Soft palate angle and basihyoid depth increase with tongue size and with body condition score in horses

Abstract

Background: Obesity has been associated with human obstructive sleep apnoea and canine brachycephalic obstructive airway syndrome. The effect of body condition score (BCS) on structures of the oropharynx, nasopharynx and upper airway of the horse has not been investigated.

Objectives: To investigate the effect of BCS on tongue measurements, soft palate angle and basihyoid depth in horses.

Study design: Retrospective, analytical, cross-sectional.

Methods: Computed tomographic (CT) images of the head of 58 horses were assessed. DICOM viewing software was used to measure head length, basihyoid-skin depth, soft palate angle (SPA), midline tongue area, dorsoventral height (DVH) of the tongue in two locations and head angle. BCS were assigned during CT examinations. Associations between measurements were tested and following initial calculations, further associations with tongue measurements as a ratio of head length were assessed.

Results: For initial measurements, 44 horses met the inclusion criteria. Addition of head length ratios to tongue measurements resulted in 24 of 44 horses meeting the inclusion criteria for the second set of calculations. Increased BCS led to an increased mean SPA (mean difference = 2.56 ${}^{°}$ ; p = 0.02) and increased median basihyoid depth (mean difference = 0.246 cm; p = 0.006). Following adjustments made for the effect of head length on tongue measures, significant correlation was identified between SPA and tongue area (Spearman's r = 0.544; p = 0.007); SPA and DVH of the tongue at the level of the hard palate (Spearman's r = 0.562; p = 0.004) and SPA and DVH of the tongue at the lingual process of the basihyoid bone (Spearman's r = 0.690; p < 0.001). No significant correlation was identified between variables and sex.

Main limitations: The sample size was small and the effect of breed on measures was not studied. Measurements were acquired on a single sagittal CT plane. The investigator collecting CT measures was not blinded to BCS. All horses were sedated for the CT procedure which may have affected measures obtained.

Conclusions: Increased BCS increases SPA and basihyoid bone depth. Increases in tongue size measurements increase SPA. Results from this study warrant further investigation into the clinical significance of the effects of BCS on the upper airways of the horse.

Keywords: basihyoid; body condition score; horse; obesity; soft palate; tongue.

FIGURE 1

(A–D) 3D multiplanar reconstructions sagittal (A and C) and transverse plane (B and D) CT images in a soft tissue window, WL 35, WW 350 of a horse BCS 1 (A and B) and of a horse BCS 5 (C and D). B and D are at the level of the rostral tip of the lingual process of the basihyoid bone. Sagittal images rostral is on the left and the top is dorsal. Transverse plane images, left is on the right and top is dorsal. Examples of measurements obtained in the sagittal plane images. Head length = green line, soft palate angle with hard palate = purple lines, area of tongue = orange line, DVH of tongue at caudal end of hard palate = red line, DVH of tongue at tip of lingual process = blue line, depth of the basihyoid bone from the ventral skin margin = yellow line. Areas of hypoattenuating fat were clearly seen within the tongue (red arrows). The increased area of fat within the tongue was best appreciated on the transverse plane.

FIGURE 2

Flow chart illustrating inclusion and exclusion criteria of horses at each stage of the study.

FIGURE 3

(A). Box and whisker plot showing the distribution of soft palate angle measurements within each body condition score. BCS, body condition score. (B). Box and whisker plot showing the distribution of basihyoid depth measurements (cm) within each body condition score. BCS, body condition score; cm, centimetres. * = outlier.

FIGURE 4

(A) A scatterplot showing the relationship between soft palate angle (°) and tongue area (cm²) as a proportion of head length (cm). cm, centimetres. (B) A scatterplot showing the relationship between soft palate angle (°) and dorsoventral height of the tongue at the level of the hard palate (cm) as a proportion of head length (cm). DVH, dorsoventral height; cm, centimetres. (C) A scatterplot showing the relationship between soft palate angle (°) and dorsoventral height of the tongue at the level of the lingual process of the basihyoid bone (cm) as a proportion of head length (cm). DVH, dorsoventral height; cm, centimetres.

FIGURE 5

Photograph of gross post‐mortem dissection of the cut section of the tongue of an obese horse unrelated to the CT study. Adipose tissue is identified infiltrating the intrinsic muscles of the tongue (arrows). Blue star = Soft palate; blue cross = hard palate.