The sphenoid sinuses are the third largest of the sinuses. They are either very small or absent at birth. They reach adult size by the late teen years. The sphenoid sinuses lie deep in the skull close to the optic nerve, behind the ethmoid sinuses. Infection in the sphenoid sinus can spread to blood vessels in the brain, causing a brain infection (abscess or encephalitis) and/or a deep vein clot within the brain. Sphenoid sinus pain is often felt around the eyes, behind the eyes and at the top of the skull.