Sylvia Masson
Canine hypersensitivity–hyperactivity syndrome is increasingly recognized as a neurodevelopmental disorder that may be misinterpreted as normal juvenile behaviour at adoption. This case reports the long-term management of Middle, a male Bernese Mountain Dog presenting from early puppyhood with impulsivity, object ingestion, sleep reduction, hyperreactivity, and poor inhibitory control. Following behavioural assessment, fluoxetine was initiated and led to marked functional improvement, including cessation of object ingestion, improved sleep regulation, reduced impulsive interactions, and restored safety in daily life. During adolescence, despite ongoing treatment, renewed agitation, reduced concentration, and increased urinary marking were observed, suggesting a hormonal influence; therefore, a deslorelin implant was placed at 30 months of age. Unexpectedly, hormonal suppression induced paradoxical behavioural deterioration, including increased arousal and polyuria–polydipsia, requiring temporary treatment adjustment. These effects persisted throughout the duration of the implant and resolved abruptly after hormonal activity ceased. After implant cessation, Middle returned to a stable functional state, allowing long-term maintenance and subsequent reduction of fluoxetine dosage. The reversibility of the deslorelin implant enabled identification of its behavioural impact, which would have been more difficult to identify following irreversible surgical castration. This case highlights the importance of individualized, longitudinal management and the clinical value of reversible hormonal interventions in dogs when suspecting hormonal influence.