Ophthalmic parameters in adult Pomeranian dogs

The objective of this study was to establish ophthalmic parameters in the Pomeranian dog breed. Schirmer tear test (STT), tear breakup time test (TBUTT), Jones test (JT), aesthesiometry, biomicroscopy, tonometry, non-contact specular microscopy (SM), ophthalmoscopy and ultrasonography (US) were carried out 48 eyes of 24 male and female Pomeranian dogs from a breeding kennel. Descriptive statistics was applied for data analysis. Means and standard errors found for STT, TBUT, JT, esthesiometry and tonometry were 24.310±0.448mm/min, 14.570±0.406s., 8.420±1.571min, 3.667±0.157cm and 17.960±0.697mmHg, respectively. For biomicroscopy, 43.75% of the eyes presented caruncle trichiasis, 25% chromodacryorrhea, 25% medial entropion and 10,42% distichiasis. SM showed endothelial cells density of 1958.285±36.760 cells/mm². For hexagonality, endothelial cells area and central corneal thickness were 62.229±2.183%, 519.231±9.853mm², 0.567±0.006mm, respectively. Ophthalmoscopy presented a pattern described in other breeds. Amelanotic fundus was reported in 8.33% of the eyes. The US means and standard errors for axial length of the eyeball, lens thickness, anterior and vitreous chamber depth were 16.859±0.148mm, 6.651±0.081mm, 2.673±0.091mm and 7.485±0.101mm. Mild changes in the lacrimal evaluation parameters were influenced by eyelid changes in STT type I. Increased corneal sensitivity was observed in the breed.


INTRODUCTION
There is a growing popularity of Pomeranian dogs in the current decade, a factor linked to the characteristics of these animals, such as ease of learning, constant activity, dedication to the owner, indifference to the climate and longevity (Fédération Cynologique Internationale, 2013).The maintenance of breeds invariably causes a reduction in genetic diversity and increases the repeatability of specific racial patterns, which favors the chances of genetic disorders, altering expressed phenotypes and impacting health (Mellersh, 2008).The Fédération Cynologique Internationale disqualifies animals of the breed that have entropion or ectropion.Presence of 58% of hereditary retinal degeneration cases has been reported in the breed (Kelawala et al., 2017).There is no data in the literature standardizing the ophthalmic parameters in the studied breed, as well as there is a scarcity in the description of the main pathological and anatomical alterations that can be found more frequently.Thus, the aim of the study was to establish these parameters in Pomeranian dogs, identify breed characteristics and describe possible common pathologies.

MATERIAL AND METHODS
Upon approval by the Ethics Committee for the Use of Animals (CEUA) (protocol n° 000316/19), 48 eyes were studied in 24 Pomeranian dogs (Canis familiaris), 12 males and 12 females, aged from 1 to 7.5, mean weight of 3.804±1.324kg,from the breeding kennel.Those who showed clinical signs of systemic disease or clinical or hematological changes were excluded from the study.
Subsequently tear breakup time test (TBUTT) was evaluated by applying fluorescein sodium (Fluorescein Strips, Ophthalmos, SP, Brazil) and the measurement of the tear film breakup time was analyzed with a cobalt blue filter light and slit lamp, with the aid of a stopwatch (Ollivier et al., 2007;Andrade, 2008;Kobashigawa et al., 2015).
Continuing the evaluation, Jones test (JT) was performed by applying fluorescein sodium without subsequent rinsing and later timing of the passage of the dye through the nasolacrimal system to the ipsilateral nostril.Animals with a positive test were those in which the dye passage occurred between 0 seconds and 30 minutes, and as negative, those that the dye was not seen in the nostrils, after 30 minutes (Ollivier et al., 2007;Maggs, 2008;Martin, 2009;Binder and Herring, 2010;Featherstone and Heinrich, 2013).
The non-contact specular microscopy was used (Specular Microscopy SP-300P, Topcon, Japan) under delicate physical restraint to obtain specular images of the central corneal area, considering the density (cells/mm²) (DE), the hexagonality (%) (HEX), corneal endothelial cells area (µm²) (EA) and the central corneal thickness (mm) (CT).The minimum count was 50 cells, and the values were tabulated in Cell Count software and the count was corrected manually (Nagatsuyu et al., 2014;Kobashigawa et al., 2015).
Thirty minutes after instillation of one drop of tropicamide 1% (Tropinom, Genom Oftalmologia®, São Paulo, Brazil), lens were evaluated and images were obtained by ophthalmoscopy (Retinal Camera TCR-50DC, Topcon, Japan) for evaluation of the posterior segment of the eye structures and image recording of the retina and optic disc (Hubbard et al., 1999;Li et al., 2012;Kobashigawa et al., 2015).
Finally, with the use of ultrasound (Ultrasound Ultrascan A/B, Alcon®) and through the transcorneal method, ocular ultrasound images were obtained in a low-light environment, after instillation of tropicamide 1% and tetracaine hydrochloride 1% + phenylephrine hydrochloride 0.1% (Anesthetic Allergan Inc, São Paulo, Brazil) in both eyes.A-mode and B-mode were used simultaneously, using a 20 MHz circular probe and sterile gel (Supra Gel, Adlin Plásticos, LTDA, Jaraguá do Sul, Brazil).Assessments of axial length of the eyeball (AL), lens thickness (L) and lengths of the anterior (AC) and vitreous (VC) chambers were performed (Martin, 2009;Guimarães, 2011;Kobashigawa et al., 2015).
The variables were tested for statistical normality (Microsoft Excel 2016) through the Kolmogorov-Smirnov test.Statistical significance was set at p<0.05, with a 95% confidence interval.Data were represented as mean, standard error and median.Box-plots graphics were constructed to summarize the data distribution.

RESULTS
The means, standard errors and medians were represented in Tab. 1.In the data distribution of STT mean of 24.310±0.448mm/min and median of 24.500 mm/min were found the specimen that presented eyelid disorder, there was a statistical difference regarding the s that presented medial entropion (25.562±2.828mm/min).The TBUTT data showed a mean of 14.570±0.406sand median of 14.170s.In the interval of 0 to 30 minutes, 62.50% (n=30) of the eyes presented positive JT, while 37.50% (n=18) showed negative results.Among the positive samples, the mean was 8.420±1.571min and median 6.840 min.

Arq
Figure 1.Images of normal eye fundus of adult Pomeranian dogs.In A, the left eye of a male, an orange tapetal fundus and a circular disc at the nontapetal fundus.In B, right eye of a female dog, yellow tapetal fundus, circular disc at the nontapetal fundus.In C, right eye of a female, orange tapetal fundus, triangular disc at the border of the tapetal and nontapetal areas.In D, left eye of a female, with an amelanotic fundus and triangular disc.
Ultrasonographic results in A-mode (Table 1) were identified.The axial length of the eyeball presented mean and median of 16.859±0.148mmand 16.900mm, respectively.Lens thickness had mean and median of 6.651±0.081mmand 6.840mm, and anterior chamber depth with mean of 2.673±0.091mmand median of 2.650mm.Finally, mean and median obtained for vitreous chamber depth was 7.485±0.101mmand 7.560 mm.There was no statistical difference, except for lens thickness.Regarding B-mode ultrasound images, no changes were identified.

DISCUSSION
Reduction in STT values is described with advancing age of the individual (Hartley et al., 2006) In this study, patients aged over 5 years had mean of 23.550±3.576mm/min,a value lower than that found for those aged under 5 years, 24.857±2.649mm/min,but without statistical difference (p>0.05).Studies indicate an STT value for young animals 15.930±5.750mm/min and 15.580±5.900mm/min,for right and left eye, respectively (Broadwater et al., 2010).
In the present study, a mean of 24.310±0.448mm/minwas identified.This value is close to the upper limit for animals with mean age of 4.463 years (Featherstone and Heinrich, 2013).High STT values can correlate with some changes found in the research, such as caruncle trichiasis, chromodacryorrhea, medial entropion and negative test for Jones (Grahn and Sandmeyer, 2013).Accordingly, STT mean in samples with eyelid alterations was higher than STT mean in eyes without eyelid alterations, with statistical significance only for eyes with medial entropion in relation to healthy eyes.This data corroborates with the hypothesis that the conditions found resulted in STT mean of the study close to the upper limit.
The TBUTT value of 14.570±0.406swas lower than the described reference value of 19.7±5s (Ollivier et al., 2007).However, it was like that demonstrated in Shih Tzu dogs, 13.66±0.53s,a brachycephalic breed with multifactorial alterations that corroborate to lower TBUTT, unlike Pomeranian that has mesocephalic cranial conformation (Kobashigawa et al., 2015).It is valid to consider that the aforementioned test has assessment subjectivity, which can interfere with the results obtained.
It is known that, in the Jones test, the average time of fluorescein passage through the nasolacrimal canal is 248 s or 4.13 min to dogs (Binder and Herring, 2010).Dolichocephalic dogs present lower time than mesaticephalic and brachycephalic dogs (Binder and Herring, 2010).Contradictorily, in the present study, mean of 8,420±1,571 min or 505 s was obtained, values that can be attributed to the changes previously described, since they hinder lacrimal drainage, such as medial corner entropion, in addition to abnormalities of the nasolacrimal duct not detected (Grahn and Sandmeyer, 2013).
Pomeranian dogs had a high mean for C-B, 3.667±0.157cm,when compared to the values described in Beagles of 1.900 cm, also mesocephalic dogs (Lisbão et al., 2012).This data represents increased corneal sensitivity in the breed, which may derive from adequate corneal innervation (Featherstone and Heinrich, 2013).The results were superior to those found in Shih Tzu dogs (2.395±0.071cm)(Kobashigawa et al., 2015), agreeing with the already known, brachycephalic breeds have decreased corneal sensitivity, differently from what was observed in dogs in this study (Kafarnik et al., 2008).
Animals with alterations such as entropion are disqualified by the International Federation of Cynology (2013).In this research, mild medial entropion was diagnosed in 25% of the individuals, which, supposedly, indicates the existence of management failures.It is important to consider that alterations like these can lead to more serious changes.Furthermore, it should be noted that the group studied has high genetic variability, since the person responsible for the kennel has a considerable collectives of Pomeranian dogs (n>100), in addition to crossbreeding with partner kennels to increase variability.
The use of TonoVet Plus to measure IOP was efficient, given the small diameter of the eyes of the individuals studied and the more accurate precision it provides (Spiessen et al., 2013).The IOP values of 17.960±0.697in the Pomeranian are according to what has already been described for dogs, from 15.000 to 18.000 mmHg (Featherstone and Heinrich, 2013).
The optic disk in dogs is located in the center of ocular fundus and may be in the tapetal or nontapetal region, a factor that varies according to the tapetum extension (Narfström and Petersen-Jones, 2013).The study demonstrated a predominance of the optic disc in the non-tapetal area (66.670%), similar to what has been described in another study (Kobashigawa et al., 2015).In a distinct study, subalbinotic pattern fundus was described in 7 eyes, in addition to an optical disc located predominantly at the junction of the non-tapetal area and the tapetal area (Sini et al., 2016), while in this paper 4 specimen (8.330%) of amelanotic fundus were found.
Ultrasonographic measurements are within known normality standards, although there is no variety of studies other than brachycephalic (Kobashigawa et al., 2015;Abreu et al., 2018).
Control or eradication of hereditary ocular disorders is dependent on the ability to diagnose the disorder and its mode of inheritance, considering factors such as the age at which the condition can develop and from which it will no longer develop.When conditions are identified prior to the reproductive age, control is facilitated.The difficulty is trying to persuade breeders, especially when the condition is not considered severe (Barnett, 1988).

CONCLUSION
Mild changes were observed in parameters of lacrimal evaluation.Considering the values of STT, the parameters were influenced by eyelid changes.Higher corneal sensitivity was noted in this breed.Pathologies such as mild caruncle trichiasis, mild medial corner entropion and chromodacryorrhea were diagnosed.

Table 1 .
The mean, median and standard error values for Schirmer tear test; breakup time test; Jones test Cochet-Bonnet esteshiometry; intraocular pressure; laser flaremetry; non-contact specular microscopy and the ultrasound measured in 24 eyes of adult Pomeranian dogs from a kennel.