VN August 2023

Vetnuus | August 2023 36 We are all aware of the well-recognised disorders associated with the respiratory system in brachiocephalic dogs (brachycephalic obstructive airway syndrome, i.e. , BOAS)]. Not to be outdone by our medicine and surgical colleagues, veterinary ophthalmology has recently accepted the terminology of Brachiocephalic Ocular Syndrome [BOS] . This syndrome refers to a cluster of eye abnormalities commonly found in brachycephalic breeds due to their distinct facial conformation. The defining features of these breeds, such as their shortened muzzles, wide heads, and prominent eyes, contribute to the development of various eye issues. The Brachycephalic dog breeds, such as Bulldogs, Pugs, Pekingese, Shih Tzu and French Bulldogs, have a surprisingly high incidence of periocular and ocular disease associated with brachycephaly. These eye-related problems can significantly impact the well-being and quality of life of affected dogs. Brachycephaly has long been thought to be a semi-dominant trait, its exact genetic cause is unknown, and several candidate genes and mutations have been proposed. In this article, we will delve into the details of Brachycephalic Ocular Syndrome, its causes, symptoms, and management. BOS concentrates on three main factors. These include: 1. Anatomical abnormalities [Skull, eyelids, hairs and cilia, corneal innervation, limbal stem cells] 2. Physiological abnormalities [Blinking, tear film, ageing ocular surface] 3. Ocular surface disease [Dry eye, ulcerative keratitis, pigmentary keratitis, prolapsed gland of the third eyelid, proptosis] Anatomical abnormalities: The brachycephalic heads generally have narrow orbits, which predispose to abnormally protruding eyes (i.e., exophthalmos) that appear larger despite relatively little variation in globe size between dog breeds. Frequently this is confused with buphthalmia from glaucoma. Eyes in these breeds tend to have an outward deviation of the visual axis (i.e., lateral strabismus, or exotropia) that further exposes the medial aspect of the globe. This could potentially be a reason for secondary medial pigmentary keratitis developing. The short muzzle of brachycephalic dogs is often accompanied by excessive skin (i.e.nasal folds) from which hair can rub against the prominent eyes and cause irritation. This often is the problem in Pekingese and Shih Tzu breeds. The head conformation also affects the lacrimal duct system of these dogs. Research has shown that brachycephalics have shorter nasolacrimal ducts and longer lacrimal canals than normocephalic dogs. The duct has to follow a steeper path before turning rostrally. This does not seem to affect them as much as brachycephalic cats. In dogs showing the presence of a nasal fold and macropalpebral fissure, the latter tripled the risk of corneal ulceration when there was a 10% increase in relative palpebral aperture. Concerning the palpebral fissure [ie the opening between the upper and lower eyelids], brachycephalics usually have a large opening referred to as macroblepharon and this, together with the conformational factors mentioned above often result in poor eyelid closure and thus lubrication and protection of the eyelid surface. A 10% increase in relative eyelid aperture width more than triples the risk of corneal ulcers. This problem also increases the risk of entropion. Probably the most common form of entropion is the medial lower lid entropion where one may get a tight medial canthus ligament. Often a serous ocular discharge is seen from irritation and a medial canthus / facial fold dermatitis may be noted. Regulars I Ophthalmology Column Brachycephalic Ocular Syndrome (BOS) Dr Antony Goodhead, Specialist Veterinary Ophthalmologist, Johannesburg and Cape Animal Eye Hospitals, www.animaleyehospital.co.za