Paranasal sinus cysts are single or loculated fluid-filled cavities with an epithelial lining that contain yellow acellular fluid (10). They are reported to be the second most frequent cause of secondary sinusitis in horses (4). The cysts can be seen in horses of all age groups (10,17,21) but in some studies, the cysts were more common in pre-weaning foals, young adult horses, and adults above the age of 10 years (10,12). The exact etiology and pathogenesis are unknown (12).

The major clinical signs include facial swelling, nasal discharge, dullness on percussion, and difficulty in breathing due to partial airway obstruction (3). The nasal discharge is rarely malodorous or hemorrhagic (10). Also, abnormal respiratory noises, epiphora, exophthalmos, blindness, head shaking, and exercise intolerance are observed (4,10,21). The large size and complex anatomy of the sinuses allow the pathologic process to persist for weeks or months before any external signs, such as facial swelling or nasal discharge, are noticed by the owner or veterinarian. This can negatively affect the prognosis with poor outcomes (20).

Diagnosis can be made from the history, clinical signs, physical examination, endoscopy to determine the origin of a nasal discharge or to evaluate the integrity of the nasal passage of the affected side, and radiography to assess the skull, nasal cavity, dental structures and paranasal sinuses (1,3,5,21). Radiography is the most useful diagnostic procedure to establish the location and extent of paranasal sinus disease but may be nonspecific due to the complex skull anatomy and superimposition of surrounding structures, especially when complicated with secondary sinusitis (12,13,19). Recently, the use of cross-sectional imaging modalities including computed tomography (CT) scan and magnetic resonance imaging (MRI) is increasing in the diagnosis and management of equine patients. CT imaging of the head offers the distinct advantage of detailed information, especially on the complex osseous and dental structures, and the nasal cavities (19).

The cysts are highly amendable to surgical treatment (2,13). Surgical access to the paranasal sinuses can be obtained by either trephination or bone flap techniques (6). The bone flap techniques are superior to trephination because they provide better exposure, visualization, and access for surgical manipulation within the sinuses (12). The frontonasal bone flap allows a thorough exploration of most of the paranasal sinuses including the ventral conchal and rostral maxillary sinuses and better exposure for surgical manipulation (6). In one long-term retrospective study, complete resolution of clinical signs without recurrence was seen in 45 out of 48 horses (21), and in another study complete remission of clinical signs without recurrence was seen in 23 out of 28 horses (18).

The purpose of this report is to describe the use of CT scans to confirm the presence of a paranasal sinus cyst and its treatment with a frontonasal bone flap in a horse.

A 3-year-old Thoroughbred gelding was referred to the Jeju National University Equine Hospital with prolonged unilateral nasal discharge for several months. Prior to referral, the horse had been hospitalized for treatment of a fracture of the third phalanx and showed unilateral nasal discharge during hospitalization. The horse was treated with systemic antibiotics such as ceftiofur, anti-inflammatory drugs such as flunixin meglumine, and given nebulization with gentamycin, acetylcysteine and fluticasone propionate.

On clinical examination, the horse showed facial swelling, sinus dullness on percussion, and mucopurulent nasal discharge with a weak odor from the left nostril. Hematological and serum chemistry results were within the normal range (Supplementary Tables 1, 2). On endoscopic examination, the nasal passage appeared narrow due to the distension of the ventral conchae and the mucopurulent exudate was identified (Fig. 1). Radiographic examination showed no fluid line but revealed a radio-opaque round mass in the left maxillary sinus causing a distortion of the nasal septum (Fig. 2). CT scan was performed to confirm and determine the degree and extent of the lesions.

Figure 1. Endoscopic examination of the left nasal passage showing mucopurulent discharge.

Figure 2. Radiography before surgery. (A) Dorsoventral view of the ventral conchal sinus. (B) Lateral view of radiographs with soft tissue opacity filling within the left rostral and caudal maxillary sinuses (dotted line).

The CT scan was performed under general anesthesia. After sedation with detomidine 0.002 mg/kg, IV (Detomidin^®, Provet Veterinary Products Ltd.; Istanbul, Turkey), diazepam 0.03 mg/kg, IV (Diazepam inj., Samjin pharm. Co., Ltd.; Seoul, Korea), and ketamine 2.2 mg/kg, IV (Ketamine 50 inj., Yuhan; Seoul, Korea) were administered to induce anesthesia. The horse was placed in dorsal recumbency on a custom-built CT table. The CT scanner (Aquilion Lightning, Canon; Otawara, Japan, 32 multislice CT) was operated in a helical manner. Anesthesia was maintained with isoflurane (Ifran^®, Hana Pharm. Co. Ltd.; Kyonggi-Do, Korea) in oxygen. On examination of the CT scan, a cyst (7.5 × 8.0 × 10.0 cm) located between the rostral maxillary sinus and the frontal sinus which was causing distortion and remodeling of the ventral nasal concha was seen, in addition to the normal internal structures of the sinus, and the maxilla. Swelling of the overlying cutaneous tissues and a slight deviation of the nasal septum were also observed. Three-dimensional (3-D) images were reconstructed with a slice thickness of 1 mm using an image viewer (Xelis; INFINITT Healthcare. Co., Ltd.; Seoul, Korea) (Fig. 3). No dental involvement or orbit distortion was identified.

Figure 3. Images of computed tomography (CT) scan of the affected head of the horse indicating paranasal sinus cysts (arrow). (A) The hyperattenuating sinus wall in the maxillary sinus was seen in the transverse image. (B) The lateral image. (C) 3-D reconstruction of a paranasal sinus cyst filling of transverse view. (D) 3-D reconstruction of a paranasal sinus cyst filling of lateral view. Note the capsule line and fluid accumulation in the affected sinuses.

It was decided to surgically remove the cyst using the frontonasal bone flap. The procedure was performed in a standing position to minimize the possible complications of general anesthesia. The horse was restrained in the stock and sedated with 0.01 mg/kg of detomidine (Provet Detomidin, JSK Co., Ltd.; Gyeonggi-do, Korea) combined with 0.01 mg/kg of butorphanol tartrate (Butorphan, Myungmoon; Seoul, Korea) intravenously, and two additional half doses of detomidine during the surgery, which was maintained with constant rate infusion (CRI) of detomidine (0.01 mg/kg/hr). Aseptic preparation following local anesthetic infiltration of the incision sites with 2% lidocaine HCl (Lidocaine, Daihan; Korea) was performed. The frontonasal bone flap (5 × 10 cm in size) was created using an oscillating bone saw and the cyst filled with pus was removed from the exposed opening of the sinuses (Fig. 4A, B). To support the approach to the maxillary sinus, trephination (1.5 cm in diameter) was performed. After the removal of the cyst, the sinus was irrigated with povidone-iodine (0.05%) and isotonic saline, and an indwelling lavage catheter was placed (Fig. 4C). The closure was initiated by closing the periosteum, and the subcutaneous tissue and skin were closed in a routine manner. To prevent contamination and protect the surgical sites, a protective hood made from stockinet was applied (Fig. 4D).

Figure 4. Frontonasal bone flap procedure. (A) A cyst filled with pus was exposed through the created frontonasal bone flap. (B) Removed cyst. Note the hard and thickened cyst wall. (C) A lavage catheter was installed after removing and irrigating the site. (D) A protective stockinet hood was applied to prevent contamination.

The result of the microbiologic examination of the sinus fluid which was obtained intraoperatively confirmed the presence of Streptococcus spp. The horse was treated with sinus lavage via the installed catheter and nebulization. Systemic antibiotics including penicillin G and dihydrostreptomycin 0.05 mL/kg intramuscularly (PPS, Daesung; Seoul, Korea) and gentamycin 6.6 mg/kg intravenously (Samwoo Gentamycin, Samwoo median; Seoul, Korea), and nonsteroidal anti-inflammatory drug phenylbutazone 10 mg/kg intravenously (Arthridine, Virbac; Seoul, Korea) were administered once a day for 7 days and the antibiotics continued for an additional 10 days.

The horse was discharged 18 days after the surgery without significant complications. There was nasal discharge and facial swelling after the long-distance transportation (12 hours of transport time), whereas no signs of complications were seen at the time of leaving the hospital. The swelling and nasal discharge was treated after arrival. Two months after surgery, the horse started training again. However, 5 months later, the horse died from an unrelated cause viz., colic.

To identify potential pathogens in the present case, the sinus fluid was obtained intraoperatively and Streptococcus equi was cultured from the sample. In previous studies, the most frequently isolated bacteria in bacterial sinusitis in horses were mixed aerobic bacteria or pure cultures of Streptococcus spp. (7,15,17). Environmental factors such as dust or fungi could cause infection which could lead to a chronic condition. In this case, the horse had been rested in the stables for several months due to the fracture of the third phalanx. It is possible that the horse developed an infection from the surrounding environment during convalescence (7).

Radiography has been the primary imaging technique for assessing the skull, nasal cavity, dental structures, and paranasal sinuses (11). Due to complex skull anatomy and superimposition of surrounding structures, it may be difficult to radiographically distinguish paranasal sinus cysts from other sinus diseases (13). Therefore, the use of CT scan is increasing in equine practice these days. The tomographic nature of the CT images allows the evaluation of each sinus separately, avoiding the superimposition of surrounding structures (11). CT imaging is clearly a superior diagnostic tool and can also be used in preoperative planning and prognostication and could potentially prevent complications (4,8). We established the CT facility to support the diagnosis and treatment of horses. In this case, we were able to use the resource to confirm the detailed structure of the lesion and thus diagnosed a paranasal sinus cyst through a CT scan for the first time in Korea.

Due to the CT scan showing a significantly large lesion, we chose the frontal flap technique to access and remove the cyst completely (14). Also, this was combined with the trephination technique to compensate for the limited access to the most rostral extent of the rostral maxillary sinus. This was the appropriate choice of surgical technique which enhanced the outcomes.

Surgery of the paranasal sinuses is, by convention, performed with the horse anesthetized and recumbent. However, performing surgery of the sinuses with the horse standing eliminates the risks involved with general anesthesia, reduces the expense of surgery, and increases visibility during surgery by decreasing hemorrhage (1,16). In this case, the standing surgery under sedation with a local anesthetic was successful. We achieved early recovery, avoided complications associated with general anesthesia, and reduced the financial burden on the owner. It would therefore be a useful method for such surgeries in the future.

After leaving the hospital, the horse showed symptoms such as facial swelling and nasal discharge. We assume that transportation over a long distance could have caused complications in this case (9). During post-surgical recovery, it was necessary to minimize the stressful environment and stabilize the horse. It was a limitation of the study that we could not observe the horse later to assess the long-term prognosis of the treatment.

In this case, we diagnosed a paranasal sinus cyst with the CT scan of a horse showing unilateral nasal discharge and facial swelling. A frontonasal bone flap combined with the trephination technique was used to expose and remove the paranasal sinus cyst, and the horse made a successful recovery. This case showed that CT scans could be valuable in the diagnosis and surgical management of paranasal sinus cysts. We recommend that a frontonasal bone flap combined with trephination under sedation be considered a useful treatment option for paranasal sinus cysts in horses in the future.

The authors have no conflicting interests.