Beyond the Ocular Surface: Nasal Sensory Input as a Driver of Reflex Lacrimation in Dogs

Objective: To evaluate the effect of nasal mucosal anesthesia on aqueous tear secretion in dogs and to compare responses between brachycephalic and non-brachycephalic breeds. Animal Studied: Twenty healthy dogs (10 Australian Shepherds, 10 Boston Terriers). Procedures: All dogs received 0.5 mL of 10% lidocaine or saline into one randomly selected nostril. The alternate solution was administered in the same nostril 2 weeks later. Schirmer tear test-1 (STT-1) was performed bilaterally before and 15 min after nasal administration. Tear strip wetting was recorded every 10 s for 60 s; the initial uptake phase (0–10 s) reflected uptake of pre-existing tears, while the active secretion phase (10–60 s) represented reflex tearing. Statistical comparisons included paired t-tests and linear mixed-effects models. Results: In non-brachycephalic dogs, lidocaine significantly reduced STT-1 values in the treated side by 11.5% (20.0–17.7 mm, p = 0.045) and did not cause a significant change in the contralateral side (21.7–20.1 mm, −7.4%, p = 0.280). Reflex tear slope decreased by 21.7% (0.23–0.18 mm/s, p = 0.004), while the initial phase slope remained unchanged (0.84–0.88 mm/s, p = 0.653). In brachycephalic dogs, lidocaine had no significant effect in either eye or tear phase (p ≥ 0.132). Saline caused mild, non-significant increases in STT-1 across all groups (+0.4% to 8.4%, p ≥ 0.116). Mixed-effects analysis identified skull type as the only significant predictor of treatment response (p = 0.047). Conclusions: Nasal mucosal anesthesia reduced reflex tear production in dogs, particularly in non-brachycephalic breeds. These results confirm the presence of a functional nasolacrimal reflex in dogs and suggest diminished nasal sensory input in brachycephalic breeds.