The horse’s brain is a remarkably small structure — not much bigger than a lemon in the average Thoroughbred — and it is the sinuses that fill much of the head. The sinuses are air-filled chambers that are connected to the respiratory system. They are situated at the front and sides of the head, around eye-level. There are five sinuses on each side, most of which interlink and ultimately drain into the nasal passages. If the skull were solid bone, it would be too heavy for the horse to lift, hence the sinuses to lighten the load.
The horse, being a herbivore, requires large cheek teeth to grind grass effectively, and it is the sinuses that allow the skull to accommodate this while making the head light enough to be easily raised to search for predators in the wild. Every year, several millimetres of tooth are ground away and more is pushed out into the mouth. In a young horse, the sinuses house the long reserve crown, which will be pushed into the mouth in later years. Consequently, the older the horse, the greater the volume of air in the sinuses.
Inflammation of the sinuses (sinusitis) can result from several different disease processes, including:
- infectious diseases of the upper respiratory tract
- as a consequence of dental disease
- sinus cysts
- ethmoid haematomas or neoplasia (a tumour)
- fungal infections
All these conditions result in a discharge accumulating in the sinuses. This either drains out via the nasal passages to the nostril or, if the drainage exit is blocked, accumulates and fills the sinus.
Usually, the sinusitis involves only one side of the head, so discharge is seen at only one nostril. A horse with a sinus disease may show any of the following signs:
- a one-sided nasal discharge of thick, purulent material or blood
- a distinctive and unpleasant smell at the nostril
- a facial deformity
- a discharge at the corner of the eye
- difficulty chewing food
- laboured breathing
The discharge at the nostril may vary in its colour, thickness, and smell according to the disease process going on in the sinus. A strong smell is most frequently associated with a sinusitis secondary to a tooth-root abscess or thick pus that has been present for many weeks.
If the horse’s face starts to appear “pushed-out” over the sinus area, it is likely that an expanding mass (such as a cyst or, occasionally, a tumour) is present, or the drainage exit is obstructed and the fluid is building up in the sinus under pressure. In some extreme cases, where facial deformity is seen, the mass may also expand inwards, narrowing the airway and causing a respiratory noise, which can sometimes be heard even at rest. The duct draining tears away from the eye lies in the wall of the sinus, and inflammation in the sinus may block the duct and result in discharge overflowing from the eye.
So how can we investigate a sinus problem? It is easy to compare the contours of the affected and normal side. Percussion (tapping gently) over the sinuses should result in equal resonance of both sides of the head. Reduced resonance will result from fluid or a space-occupying mass within the sinus. The nostrils should be checked for equal airflow and the mouth examined for any problems with the crowns of the teeth, for example, fractures.
X-raying the sinuses is a very useful way of giving a picture of any disease. The air-filled sinuses and the surrounding dense bone provide a very good contrast and hence a clear image. Endoscopy is also helpful.
Sometimes it is necessary to drill a small hole into the sinus to take a sample of the fluid for analysis or to biopsy a soft-tissue mass within the cavity. If a slightly larger hole is made through the bone, an endoscope can be used to see the inside of the sinus.
Occasionally, an MRI scan is carried out to provide an extremely detailed picture of the sinus and its contents, but this requires the horse to have a general anaesthetic and the equipment is available at only a few specialist referral centres.
Acute or primary sinusitis is a relatively common problem and may follow an upper respiratory infection. In many cases, the disease will be cured with a course of antibiotics, steam inhalations and mucolytics (medication to thin the discharge and aid its drainage). It is helpful to keep the horse in light exercise to encourage drainage. Some horses will require daily flushing via a catheter placed directly into the sinus through a small hole in the bone.
Some cases of sinus disease will be secondary to another disease process. If a strongly smelling one-sided nasal discharge is seen, an underlying dental problem will be suspected. This may involve a tooth-root abscess or a fractured or displaced tooth. In these cases, the affected tooth will almost always have to be removed before the sinus problem can be resolved.
The procedure may involve knocking the tooth out via the sinus, removing it via a hole made in the cheek or, in older horses, extracting the tooth via the mouth. Any resulting communication between the mouth and the sinus has to be sealed to prevent food entering the cavity and subsequently reinfecting it. The sinus has to be flushed daily in the post-operative period.
Horses are renowned for causing trauma to themselves, and injury to their heads is no exception. The bony wall of the sinuses is relatively thin, so knocking the head may cause fractures and a resulting infection in the sinus. Some “pushed-in” fractures require surgery to elevate the bone and stabilise the pieces with surgical wire, as well as treating any secondary sinus infection.
Other diseases with expansive masses in the sinus, for example, sinus cysts or ethmoid haematomas, require manual removal. This requires a flap of bone to be removed to allow access. The sinuses are vascular and any surgery in this area is very bloody, but it is often possible to get good visibility via a hole on the front of the head, with the patient standing. For more extensive lesions, a general anaesthetic is required.
After a surgery involving a sinus flap, the long-term cosmetic result is surprisingly good, with only a shallow depression present where the bone was removed. The prognosis after removal of a sinus cyst is usually very good, but recurrence can be a problem with ethmoid haematomas.