Your sphenoid sinuses are located behind your nose and eyes. In fact, your sphenoid sinuses are the only sinuses located somewhat removed from your face, as they are located at the base of your brain, close to the ear. You have one sphenoid sinus for each side of your head and each sinus is about the size of a grape. The two sphenoid sinuses are positioned beside each other but are separated by a thin bone called the septum [source:

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Your sphenoid sinuses are located very close to nerves associated with sight. The sphenoid sinuses are also close to large blood vessels that are responsible for bringing blood to and from the brain. Symptoms of sphenoid sinusitis typically include headaches, coughing, pain behind the eyes or across the forehead, fever, and nasal discharge. Symptoms may also be worse when you lie on your back or when you bend forward.