Slight facial asymmetry may be difficult even for very diligent horse owners to identify in the absence of trauma. In some cases there is gross massive distortion of the bones around the sinus with impediment of airflow before anyone realizes something is wrong.
Appropriate diagnosis of the disease causing deformation of the maxillary and frontal bones, sometimes accompanied by lacrimal and nasal bone involvement, may be delayed if the owner assumes the deformity is due to a untreatable disease. Indeed, squamous cell carcinoma, the most common malignant neoplasia of the nasal passages and sinuses,1 carries a poor long-term prognosis, especially if it’s associated with bony involvement. However, deformity of the bones around the sinuses is often secondary to more benign and treatable disease such as sinus cysts, suture periostitis and, less commonly, ethmoid hematoma.2
Rarely do periapical intrasinus tooth root abscesses or primary bacterial sinusitis cause deformity, but veterinarians should also consider these differientials. The underlying cause of facial deformity can absolutely be diagnosed on the farm with minimal equipment. Treatment sometimes requires referral to a surgical facility, but it can often be performed with the horse standing under routine sedation and local anesthesia.
Equine sinus: a review of the anatomy
The paranasal sinuses in the horse are made of six paired sinuses (frontal, maxillary, dorsal, middle and ventral conchal, and sphenopalatine), each intricately associated with the others, either directly or indirectly. The nasal septum divides left from right sinuses. The rostral and caudal portions of the maxillary sinus are divided by a usually complete unfenestrated bony septum that may become disrupted by the underlying cause of distortion. These portions of the maxillary sinus communicate only indirectly through the opening into the nasal passage, i.e. the nasomaxillary aperture.
There is no division between the dorsal aspect of the frontal sinus and the dorsal conchal sinus (this space is therefore often called the “conchofrontal sinus”) and it readily communicates with the caudal maxillary sinus through the large frontomaxillary opening. The thin bone separating the frontal sinus from the ventral conchal sinus can easily be broken down with a sponge forceps through a large frontal trephine or flap. The ventral conchal sinus communicates with the rostral maxillary sinus on its lateral border at the dorsal aspect of the infraorbital canal. The sphenopalatine sinus sits ventrally and axially to the other sinuses but is not usually involved in diseases causing facial distortion.
Exam and imaging
As always, begin with a good physical exam, making special note of any cranial nerve abnormalities, exophthalmos, or uneven airflow through the nares before imaging. I also recommend a careful oral exam, as squamous cell carcinoma may invade from the sinus into the oral cavity. Invasive neoplasms may also cause loosening of teeth.
Radiographs. Good-quality radiographs are essential for diagnosis of a space-occupying lesion, especially if diagnosis is to be made on the farm through needle trephination. At the very least, take lateral, dorsoventral and both oblique views to visually separate the sinuses from each other. Bones deformed by sinus cysts are often thinned, but lytic lesions are more consistent with aggressive neoplasia or fungal disease.
Suture periostitis, arising from the intersection of the nasal and frontal bones, is usually identified on a lateral view, as are ethmoid hematomas. The dorsoventral view is most helpful in identifying soft tissue opacity in the ventral conchal sinus, which runs axial to the dental arcades on a well-taken radiograph. Radiographs may also aid in determining sinus margins when the lesion distorts normal anatomy, although cross-sectional imaging may be necessary if distortion is severe.
Endoscopy. Endoscopy is not necessary but is useful as an ancillary diagnostic technique, especially if radiographs are equivocal. The nasomaxillary aperture can be evaluated, as can patency of each nasal passage. Ethmoid hematomas are often more readily identifiable on endoscopy than radiographically and may be treated with 10% formalin infusion through an endoscopic needle.
Computed tomography (CT). Cross-sectional imaging is useful for surgical planning but is available only at referral institutions. CT is indicated if more than one mass is suspected or if facial deformity is extreme.
To perform trephination you will need:
- Sedation for a 10- to 15-minute procedure
- 1 to 3 ml 2% lidocaine or carbocaine
- 14- to 16-ga needle and mallet or Steinmann pin and Jacob’s chuck
- 16- to 18-ga catheter or fresh needles for sampling
- No. 15 blade
- Syringes for collection and flushing
- Sterile saline
- Extension set
- Skin staples.
Again, radiographs will aid in determining an appropriate sample site. Take care to avoid the cheek teeth, especially in horses younger than 6 years old. Also, keep in mind that trephination may be contraindicated when a mass of bony opacity is viewed on radiographs.
With the horse sedated, clip and sterilely prep the area overlying the mass visualized on radiographs or at the area of maximal distortion over the frontal or maxillary sinus.
Inject 1 to 3 ml of local anesthetic into the trephination site. Make a stab incision through the skin and periosteum with the blade. With the Steinmann pin loaded into the Jacob’s chuck with only about 2 to 3 cm exposed, apply firm pressure to the bone while holding the pin exactly perpendicular and applying a slow twisting motion. You’ll feel a sudden loss of resistance when the bone is penetrated.
If you are using a needle as the trephination instrument, tap the needle gently with the mallet. It is important to hold the needle perpendicular to the skull, otherwise the hub may break off. Once the site is trephinated, remove the pin or needle if there is debris within the lumen and insert a 14-ga catheter or a fresh needle, advancing the catheter off the stylet once the catheter has advanced past the trephine hole. A catheter affords greater length and less trauma to underlying tissue, but a needle may be needed for aspiration of soft tissue masses.
After sampling, flush the sinus through with nonsterile or sterile saline. If the saline does not readily come out the ipsilateral nare, consider obstruction of the nasomaxillary aperture or frontomaxillary opening (if the frontal sinus is the site of trephination). After flushing the sinus, I choose to place one skin staple over the trephination site, both to prevent subcutaneous leakage of pus should sinusitis be present and also to mark the spot of trephination should the site need to be revisited.
Sinus cysts are one of the most common causes of facial distortion in any age of horse. While extirpation and resolution can be achieved through larger-bore trephinations using a Galt trephine of 6.4 to 25 mm, surgical removal of the cyst lining is most effective through a frontonasal flap if it is in the frontal or caudal maxillary sinus and will avoid trauma to the tooth roots and infraorbital canal. Likewise, palliative care may be enhanced with debulking of a carcinoma through a flap. These flaps may be done standing, but referral to a surgical facility is recommended.
A soft tissue opacity visible on radiographs is determined to be a sinus cyst when trephination produces honey-colored translucent fluid. Trephination in the field is most valuable for those owners who are open to surgery for a condition with a good to excellent prognosis, as with sinus cysts. Surgical debridement of the cyst lining is curative and airflow through an effective nasal passage may improve through remodeling, although the deformity will still be visible. Many neoplasias of the head, the most common of which is squamous cell carcinoma, carry a poor prognosis, and debulking of the mass should be considered merely palliative.
Sinusitis arising from inadequate drainage through the nasomaxillary aperture is common secondary to facial deformity. The nasal discharge is usually unilateral and is normally not malodorous, depending on the presiding secondary infection. Rarely is it green or fetid, which more often arises from tooth root abscesses or orosinal fistulas. Anticipate the need for sinus flushing through needle trephination to treat secondary sinusitis after surgical treatment (flap) for a condition causing facial deformity, as well as a two-week course of oral broad-spectrum antibiotics.
Suture periostitis is usually transient and only an aesthetic concern, although application of small plates can limit motion between the nasal, maxillary and incisive bones. Diagnosis can be made by radiography without any need for trephination (Figure 4).
All in all, many horses with gross facial distortion are often not treated immediately because of misconceptions about the underlying cause. Differentials include many treatable diseases, and diagnosis often can be made on the farm.
1. Head K, Dixon P. Equine nasal and paranasal sinus tumours. part 1: Review of the literature and tumour classification. Vet J 1999;157(3):261-279.
2. Tremaine W, Dixon P. A longâterm study of 277 cases of equine sinonasal disease. part 1: Details of horses, historical, clinical and ancillary diagnostic findings. Equine Vet J 2001;33(3):274-282.