Sinuses are extensive air-filled cavities in the horse. They probably evolved to allow the horse’s head to achieve the size needed to accommodate its large array of molars and premolars, but with minimal added weight. Sinus cavities are located on each side of the horse’s head, above, below and between the eyes, and extending down the face to a point level with the end of the very obvious facial crest. Most of the horse’s cheek teeth, premolars, and molars extend into the sinus cavities and are contained within bone that forms the floor of the sinuses. Because the bone over the sinuses is thin, it is readily distorted by internal pressure from a disease process.
Anatomy of the Horse’s Sinuses
Diagram by David Freeman, MVB, MRCVS, ACVS, Professor, Large Animal Surgery
- Figure A: Dorsal View (above or behind)
- Figure B: Lateral View (side)
1. Rostral Maxillary Sinus
2. Caudal Maxillary Sinus
3. Ventral Conchal Conchal Sinus
4. Sphenopalatine SinuS
5. Frontal Sinus
6. Ethmoid Labyrinth
7. Frontomaxillary Opening
8. Dorsal Conchal Sinus
9. Infraorbital Canal
10. Septum between Maxillary Sinuses
11. Caudal Bulla of Ventral Conchal Sinus
Blue – Conchofrontal Sinus
Red – All Others
One of the most common diseases of the sinuses is sinusitis, which is an infection, usually of all the sinus cavities on one side of the horse’s head. There are two major causes of this infection. One is primary sinusitis, in which a bacterial infection invades the delicate lining of the sinus and causes a build-up of pus in the sinus cavity. These horses will have a unilateral (one-sided) nasal discharge and the diagnosis can be made by endoscopy, radiographs, and by sampling fluid from the sinuses. Another cause of sinusitis is called secondary sinusitis and this arises from a diseased cheek tooth. In these cases, the infection cannot be resolved until the affected tooth has been removed because the damaged tooth allows food particles to contaminate the sinuses. These horses have the same signs as a horse with primary sinusitis, although more severe, and they are more likely to have malodorous breath.
- Progressive ethmoid hematoma is another well-known sinus disease in horses. The cause of this is not known, but affected horses develop a mass of tissue composed mostly of a mature blood clot surrounded by a membrane within the affected sinus. This mass typically extends from the sinus cavity into the nasal passage where it can be seen on endoscopic examination. The most common clinical sign of this disease is a persistent and intermittent trickle of blood from one nostril in a mature horse that is otherwise healthy. Blood loss is rarely if ever severe. Whereas sinusitis can cause the outward surfaces of the sinuses to swell, swelling is very rare with ethmoid hematoma. Treatment of this condition involves surgical removal, which is usually successful.
- Wounds and fractures of the sinuses are quite common and are caused by a kick or other blunt trauma. In some cases, the skin may be broken so that there is a hole from the outside into the sinus cavity. In some horses, the skin remains intact and the external surface of the sinus appears normal. However, when it is palpated, some air can be felt trapped in the tissues between the skin and underlying bone. Also, the normal boney surface cannot be easily palpated because it is depressed into the sinus cavity. A variable amount of nasal bleeding will be evident. These fractures are called depression fractures and they require surgical repair. If surgery is not performed promptly with these fractures, the depressed segment of bone will heal and form an obvious concave depression and a bump on the horse’s head. This may not be cosmetically acceptable and can be difficult to repair later.
- Sinus cysts develop and invade many of the sinus cavities on one side of the horse’s head. These usually develop from normal sinus structures in a way that traps straw-colored secretions within large, thin-walled cavities within the affected sinus. The cause is not known, although a developmental problem is most likely. There is no evidence that they originate from dental structures, although this is always possible. Typical clinical signs are a persistent discharge of mucous from the nostril on the affected side and swelling on the outer surfaces of the sinuses. These horses will also have swelling on the inside of the sinuses that partly closes the nasal passage. These horses will then have decreased air flow through the nostril on the affected side, reduced exercise tolerance, and an abnormal noise. Sinus cysts respond very favorably to surgical removal.
Less common diseases of the sinuses are a variety of cancers and fungal infections. Typically these develop in older horses and can have a mixture of clinical signs. There may be outward swelling on the face over the affected portion of the sinus, a persistent mucous discharge, a persistent and intermittent trickle of blood from the nose on the affected side, sometimes a bad odor, and some horses might have a fever and be depressed. Benign tumors can also develop and these usually respond well to surgical treatment. However, malignant tumors can rapidly invade all the sinus cavities on one side of the head, cause considerable damage, and are rarely responsive to surgical treatment, without radiation therapy. Severe fungal infections can also be very difficult to treat.
Surgical Options for Sinus Horse Diseases
Surgeons at the University of Florida have a strong interest and considerable experience with sinus surgery in horses. Some of the more commonly used surgical methods for diseases in the sinuses have been developed by Dr. David Freeman at the University of Florida. The University of Florida also has state of the art imaging methods that are critical to the accurate diagnosis and localization of sinus diseases and for surgical planning. In 2008, Dr. Freeman and colleagues at the University of Florida successfully treated a dressage horse from the Brazilian Olympic team that had a very destructive sinus infection.
Return to Equine & Large Animal Surgery
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