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

## TL;DR

This study shows that numbing the nasal lining in dogs reduces tear production, especially in non-brachycephalic breeds, indicating a functional link between nasal sensation and tear reflex.

## Contribution

The study identifies nasal sensory input as a driver of reflex tearing in dogs and reveals breed-specific differences in this reflex.

## Key findings

- Nasal mucosal anesthesia reduced reflex tear production in non-brachycephalic dogs.
- Brachycephalic dogs showed no significant change in tear production after nasal anesthesia.
- Skull type was the only significant predictor of treatment response.

## Abstract

To evaluate the effect of nasal mucosal anesthesia on aqueous tear secretion in dogs and to compare responses between brachycephalic and non‐brachycephalic breeds.

Twenty healthy dogs (10 Australian Shepherds, 10 Boston Terriers).

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.

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).

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.