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Fig 1.

Methods to measure angulation, length, and slope of the nasolacrimal duct (NLD).

The images depict the measurement methods used to determine the angulation, length, and slope of the NLD. The viewing perspective is represented by the heads in the picture. (a) Angulation: A 3D illustration of the complete nasolacrimal drainage system (NDS) (unilateral) is shown. Visible are the three points used for angle determination: the upper lacrimal punctum, the caudalmost point of the NLD (vertex), and the nasolacrimal ostium. LC = lacrimal canaliculi; LS = lacrimal sac; NLD = nasolacrimal duct. (b) Length: In a CT sagittal section of a Pug, the different parts of the NDS can be seen (bilateral) marked in different colors. The NDS was marked manually point by point along its entire course using the colored markers shown. The lengths of the individual segments were determined separately. All straight distances between the individual markers were then added to the total length of one segment of the NDS. (c) and (d) Slope: The picture shows the NLD bilaterally (not the entire NDS) in a three-dimensional coordinate system. The coordinates of the NLD are illustrated as dots. The triangle represents the alignment of the hard palate. The green dots show the area included in the measurement. The orange dots represent the lowest and highest points. The grayish dots are outside the area of interest. The gradient was obtained starting from the lowest point of the NLD shortly before leaving the osseous lacrimal canal. The second measuring point was the peak of its middle portion. The lowest and highest points of the NLD were determined in relation to the alignment of the hard palate (triangle), so the results were unaffected by head positioning.

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Fig 1 Expand

Fig 2.

The NDS of a brachy- and normocephalic dog in CT cross-sectional images.

The picture illustrates the different, contrast medium-filled parts of the NDS in CT cross-sectional images. Figures (A)—(E) show a French bulldog, (a)—(e) a normocephalic mixed breed dog in comparison. (A) and (a) focus on the lacrimal canaliculi. It is noticeable that the canaliculi in the brachycephalic dog, due to their backward course, are shown in cross-section (punctiform), while in the normocephalic dog they are shown in longitudinal section. (B) and (b) depict the lacrimal sac. While the lacrimal sac in the normocephalic dog appears as a well delineated, drop-shaped structure, in the French bulldog it is clumsy and poorly demarcated. (C) and (c) show the nasolacrimal duct (NLD) within its bony canal. The different course of the NLD is also noticeable. (D) and (d) illustrate the middle portion of the NLD below the basal lamina of the ventral nasal concha. Here the NLD has a larger diameter than the initial part. (E) and (e) show the end portion of the NLD as it runs deep below the mucous membrane just before it emerges from the lacrimal foramen. Please note that in the French Bulldog different parts of the NDS are visible within one CT cross-sectional image. This is never the case with the images of the normocephalic dog. Pictures (A) and (D) show both the canaliculi and the middle portion of the NLD. Image (B) depicts the lacrimal sac and the end of the initial part of the NLD.

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Fig 2 Expand

Fig 3.

Comparative course of the nasolacrimal drainage system (NDS) in a brachy- and normocephalic dog.

The NDS is illustrated three-dimensionally, in green color, inside the skull of an English Bulldog and a Rhodesian Ridgeback. In the English Bulldog, the lacrimal canaliculi and initial part of the nasolacrimal duct (NLD) are directed backwards, the lacrimal canaliculi have much larger dimensions than those in the normocephalic dog, and the NLD has to cross beneath the maxillary canine root to reach the nasal cavity. (1a) upper lacrimal canaliculi, (1b) lower lacrimal canaliculi, (2) lacrimal sac, (3) nasolacrimal duct.

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Fig 3 Expand

Fig 4.

Comparative course of the nasolacrimal drainage system (NDS) in three brachycephalic dog breeds.

The NDS is illustrated three-dimensionally, in green color, inside the skull of an English Bulldog, a French Bulldog, and a Pug. The viewing perspective is represented by the head at the bottom of the picture (dorsal view). The backwards orientated parts of the NDS (canaliculi, initial part of the nasolacrimal duct) are smallest in English Bulldogs and largest in Pugs.

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Fig 4 Expand

Fig 5.

Gradient of the nasolacrimal duct (NLD).

The slope in percent of the NLD in 10 English Bulldogs (EB), 18 French Bulldogs (FB), 20 Pugs, and six normocephalic dogs (NC) presented in a box-and-whisker diagram.

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Fig 5 Expand

Table 1.

NLD gradient percentage in 10 English Bulldogs (bilaterally in all cases), 18 French Bulldogs (17 bilaterally, one unilaterally), 20 Pugs (18 bilaterally, two unilaterally), and six normocephalic dogs (bilaterally in all cases).

The NLD of normocephalic dogs ascends slightly in comparison to the ducts of English Bulldogs, French Bulldogs and Pugs, which exhibit a steep slope.

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Table 1 Expand

Fig 6.

Angulation of the nasolacrimal system (NLS).

The angle in degrees in 10 English Bulldogs (EB), 17 French Bulldogs (FB), 22 Pugs, and six normocephalic dogs (NC) presented in a box-and-whisker diagram.

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Fig 7.

The different shapes of the tear drainage system (NDS) in normocephalic and brachycephalic dog breeds.

The contrast-filled bilateral NDS, viewed from the side, is shown in green. At the top of the picture, it is shown outside its bony environment, while below it appears inside the skull. The given degrees reflect the angulation of the NDS. The NDS in normocephalic breeds has an obtuse-angled character, while brachycephalic dogs exhibit an acute-angled NDS, with the Pug showing the smallest values. These values support the depicted inversely directed course of the initial parts of the NDS in brachycephalic dog breeds. The very small angle in Pugs shows that the NDS is severely compressed in the vertical plane, with the lacrimal canaliculi and end portion of the nasolacrimal duct moving towards each other. In the lateral view, the characteristic shape of the NDS resembles the letter L in normocephalic and a U in brachycephalic dogs. In the case of a Pug, the compressed, clearly acute-angled appearance is more like a V than a U. Also noteworthy are the very long lacrimal canaliculi in brachycephalic dogs compared to the Rhodesian Ridgeback.

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Table 2.

Angulation of the nasolacrimal drainage system in 10 English Bulldogs bilaterally, 17 French Bulldogs (15 bilaterally, two unilaterally), 22 Pugs (19 bilaterally, three unilaterally) and six normocephalic dogs (bilaterally in all cases).

Measurements of NDS angulation revealed mean degrees of 84.3 in English Bulldogs, 80.4 in French Bulldogs, 59.7 in Pugs, and 112.0 in normocephalic dogs.

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Table 3.

Lengths of the NLD and the inferior and superior lacrimal canaliculi (median values).

The mean length of the NLD including the lacrimal sac is 55.2 mm in English Bulldogs, 43.7 mm in French Bulldogs, 40.1 mm in Pugs, and 94.0 mm in the normocephalic control group.

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Fig 8.

Accessory opening of the nasolacrimal system (NLS).

(a) Three-dimensional illustration of the entire, contrast agent-filled bilateral NLS. The viewing perspective is represented by the head at the bottom of the picture. The white arrows point to the accessory opening of the left nasolacrimal duct. The contrast agent (shown in green) emerges from the opening. (1a) right upper lacrimal canaliculi, (1b) right lower lacrimal canaliculi, (2) right lacrimal sac, (3) right nasolacrimal duct. (b) Transverse CT-image of the nose. The white arrows point to the accessory opening, from which contrast agent emerges into the nasal cavity. From there, the contrast medium flows along the lateral nasal mucosa and collects on the floor of the nasal cavity.

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