Shih-Tzu dogs show alterations in ocular surface homeostasis despite adequate aqueous tear production

Shih-Tzu dogs are frequently affected by ocular surface disorders such as corneal ulceration and dry eye disease (DED). The aim of this study was to evaluate ocular surface homeostasis in Shih-Tzu dogs that have adequate aqueous production. Twenty-eight dogs were subjected to eyelid blink counting, Schirmer tear test (STT-1), ophthalmic evaluation, tear film break-up time (TBUT), fluorescein test and Masmali tear ferning (TF) grading scale.

Of the 28 animals evaluated, the median value of incomplete eyelid blinks/min (median = 15.0 blinks/min; Interquartil interval - IQR = 8.7 blinks/min - 19.5 blinks/min) was higher than the complete blinks/min (median = 2.5 blinks/min; IQR = 1.6 blinks/min - 4.3 blinks/min), with statistically significant difference. The Schirmer tear test had a median value of 25.0 mm/min (IQR = 22.7 mm/min - 27.5 mm/min), considered within the normal range for the species. On ophthalmic examination, all dogs had trichiasis of the caruncle and medial lower eyelid entropion. Lagophthalmos was the third most common alteration observed (71.4%; 20/28). The median of TBUT was 4.0 s; (IQR = 3.0 - 6.0 s). All the animals were negative to the fluorescein test and the TFT indicated that the majority of the eyes (51.8%; 29/56) were classified in abnormal grades 3 and 4 according to the Masmali tear ferning (TF) grading scale.

Although the Shith-Tzu dogs had STT-1 values within the normal range for the species there was high prevalence of abnormal TFT grades and low TBUT in all dogs, showing that despite adequate aqueous production, these dogs have poor precorneal tear film quality. In addition, the dogs showed few complete eyelid blinks and ophthalmic alterations, promoting poor tear film diffusion. All these findings, isolated or together, can result in DED.