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

Nasal sensory input drives reflex tearing in dogs, with diminished response in brachycephalic breeds.

This study investigated the nasolacrimal reflex in dogs by evaluating how nasal mucosal anesthesia affects tear production. Twenty healthy dogs (10 Australian Shepherds, 10 Boston Terriers) received intranasal lidocaine or saline with Schirmer tear test-1 (STT-1) measurements taken before and 15 minutes post-administration. In non-brachycephalic dogs, lidocaine significantly reduced STT-1 values by 11.5% on the treated side and decreased reflex tear secretion slope by 21.7%, while the initial tear uptake phase remained unchanged. Brachycephalic dogs showed no significant response to lidocaine in either eye or tear phase. Saline administration produced only mild, non-significant increases across all groups. Mixed-effects analysis confirmed skull type as the only significant predictor of treatment response. These findings demonstrate a functional nasolacrimal reflex in dogs mediated by nasal sensory input, with notably diminished nasal sensory contribution to tear production in brachycephalic breeds. This suggests breed-specific anatomical or neurological differences in reflex tear mechanisms.

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