Alteration of the TF is a frequent finding in any disorders affecting the lacrimal functional unit, and it is a common cause of ophthalmic examination in the dog.
The most common findings in our study were MGD (70%), followed by macroblepharon (16.7%) and eye exposure keratitis (12.7%), also considered potential causes of altered TF distribution.
Several techniques have been used to study the meibomian gland function. Slit-lamp biomicroscopy allows the assessment of the meibomian glands openings and may provide morphological evaluation of the glands in non-pigmented eyelid; however, it is limited as an indirect measure of meibomian gland structure and function . Meibometry is a technique developed to measure basal meibum levels at the eyelid margin , and it has been used in dogs to quantify meibomian lipid secretion [14, 15]. However, a large range of meibometry values has been reported with low repeatability, and, thus, it is no longer considered clinically relevant in veterinary medicine . Meibography is an in vivo technique that allows the visualization of meibomian glands morphology, including the glandular ducts and acini, and it may be performed by contact and non-contact techniques. The contact technique consists of the transillumination of the everted lid over a source of light allowing the visualization of the meibomian glands from the conjunctival surface of the eyelid . In non-contact infrared meibography, the eyelids are everted and the meibomian glands observed by means of infrared light with no contact to the instrument . In the charts herein reviewed, non-contact meibography had been performed on all dogs, and thus, prevalence of MGD among OSD cases could be accurately estimated.
Meibography had been performed on the upper eyelid of the examined eyes. Human studies on MGD demonstrated no significant differences on examining either the upper or lower eyelid for MGD diagnosis, since when MGD was present, there was a good correlation between upper and lower meibomian gland loss . Further studies are currently being conducted to find out if dogs with MGD are more frequently affected on the upper or lower eyelid.
The risk of developing MGD in humans increases with age, androgen deficiency and several other factors, including ophthalmic risk factors like dry eye and blepharitis [18–20].However, according to a recent human study, gender has not shown any association with MGD . In the caseload of dogs presented herein, not only age but also gender was significantly associated to MGD. Neutered dogs were not separated from intact animals, and thus, a study separating both groups of animals is being conducted to investigate the effect of neutering on MGD.
Interferometry allows visualization of the kinetics of the oily layer of the TF that can be partially influenced by its composition, not only by its thickness [21–24]. LL composition, probably more than LL thickness, is highly correlated with the TF thinning rate caused by evaporation [25–27]. The current study shows that the presence of MGD is significantly associated with lower grade 0 of interferometry, while only in grade 4 no eyes were affected by MGD.
Brachycephalic dogs are reported to be prone to OSD ; however, in our study population no association between MGD and skull conformation was observed. Meibomian glands abnormalities in the brachicephalic breed Shih Tzu are more frequent in dogs with keratoconjunctivitis sicca than in control dogs ; thus, the association between skull conformation and OSD might include pathologies other than MGD.
In the presence of MGD, no association was observed between tear production (measured by STT–1) and grades of interferometry. A compensatory system has been described in humans by which, the reduced oily layer of the TF, in case of MGD, is compensated by an increased secretion of aqueous component . In the current study, we observed patients with MGD and a thin LL that can present a STT–1 > 15mm/min, which could be explained by the above reported compensatory process. However, we also found eyes affected by MGD with thin TF-LL (grade 0 to 1) but low STT–1 (<10 mm/min). Excessive thinning of the aqueous phase increases lipid contamination of the mucus layer present over the corneal surface epithelium, rendering it hydrophobic and less able to retain a stable TF , thus, in severe cases, the compensatory mechanism might not be sufficient. Otherwise, as described in humans , we observed patients not affected by MGD and with low STT–1 that can present a thick LL (61 - 100 nm), in this case a thicker LL could initially compensate the decreased function of the aqueous layer.
The balance of TF components is important for TF stability, and a compensatory system is thought to operate in response to changes in these components . Further investigations, involving all components, are required to find out the mechanism of the TF compensatory system.