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Brachycephalic eyes – why do they get so many ulcers?

Brachycephalic ocular health is precarious and there are multiple factors which result in these breeds being overrepresented with ocular problems

07 December 2020, at 7:40am

Ophthalmic disorders are extremely common in Pugs and other brachycephalic breeds. Indeed, along with obesity and otitis externa, they were the most preva­lent conditions recorded in a recent study on the demogra­phy and health of Pugs in England (O’Neill, 2016). The breed is approximately eight times more likely to develop corneal ulcers compared to dogs without flat faces. In addition, the ulcers they suffer from are often deeper than those in non-brachycephalic breeds (Iwashita et al., 2020) (Figure 1). Table 1 summarises the common causes of corneal ulcers.

TABLE (1) There are many examples of causes of ulceration. Conditions in bold are particularly common in brachycephalic breeds
TABLE (1) There are many examples of causes of ulceration. Conditions in bold are particularly common in brachycephalic breeds

When we look at why these breeds get so many ulcers, there are several factors which make brachycephalic animals predisposed. The first thing to consider is the skull. Compare the general head shape of a Pug (Figure 2) and a German Shepherd Dog (Figure 3) for example – the difference is dramatic to say the least! The orbit is the bony fossa which separates the eye from the cranial cavity. Many bones contribute to it: the frontal, lacrimal, maxillary, zygomatic, pterygoid, palatine and sphenoid. The orbit sur­rounds and protects the globe while providing a route for vessels and nerves to access the eye (via various foramina). In addition to the eye itself, the orbit contains the extraoc­ular and retrobulbar muscles, optic nerve, lacrimal and nictitans glands and part of the zygomatic salivary gland, along with the blood vessels and nerves (cranial nerves II, III, IV, V, VI and VII). Connective tissue and fat are also present. Thus, there is quite a lot of soft tissue to fit into this space. This is where the first brachycephalic problem arises: the orbit in these breeds is much shallower than in doliocephalic breeds – I describe it to owners as being more the shape of a teaspoon than an ice-cream scoop. No wonder the eye is not well protected. The orbital rim should be palpated as part of the ophthalmic examination and the variation in its position will become apparent; with extreme skull shapes, such as the Pug and Pekingese, the orbital rim is markedly posterior to the eyeball itself, so really provides no protection to the globe, and appreciation of this makes it easy to understand how both injuries to the cornea and proptosis can occur so easily in such breeds.

In addition to the shallow orbit, brachycephalic breeds have very large palpebral fissures (macropalpebral fissure or euryblepharon) which contribute to lagophthalmos (incomplete eyelid closure and globe coverage). The large lids and shallow orbit result in marked “scleral show” (ie visible “white of the eye”). This is said to be what makes humans find these breeds so attractive; they look doll- or baby-like, nurturing our desire to care for them. However, it is not necessarily good for the health of the cornea. Ask own­ers to check if the dog closes its eyes fully when it is asleep (many do not and it is something some owners also find “cute – he’s always keeping an eye on us”). During your ophthalmic examination, monitor how many blinks are complete and how many are partial. Poor spreading of the tear film due to inad­equate blinking is a major factor in brachycephalic ulcers.

The “flat face” of brachycephalic dogs further predisposes to corneal ulceration. Think of a Labrador snuffling though the leaves in the park: his eyes are several centimetres cau­dal to his nose, but the Pug or French Bulldog has no such distance to protect and thus can very easily scratch his corneas inadvertently (Figure 4). Likewise, when rubbing the mouth after eating or scratching the ears for example, bystander damage to the globe is a daily hazard. Foreign bodies can easily lodge on or in the cornea (Figure 5) as well, since the eyeball is so prominent.

The next brachycephalic anatomical feature to consider is corneal sensitivity. A seminal paper back in 1991 (Barrett et al., 1991) demonstrated that brachycephalic breeds have the lowest corneal sensitivity compared to doliocephalic and mesaticephalic skull types. They have a smaller number of trigeminal nerve fibres crossing into the cornea from the limbus and these nerves then have fewer branches com­pared to other skull shapes. So, our poor Pug has an exposed globe, does not blink properly but also cannot feel his cornea so well as other breeds. One might think that these “design faults” were enough to contend with, but unfortunately not!

Corneal pigmentation is very common in Pugs – the incidence reported varies from 70.0 to 87.8 percent (Labelle et al., 2013; Krecny et al., 2015; Vallone et al., 2017; Maini et al., 2019). Many brachycephalic breeds, Pugs in particular, have some degree of medial lower lid entropion and the resultant chronic low-grade irritation can cause pigmentary keratitis to form (Vallone et al., 2017) although in the earlier paper by Labelle et al. (2013), there was no association with eyelid conformation and presence of corneal pigment iden­tified. The pigment can be accompanied by visible vascular­isation and also a whorl shape of corneal opacity (Figure 6). The caruncle, which is the small fleshy protrusion just inside the medial canthus, frequently has hairs growing on it, which can also contribute to both the development of pigment on the cornea but also to ulceration – especially in longer-coated breeds such as the Shih Tzu and Pekingese. Other cilia-related abnormalities which can be detrimental to brachycephalic ocular health include distichiasis, ectopic cilia and nasal fold trichiasis.

BOX (1) Clinical signs of keratoconjunctivitis sicca (KCS)
BOX (1) Clinical signs of keratoconjunctivitis sicca (KCS)

No discussion of brachycephalic eye problems would be complete without mentioning keratoconjunctivitis sicca (KCS, dry eye). Box 1 lists the common clinical signs of KCS. Quantitative KCS results from a reduction in the aque­ous portion of the tear film and is diagnosed by performing a Schirmer tear test (STT) (readings less than 15mm/min are classified as KCS). Aqueous tear production has been demonstrated to be lower in brachycephalic dogs compared to non-brachycephalics (Bolzanni et al., 2020) and this is thought to be linked to the reduced corneal sensitivity we have already considered. An incidence of KCS in Pugs of approximately 15 percent has been reported (Krecny et al., 2015) and there was a significant association between KCS and corneal pigmentation in this study. Corneal vasculari­sation is another feature of KCS, as is ulceration, which can be rapidly progressive and severe (Figures 7 to 9). Given the lagophthalmos we have already mentioned, evaporation of tears from the central cornea is a significant problem, and so supplementing with artificial lubricants in dogs with border­line STT readings, and starting lacrimostimulants (eg ciclo­sporin or tacrolimus) sooner rather than later is sensible.

Along with the corneal pigment and medial entropion, there is a large strand of mucus on the medial cornea which is very common in pugs and can be an indication of a qualitative tear film abnormality
Along with the corneal pigment and medial entropion, there is a large strand of mucus on the medial cornea which is very common in pugs and can be an indication of a qualitative tear film abnormality

Qualitative KCS occurs as a result of tear instability due to mucin or lipid abnormalities. STT readings are normal in these patients, and diagnosis is by careful examination of the lid margins and meibomian glands (from where the lipid layer of the tear film is secreted) and the conjunctiva, especially in the ventral fornix, where the goblet cells which produce the mucin are most numerous. Evaluation of the tear break-up time (Table 2) is also essential in the diag­nosis of qualitative KCS. Pugs often have mucus plugs and strands across their corneas, suggestive of an imbalance in tear film composition (Figure 10).

TABLE (2) How to perform a tear break-up time (TBUT) test
TABLE (2) How to perform a tear break-up time (TBUT) test
BOX (2) A summary of factors contributing to ulceration in brachycephalic dogs
BOX (2) A summary of factors contributing to ulceration in brachycephalic dogs

Conclusion

In conclusion, brachycephalic ocular health is precarious and there are multiple factors which result in these breeds being over-represented with ocular problems, especially ulcers (Box 2). These result in the frequent need for medi­cal and surgical treatments (Figures 11 and 12), details of which are beyond the scope of this article. Recognition of the predisposing factors, together with owner education, can allow us to identify ulcers quickly, treat appropriately and hopefully reduce the number of patients who end up losing an eye through corneal rupture.

References
Author Year Title
Barrett, P. M., Scagliotti, R. H., Merideth, R. E., Jackson, P. A. and Alarcon, F. L. 1991 Absolute corneal sensitivity and corneal trigeminal nerve anatomy in normal dogs. Progress in Veterinary and Comparative Ophthalmology, 1, 245-254
Bolzanni, H., Oriá, A. P., Raposo, A. C. S. and Sebbag, L. 2020 Aqueous tear assessment in dogs: Impact of cephalic conformation, inter‐test correlations, and test‐retest repeatability. Veterinary Ophthalmology, 23, 534-543
Iwashita, H., Wakaiki, S., Kazama, Y. and Saito, A. 2020 Breed prevalence of canine ulcerative keratitis according to depth of corneal involvement. Veterinary Ophthalmology, 23, 849-855
Krecny, M., Tichy, A., Rushton, J. and Nell, B. 2015 A retrospective survey of ocular abnormalities in pugs: 130 cases. Journal of Small Animal Practice, 56, 96-102
Labelle, A. L., Dresser, C. B., Hamor, R. E., Allender, M. C. and Disney, J. L. 2013 Characteristics of, prevalence of, and risk factors for corneal pigmentation (pigmentary keratopathy) in Pugs. Journal of the American Veterinary Medical Association, 243, 667–674
Maini, S., Everson, R., Dawson, C., Chang, Y. M., Hartley, C. and Sanchez, R. F. 2019 Pigmentary keratitis in pugs in the United Kingdom: prevalence and associated features. BMC Veterinary Research, 15
O’Neill, D. G., Darwent, E. C., Church, D. B. and Brodbelt, D. C. 2016 Demography and health of Pugs under primary veterinary care in England. Canine Genetics and Epidemiology, 3
O’Neill, D. G., Pegram, C., Crocker, P., Brodbelt, D. C., Church, D. B. and Packer, R. M. A. 2020 Unravelling the health status of brachycephalic dogs in the UK using multivariable analysis. Scientific Reports, 10, 17251
Vallone, L. V., Enders, A. M., Mohammed, H. O. and Ledbetter, E. C. 2017 In vivo confocal microscopy of brachycephalic dogs with and without superficial corneal pigment. Veterinary Ophthalmology, 20, 294-303

Sally Turner, MA, VetMB, DVOphthal, MRCVS, is an RCVS Specialist in veterinary ophthalmology and runs a referral ophthalmology service in London. She does a lot of teaching and has authored two veterinary ophthalmology books. She enjoys both medical and surgical ophthalmology.

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