Equine Sinus Cysts

Warning: this post contains some graphic images of wounds and blood.

In early June this year Severn Edge Equine was presented with a thoroughbred yearling filly that had a large swelling on the left side of her face. The filly was bright and well in herself but did have a reduced air flow through the left nostril. The initial examination showed no obvious external signs of trauma around the swelling and on oral examination there was no obvious abnormalities with the dental arcade directly below the swelling.

The yearling was initially treated symptomatically with a course of antibiotics and anti-inflammatories. These had little effect, however it was noted that the swelling was increasing in size and the eye was starting to water.

The filly was brought into the clinic at Aston Munslow for further investigations. Swellings on the side of the face with no nasal discharge are usually caused by tooth root abscesses of the first 2 molar teeth. The first 2 molars are in front of the sinuses and therefore if infected will cause swelling of the bone rather than draining into the sinus and then down the nose.

Facial Crest

The swelling in this case was too far back to be associated with the first 2 molars. Sinus infection will normally produce a one sided nasal discharge (smelly) and we would not normally see a swelling unless the outflow/drainage tract was blocked. In this particular case were we dealing with a blocked infected sinus, or something more unusual like a para nasal sinus cyst, ethmoid haematoma or even a tumor.

X-ray L Oblique

Endoscopic examination of the nasal passages showed that they were particularly narrowed due to the sinus swelling, forcing the nasal passages to deviate inwards. The radiographs showed healthy tooth roots but the sinus area was filled with soft tissue opacities rather than dark areas of gas. They did not show any fluid lines which you would normally see in cases of infected sinuses. The views had what we call a ground glass appearance. A spherical outline could also be seen in the sinus on the view from above. There was also a multiloculated appearance within the sinus and also some distortion of the nasal conchae.

These signs, together with the age of the horse, and the appearance of the facial swelling made us highly suspicious of a para nasal sinus cyst. A needle was placed into the facial swelling and a yellow fluid was removed. This was not pus and again fit with the diagnosis of a sinus cyst.


It is not known what causes a para nasal cyst to occur but it is thought that trauma to the respiratory mucosal layers within the sinus can start the process of cyst development.

Surgery is the only option for treatment. If ignored the cyst would start to cause more space occupying problems to the nasal cavity as well as affecting the left eye and associated nerves within the sinus. The surgery is carried out in the standing sedated horse as it carries less risks than a full general anaesthetic.


The filly had a drip placed so sedative could be administered slowly throughout the procedure.  Local anaesthetic was then placed under the skin before a maxillary sinus flap was created by cutting the skin then using an oscillating bone saw to cut through the sinus.

The series of photos below show that a flap was opened and the multiloculated pockets of the cyst could clearly be seen. The sinus was full of cystic material. This was carefully scraped out and removed using fingers and a bone curette. The main part of the cyst was within the frontal sinus and had completely disrupted the normal architecture of the para nasal sinus.

A frontal sinus trephine hole was created to help with removal of the cyst and also for follow up flushing. On removal of the cyst the filly was left with one large open sinus cavity rather than the normal partitioned cavities. There was some concern over disruption of the nerves lying within the sinus.

Trephine Hole

After clearing out as much cystic material as possible a drainage tract was created between the sinus and the nasal cavity as the normal one had been closed by the cyst expansion to allow flushing and draining during the aftercare. A bandage was placed in the maxillary sinus to be removed the following day from the nose. The surgical flap was then closed and a lavage/flushing system was placed into the top frontal trephine hole and sutured into place.

Trephine Hole

The filly soon recovered from the sedation and despite all the surgical intervention was bright, well and eating within hours. There was thankfully no obvious damage to any of the sensitive nerves crossing the sinus. She went home after a couple of nights in the clinic and had the sinus flushed daily. Some swelling occurred around the surgical sites initially but has since resolved.

It is 2 weeks on from the procedure and all is now progressing well. The drainage tract between the sinus and nasal passages blocked due to over granulation of the sinus mucosa. This is a recognised complication with this type of surgery and required intervention to re-open the tract. This was done via access through the frontal sinus trephine hole.

Although we are still early in the recovery process, the prognosis is good for the filly. Some horses are left with a mild but continuous nasal discharge, this however is asymptomatic. The disruption and narrowing of the nasal cavities should remodel and allow normal athletic breathing in the future.
We are keeping our fingers crossed for a full and uneventful recovery.

By Dr Mark Sanderson BVM&S MRCVS

Further Advice

Related SEV Branch

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