible to close.
Breeds such as English and French bulldogs, pugs and boxers are going blind, with many not able to close their eyes, vets have warned
It is also common for the breeds to suffer breathing issues. The vets' study examined flat-faced breeds aged up to 16, also including shih-tzus, Pekingese and Boston terriers. Of 93 dogs examined, nearly half had macroblepharon.
French bulldogs – the UK's second-favourite breed after labradors – were the worst-hit. The study was compiled by vets from Lisbon and Leipzig universities.
'As these breeds grow in popularity, veterinary hospital teams are treating more and more dogs of brachycephalic [flat-faced] breeds with a wide variety of problems caused by breeding for a characteristic short-nosed flat-face,' the vets said.
For their study, they examined a range of flat-faced breeds, ranging in age from just a few months to aged 16.
The breeds, which had been brought to two vet school clinics suffering with eye issues, were French bulldogs, Shih-Tzus, Pugs, English Bulldogs, Boxers, Pekingese and Boston Terriers.
Of the 93 dogs studied, nearly half had macroblepharon. The French bulldogs - which, according to Kennel Club figures are the UK's second favourite dog - were the worst affected by this condition.
They said over-breeding is leading to changes in skull shapes, making eyes protrude and leaving the surface too exposed
Entropion, where the eyelid turns inwards so that the eyelashes rub the eyeball, was found in 20 of the dogs, with pugs particularly afflicted.
Ulcers on the eyeball's surface - the cornea - were found in 41 of the pets, and five had them in both eyes, the vets from the University of Lisbon and University of Leipzig discovered.
Corneal pigmentation affected 33 of the dogs, with pugs suffering the most. Vision loss occurs as the blackish brown pigment progressively clouds the cornea.
Corneal fibrosis, or scarring, affected 23 dogs. Shih Tzus and French Bulldogs were the breeds most likely to have these issues.
'Their personalities, wrinkly faces and appealing large eyes have turned them into popular pets.
'This popularity is thought to exist because humans find the large and round eyes, as well as the round face very appealing.'
Their study, the vets said, has 'highlighted the importance of responsible breeding, early diagnosis and regular ophthalmic check-ups to correctly diagnose, treat and if possible prevent situations of irreversible blindness in these patients'.
As well as eye problems, these breeds commonly suffer from breathing issues - a condition known as Brachycephalic Airway Obstructive Syndrome.