The equine paranasal sinuses represent one of the most anatomically complex regions of our equine patients. The sinuses accommodate multiple causes of disease, including that secondary to cheek teeth infection, benign and malignant space-occupying intra-sinus masses, trauma, oromaxillary sinus fistulae, and mycotic infections, and in the absence of any identifiable underlying cause, the remainder are by default: primary sinusitis. Accurate differentiation of these conditions has implications for surgical planning, prognosis, and patient welfare. Conventional radiography has historically formed the basis of imaging in equine sinus disease, yet its utility is fundamentally constrained by the superimposition inherent to the complex craniofacial architecture.
The use of computed tomography (CT) has had a positive impact on the diagnosis of equine sinonasal disorders, allowing detailed imaging in multiple planes without the presence of overlapping structures. This review evaluates the CT characteristics of primary sinusitis, dental secondary sinusitis, mycotic sinusitis, paranasal sinus cysts, progressive ethmoid hematoma (PEH), sinonasal neoplasia, and traumatic sinus injury, with the emerging veterinary literature supporting CT as the imaging modality of choice.

Primary and Secondary Sinusitis
Primary sinusitis, meaning sinus inflammation in the absence of identifiable additional cause, and secondary bacterial sinusitis arising from dental, primary fungal, traumatic, or mass-associated aetiologies, share overlapping CT appearances though with distinguishing features when evaluated systematically. In primary or dental sinusitis, soft tissue attenuating to fluid material, with decreased volume of air is present within the sinus, associated with exudate caused by bacterial infection and adjacent mucosal thickening / oedema. The relevance of CT over radiography is demonstrated by a 97% (CT) sensitivity, facilitating the diagnosis of periapical infections that could lead to secondary sinusitis. In contrast, radiography has demonstrated sensitivities of 76–80% in clinical populations (Townsend et al.., 2011). Compartment involvement on CT has been widely characterised. The rostral, and more dependent sinus compartments are most commonly involved, with the rostral maxillary sinus affected in 284 of 300 horses (94.7%) and the ventral conchal sinus in 87% of cases. The caudal maxillary sinus (65.3%), dorsal conchal sinus (52.7%), frontal sinus (26%), ethmoidal (or middle conchal) sinus (32%), and sphenopalatine sinus (28.7%) were less commonly affected. Furthermore, there was involvement or deformity of the ipsilateral nasal conchal bullae in 56% of horses with paranasal sinus disorders. This information, limited in assessment on radiography, guides surgical approach selection and underpins the superior nature of CT in equine sinusitis evaluation. Fungal sinusitis is much less common, with suspicions raised in cases of significant osteolysis / bony thickening, and in cases with plaque like lesions which are infraorbital canal – centric in location. Dental (secondary) Sinusitis Secondary sinusitis is more common than primary sinusitis in horses, and of the causes of secondary sinusitis, dental-associated sinusitis is the most frequent, resulting from a breach of the alveolar bone overlying the maxillary cheek teeth (Triadan 08–11). CT findings associated with dental disease and secondary (dental) sinusitis include; hypoattenuation of the cementum, enamel loss / defects, infundibular changes when severe, gas within the roots or fragmentation of the root in combination with swelling / thickening of the adjacent sinus lining. Critically, CT features of sinusitis in this context include thickening of the respiratory epithelium in the rostral maxillary sinus. In some cases the facial crest or maxillary bone more generally is involved, characterised by ‘endosteal’ sclerosis, thickening, periosteitis, and / or deformation leading to facial swelling in chronic infections. Identification of the specific offending tooth guides surgical treatment, and CT images allow the most detailed and rapid assessment of the cheek teeth. Henninger et al.. (2003), in a retrospective evaluation of 18 Warmblood horses with chronic sinusitis, demonstrated that the first molar (09) was the most frequently affected maxillary cheek tooth, a finding sometimes obscured on radiographs by fluid presence within the sinus. Both related to diastemata presence, in some cases of trauma, and in cases post-extraction with complications, oromaxillary sinus fistulae can be present, and can lead to chronic – persistent – recurrent sinusitis, with or without feed material present in the sinus compartments. The CT features (Hargreaves & Dixon. 2018) are known to be extremely variable, however evaluating sockets carefully if complications are suspected can be vital, as some alveolar bone defects in particular, can be small. Additionally, some may not be visible on oroscopic examination if the alveolus is filled with granulation tissue.

Sinus Cysts
Paranasal sinus cysts represent a differential for space-occupying - expansile intra-sinus lesions and carry a favourable prognosis compared to neoplasia. Their CT differentiation from other mass lesions was documented by Ostrowska et al.. (2020), in a series of eight histopathologically confirmed cystic cases. A discrete hyperattenuating ‘wall-like’ structure was detected in the periphery of the sinus lesion in non-contrast images in 7 of 8 horses, with a similar ‘wall-like’ structure detected in only 3 of 10 horses with other sinus diseases; in these latter cases, two also had hyperattenuating regions within the contents of the lesion. The detection of this peripheral hyperattenuating capsule therefore represents a potentially useful feature on plain CT assessment. Interestingly considering the content differences at gross assessment, no significant difference in attenuation (HU) values was found when comparing the fluid or soft tissue content of paranasal sinus cysts (mean 28.9 ± SD 9.2 HU) with other sinus diseases when progressive ethmoid hematomas (PEH) were excluded (30.4 ± 12.9 HU), indicating that measurement of attenuation values of the internal contents alone is not reliable in identifying paranasal sinus cysts. However, in the authors view they often have somewhat characteristic appearances. Additional CT features include displacement of adjacent osseous structures and septal deviation (mass effect) without aggressive bone destruction. CT allows better evaluation of the lesion extent with respect to adjacent bones, the nasal septum, and paranasal sinuses than radiography. Some horses will have secondary effects on the nasolacrimal ducts and infraorbital canals. In rare cases, cysts may attain sufficient size to cause neurological compromise, as illustrated by a reported case of a sinus cyst extending into the cranium and compressing the optic chiasm (Frontiers in Veterinary Science, 2022).

Progressive Ethmoid Hematoma (PEH)
Progressive ethmoid hematoma are a locally expansile, non-neoplastic angiomatous masses of uncertain aetiology, arising from the mucosa of the ethmoid turbinates or paranasal sinus linings, characterised by recurrent epistaxis and slow mass growth. Intra-nasal portions of PEH are readily diagnosed on nasal endoscopy, however intra-sinus PEH require imaging or sinoscopy for diagnosis. CT allows differentiation between fluid and a PEH and facilitates diagnosis of lesions of the ethmoidal or sphenopalatine sinuses, even when small, contributing to surgical planning. A characteristic CT appearance, though not pathognomonic, is described: a mixed, hyperattenuating (mean 101HU, SD 37.4HU), “swirling” pattern without severe bony destruction suggests an ethmoid hematoma is highly likely. Some regional skull bone deformation due to expansion is often seen. This mixed internal attenuation pattern reflects the layered haemorrhagic contents of varying ‘age’ within the lesion through expansion over time. Textor et al.. (2012) reported CT findings in 16 horses with PEH, noting that lesions involving the sphenopalatine sinus posed the greatest imaging challenge due to limited endoscopic access and anatomical complexity.

Sinonasal Neoplasia
Sinonasal neoplasia in the horse is thankfully uncommon but associated with a poor to grave prognosis owing to locally aggressive behaviour and the advanced state of disease typically present by the time of an initial diagnosis. Several tumour types of the nasal cavity and paranasal sinuses have been described, with the most common being squamous cell carcinoma (SCC), followed by neuroendocrine tumours, carcinoma, myxosarcoma, adenocarcinoma, and hemangiosarcoma. Cissell et al.. (2012), examined CT features specifically in a case series of equine sinonasal neoplasia, documenting the appearance of malignant tumours as poorly marginated, heterogeneously attenuating (soft tissue) masses with variable osteolysis of adjacent osseous structures, including potential disruption of the cribriform plate. Contrast-enhanced CT can in such cases add further value: in neoplastic lesions, heterogeneous peripheral enhancement may be evident, reflecting neovascularity, in contrast to the largely non-enhancing fluid content of cystic lesions or the minimally enhancing central regions of PEH. Neoplasia of the sinus system in horses is rare, with tumours originating from the oral cavity, osseous or odontogenic structures extending into the sinuses being more common than tumours originating within the sinuses themselves. Despite CT’s clear superiority to radiography in characterising lesions, definitive diagnosis invariably requires histopathological confirmation, and the imaging differentiation of PEH from neoplasia remains imperfect

Traumatic Injuries
Trauma, commonly resulting from kick injuries or blunt craniofacial trauma, presents a distinct CT diagnostic context where the modality’s capabilities are particularly informative. Often, fracture presence can be identified on radiography, however very typically the extent and complexity are underestimated compared to subsequent CT findings. The most common bones involved in skull fractures are the maxilla, nasal, and frontal bones, and sometimes the periorbital structures; secondary intrasinus haemorrhage / sinusitis was present in 10 of 13 horses with skull fractures in one study, with the rostral maxillary, caudal maxillary sinus, and ventral conchal sinus being the most affected. In the early stages haemorrhage can be potentially differentiated from sinusitis due to high HU presence in blood. CT allows for precise characterisation of fracture configuration, displacement, and involvement of adjacent structures including the orbit, nasolacrimal duct, and infraorbital canal - information critical to both any form of management. In the context of chronic traumatic sinusitis, CT is additionally capable of identifying sequestrum formation, periosteal reaction, and the presence of bone fragment displacement into the sinus compartments - findings that limit the ability to achieve resolution with medical management alone.

Conclusions CT has established itself as the gold-standard imaging modality for equine paranasal sinus disease, especially considering the limited availability of MRI for the equine skull, and limitations associated with this modality. The identification of a peripheral hyperattenuating capsule favours a sinus cyst; a mixed internally heterogeneous, swirling hyperattenuating mass without aggressive osteolysis is most consistent with PEH; poorly marginated, heterogeneous masses with cribriform plate destruction raise strong concerns for neoplasia; and fluid attenuation sinus opacification with periapical gas or root fragmentation likely reflects dental (secondary) sinusitis. Traumatic pathologies are best evaluated using CT bone algorithms and window width / level settings to delineate fracture / lesion extent and/or sequestrum presence. While histopathological confirmation remains mandatory for definitive diagnosis of mass lesions, CT-guided pattern recognition provides clinically actionable diagnostic probability that directly shapes surgical approach, client counselling, and prognostic accuracy in this demanding clinical domain. In the absence of a clear inciting cause, with sinus compartments filled with fluid – exudative material, a primary sinusitis would be the likely diagnosis of exclusion (bacterial > fungal).

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