Small mucous retention cyst in the left sphenoid sinus what can be the cause?

Retention cyst: Hello, cysts are commonly found within the paranasal sinuses. It is believed that they occur from an occlusion of an outflow tract from a mucus secreting gland. This gland continues to produce mucus and before long it swells like a water balloon. These cysts can be varying sizes and as long as they don't occlude the sinus ostea, they are just identified and observed. No surgery is needed.