Sinusitis is the most common disease affecting the paranasal sinuses in horses. According to a survey performed by the British Equine Veterinary Association, which found a prevalence of 0.4 percent in 17,000 horses (Dixon et al., 2011), sinusitis is not so common among the equine population despite being a well-described pathology.
The anatomy of the equine paranasal sinuses
The complex anatomy of the paranasal sinuses can make it a challenge to treat sinusitis in horses. The paranasal sinuses are air-filled cavities formed by seven pairs of compartments, described as the frontal (F) and dorsal concha (DSC). Collectively, these are termed the concho-frontal, caudal maxillary (CMS), ethmoidal (E), sphenopalatine (SP), rostral maxillary (RMS) and ventral conchal (VCS) sinuses.
The concho-frontal sinus drains into the caudal maxillary sinus, together with the sphenopalatine and ethmoidal sinuses, whereas the rostral maxillary sinus and the ventral conchal sinus communicate with each other but drain independently to the caudal maxillary sinus through the nasomaxillary opening located in the middle meatus (O’Leary and Dixon, 2011) (Figure 1). The drainage pathway of the equine sinus is very narrow, and in cases of inflammation can get obstructed, not allowing natural drainage (Probst et al., 2005).
Additionally, we must not forget about the relationship between some of the cheek teeth and the maxillary sinus. The 2nd and 3rd molars communicate with the caudal maxillary sinus, while the 1st molar and sometimes the 4th premolar communicate with the rostral maxillary sinus. Therefore, apical infections of these cheek teeth are a common secondary cause of sinusitis (Beard and Hardy, 2010).
What are the clinical signs of equine sinusitis?
Most commonly, sinusitis presents unilaterally, although very infrequently and unusually, some cases have been described bilaterally.
Typically, horses present with unilateral mucopurulent discharge (Figure 2A), nasal airflow obstruction and lymphadenopathy. Chronic cases can be accompanied by facial swelling, deformation (Figure 2B) or even cutaneous draining tracts (O’Leary and Dixon, 2011). Often, horses present as completely normal on physical examination but have a painful response to palpation of the facial swelling or have a pungent smell. Percussion of the affected sinus produces a dull sound, indicating that it is not filled with air but filled with fluid or a mass.
What is the origin and pathology of equine sinusitis?
Equine sinusitis can have a primary or secondary origin.
Horses with an upper airway bacterial infection can develop primary sinusitis when the bacteria invade the sinus, inflaming the mucosa and impairing mucociliary function. This makes sinus drainage difficult, and leads to an accumulation of mucus and bacteria in the sinus, becoming the perfect environment for bacterial proliferation. Primary sinusitis usually responds well to antibiotic treatment, especially if there is a previous bacterial culture and sensitivity (Gergeleit et al., 2018).
Primary sinusitis usually responds well to antibiotic treatment, especially if there is a previous bacterial culture and sensitivity
When horses respond temporarily or do not show a response to antibiotic treatment, the veterinarian should look for a secondary cause as the origin of the sinusitis. The most common causes of secondary sinusitis described in horses are mycotic sinusitis, sinus cyst, progressive ethmoidal haematoma, traumatic sinusitis, oro-maxillary fistula formation, sinus neoplasia or dental sinusitis, which is the most common (O’Leary and Dixon, 2011). Therefore, it is crucial that the veterinarian conducts a complete oral examination every time they face a horse with suspected sinusitis.
It is also important to mention that the bacteria colonising the sinus tend to form a biofilm, which makes it more complicated to achieve a successful outcome with antibiotic therapy (Tan et al., 2013). This can become a nightmare for the horse, owner and veterinarian.
The bacteria colonising the sinus tend to form a biofilm, which makes it more complicated to achieve a successful outcome with antibiotic therapy
How do we diagnose equine sinusitis?
Nowadays, advanced diagnostic techniques are available, allowing the veterinarian to get a more accurate diagnosis and achieve a successful outcome.
In addition to a thorough anamnesis and meticulous physical examination, the most readily available diagnostic tools are endoscopy and radiology. Sinocentesis – a sterile collection of fluid from the sinus – should also be performed for culture and sensitivity. Endoscopy can help the veterinarian confirm whether drainage comes from the nasomaxillary opening and identify the shape or presence of a progressive ethmoidal haematoma (Figure 3).
Sometimes opacities corresponding with masses or cysts, as well as fluid lines, can be observed on a radiograph. It is very important to position the nose of the horse pointing to the ground to be able to observe a clear fluid line on a radiograph (Figure 4). Radiographs of the head are not always diagnostic as horses have many air-filled cavities, teeth and bones in the head that superimpose, so radiographs show a sensitivity of 76 to 80 percent (Townsend et al., 2011). Another available diagnostic technique is scintigraphy which has shown a lower sensitivity overall but better sensitivity to diagnose dental-related sinusitis (Barakzai et al., 2006).
The advances in technology and surgical techniques have made computed tomography (CT) and sinuscopy the gold-standard techniques to achieve the most accurate diagnosis in horses with sinusitis. CT scans have shown 97 percent sensitivity (Liuti et al., 2018), and this technique allows the veterinarian to observe the sinuses and dental structures in detail in the horse. This facilitates the diagnosis of periapical infections that could lead to secondary sinusitis (Figure 5).
How can we treat equine sinusitis?
Even if the horse presents with secondary sinusitis, it is important to treat the primary cause. Otherwise, once antibiotic therapy is discontinued, the clinical signs such as nasal drainage can reappear.
In cases of dental-related sinusitis, the extraction of the affected molar is often required. However, equine dentistry has evolved quickly in the last decade, and now less invasive techniques are sometimes possible. Therefore, it is recommended to consult a dentistry specialist or a surgeon to choose the proper technique.
If conservative treatment is elected, culture and sensitivity will help the veterinarian choose the correct antibiotic therapy, which can be combined with sinus lavages. If a sinus lavage is used, it is very important to consider which compartments are affected, as the rostral maxillary sinus and ventral conchal sinus do not communicate with the rest of the compartments.
If conservative treatment is elected, culture and sensitivity will help the veterinarian choose the correct antibiotic therapy
For this technique, a small trephination hole is made over the frontal, caudal maxillary or rostral maxillary sinus where a 24 to 28F foley catheter is placed. The affected sinus compartments should be lavaged with four to five litres of saline or diluted povidone solution two to three times a day. The frequency of lavage is dependent on the severity of the sinusitis and should be discontinued once the outflowing fluid is clean.
What are the surgical options for treating sinusitis?
If the sinus needs to be explored or a mass removed, the most common surgical techniques performed on the horse are trephinations and sinus flaps. Both surgical techniques can be performed under general anaesthesia, but nowadays, it is preferred to perform them under deep standing sedation. Besides avoiding the complications associated with general anaesthesia, bleeding is better controlled when the patient is standing; however, not all horses are good candidates to perform standing surgery.
Trephination is often used to introduce a fibro-endoscope or to perform a sinuscopy (Figure 6). Trephination can be performed with as large an opening as needed, but the surgeon must bear in mind that the bone removed is not replaced. For this reason, ideally the surgeon should make the opening as small as possible to achieve the best cosmetic result.
Trephination can be performed with as large an opening as needed, but the surgeon must bear in mind that the bone removed is not replaced
On the other hand, when trephination does not provide enough visualisation or space for manipulation, a larger opening can be achieved with what we know as a sinus flap. Sinus flaps can be performed using a frontonasal approach, which primarily allows access to the dorsal conchal, frontal and caudal maxillary sinuses (Figure 7A), or by a maxillary approach, which allows access to the rostral, caudal maxillary and ventral conchal sinuses (Figure 7B) (Dixon and O’Leary, 2012).
Are there any complications with surgical treatment of sinusitis?
As with all surgical procedures, complications can occur intra- and post-operatively.
The most common intraoperative complication is bleeding. This can be a fatal outcome, but can be prevented with good planning and the availability of a possible blood donor in case of need.
After a sinus flap, horses can develop complications such as recurrent sinusitis, bone sequestrum or sinonasal fistulation, which can prolong the treatment time and cost.
In conclusion, sinusitis is a well-described disease in horses that can be complicated to treat, especially in chronic cases where the bacteria are resistant and create a biofilm when colonising the sinuses. A prompt diagnosis and treatment by the veterinarian can help to accomplish the best possible outcome.