Histological evaluation of the soft palate in dogs affected by brachycephalic obstructive airway syndrome

To histologically examine tissue obtained from the thick rostral portion of the soft palate in severely affected brachycephalic dogs, and to quantitatively compare the histological findings with a similar region in control dogs.

Nine dogs examined for treatment of brachycephalic obstructive airway syndrome (BOAS) were prospectively recruited into the study. Four mesaticephalic dogs were also recruited following euthanasia for reasons unrelated to respiratory or gastrointestinal disease. Palatine tissue was harvested using an elliptical incision extending from the junction with the hard palate to the free caudal edge, and passing axial to the tonsilar crypts. The resected portion was fixed in formalin and stained using H&E. Tissues were examined in transverse section and assessments of the palatinus muscles, interstitium, lamina propria and salivary tissue were recorded. Quantitative analysis of the proportions of palatinus muscle, salivary tissue and stroma was performed using ImageJ image analysis software.

When compared to the control dogs, the histological findings of the clinically affected dogs showed a marked increase in acute and chronic muscle degeneration and necrosis. This was typified by swollen, hypereosinophilic fibres with centralised nuclei, myofibre atrophy, loss of cross striations and fragmented sarcoplasm. There was reduced proportions of palatinus muscle (p<0.001) and increased proportions of collagenous stroma (p=0.004) and salivary gland tissue (p=0.046) in the clinical samples, compared to control dogs.

Increased thickness of the palate in dogs with BOAS was not due to muscle hypertrophy but was due to increased stroma within the lamina propria, and increased proportions of salivary tissue. There was also a reduction of muscle mass, especially within the paired palatinus muscles, that may have consequences for palate function.

The results of this study enable us to start investigating the aetiology of the changes seen in brachycephalic compared to mesaticephalic dogs, which will impact timing and extent of treatment of clinical cases.