Some guinea pigs are very susceptible it this condition and one of the main causes for it is bedding them on sawdust and wood shavings. I always bed mine on hay and newspaper but I am very fussy about the quality of the hay I use for old and dusty hay can be as bad as sawdust or shavings.
The symptoms of sinusitis are similar to those of a human being with hay fever, sneezing, a running nose and a wheezing noise which come from the nose and can sometime reach back to the throat.
There is not a lot that can be done other than putting a dab of Vic ointment on the nose, though I have heard that the use of antihistamines can be used I have had no experience of using them. As I am uncertain of any side effects they may have, and the condition is not life threatening I have not even thought of trying them myself.
Obviously, if the bedding is suspect then it has to be removed but sometimes the source of the irritant cannot be traced so it is just a matter of trying to be observant the next time the condition occurs.