Depends what you mean by sinus. All horses have sinuses in the front of their heads which are just air filled chambers that are present to lighten the weight of the skull. Sinusitis is the term for inflammation or infection within the sinuses.
But a sinus can also sometimes refer to a tract of infection -usually following a penetrating wound for example.
Depends which problem you have as to what to do to help.
AJ
My horse has had sinusitus caused by teeth problem. The first time (before I had him) he had a supernummery tooth growing into the sinus and the second time (when I had him) he had a cracked tooth. On both occasions he had to have the tooth out, sinus flusing and antibiotics. Unfortunatly he still has the sinusitus I think and really needs to have a bone flap - however, while he is happy I am saving up for the operation! If you mean your horse has sinusitus rather than a sinus you should seek your vets advice. Have a really good smell of his breath - you may be able to smell something like bad breath, which is a sign of infection.
My horse had sinusitus too. It was caused by a fractured cheek tooth that had to come out via a bone flap. Two years on and he still has it even after the op, loads of antibiotics and two trephines to flush his sinus although it comes and goes and doesnt seem to cause him any discomfort or problems. He does stink from the nostril on the side of the problem sinus. You wont miss the smell, its awful and lingers on clothes for ages!!!
Feeding hay from the floor helps to drain my horses sinus when it flares up and touch wood it soon goes again.
It is sinusitus he has white milky stuff coming out of his left nostral in the mornings.(not every morning though) that's whats puzzling. It's not putting him up nor down, the vet said to feed him from the floor as it helps the sinuses to drain naturally. there is no smell and it's always white thick bit like porrage with out the lumps.
One of my horses had this many years ago. Vet said that the only way to sort it was by drilling into bone and flushing! I was mortified at the time and tried a homeopathist - this worked for 2 weeks but the sinusitis came back. Interestingly, from OT, he had had his teeth done by a different dentist prior to that, but no one had put the two together. Unfortunately he is no longer with us, but thats another story.
Good luck, hope you get it sorted.