Background
Transnasal sinus endoscopy (TSE), together with derived techniques such as transnasal endoscopic sinus lavage (TESL) and transnasal endoscopic sinus surgery (TESS), represents a non- to minimally invasive approach for the investigation and treatment of the equine paranasal sinuses and adjacent nasal structures. Despite its potential, this technique is not yet widely adopted in equine sinonasal imaging and surgery. The increasing availability of small diameter high-quality (single-use) videoscopes is likely to further improve its accessibility.

Objectives
To evaluate the feasibility, advantages, and limitations of transnasal sinus endoscopy in the diagnosis and treatment of equine sinonasal disease.

Materials and Methods
A retrospective evaluation was performed on 55 cases presenting with unilateral nasal discharge (including primary and dental-related sinusitis, excluding cysts and neoplasia), which were managed using transnasal techniques.

Results
Transnasal sinus endoscopy proved clinically feasible in the majority of cases, with only a limited number requiring surgical conversion such as trephination (n=4) and sinus flap surgery (n=1), with 2 documented failures (n=2). Analysis of treatment frequency per case demonstrated a median of approximately 3 interventions, with most cases managed within 2 to 4 treatment sessions. However, a subset of cases required substantially more repeated interventions, reflecting increased disease chronicity, anatomical complexity, and limitations in access.

Discussion
The technique is inherently constrained by the complex anatomy of the equine sinonasal system, particularly the narrow nasomaxillary apertures. Access to the rostral maxillary sinus and ventral conchal sinus (the anatomically distinct “rostral system”) remains especially challenging. However, advances in endoscopic technology, combined with appropriate case selection and pre-treatment, have significantly improved procedural success. Pathological remodelling and the formation of additional drainage pathways may facilitate transnasal access to otherwise difficult-to-reach regions. In addition, structures such as the dorsal and ventral conchal bullae, although not strictly part of the sinus system, are frequently involved in chronic disease and may become accessible via a transnasal approach in selected cases. While transnasal approaches may require multiple treatment sessions and can be more time-consuming than traditional surgical techniques, their minimally invasive nature and repeatability under standing sedation make them particularly valuable for selected cases, follow-up treatments, and as an adjunct to surgical approaches.

Conclusions
Transnasal sinus endoscopy represents a valuable minimally invasive diagnostic and therapeutic tool in the management of equine sinonasal disease. Its ability to provide direct visualisation of sinonasal structures allows improved localisation and characterisation of pathology, including differentiation of secretions, identification of necrotic tissue and sequestra, and assessment of mucosal changes. Although anatomical constraints limit access to certain compartments, particularly within the rostral sinus system, transnasal sinus endoscopy offers clear advantages in selected cases and complements conventional diagnostic modalities such as radiography and computed tomography. While it does not replace trephination or sinus flap surgery, it expands the diagnostic and therapeutic toolbox within a multimodal approach and allows a staged, less invasive approach in a substantial proportion of cases.

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