Are ALL of the various sinus cavities in an adult's head (i.e., maxillary, ethmoid, etc.) connected together by some mechanism -- or, instead, is each sinus an independent entity? Thus, is it (a) likely, or (b) unlikely that a bacterial infection in one sinus will result in the same bacteria invading the other sinuses?

Mark

The sinuses are interconnected, but the sinuses are also independent entities. You can certainly develop a bacterial infection in one sinus, which may or may not spread to adjacent sinuses. Both situations are fairly common (infection involving an isolated sinus and infection of multiple sinuses).

Just as the various regions of each lung are ultimately interconnected via the bronchi and trachea, the sinuses all drain into the nasal cavity. Infection can spread via this "nasal connection," and infection can also spread from one sinus directly to adjacent sinuses.

Sinuses drain into the nasal cavity through channels that are composed of bone (which in many areas is paper-thin) lined with mucosa (mucous membrane). These channels are very narrow. Infection leads to inflammation (tissue swelling), which in turn further narrows the drainage channel of the infected sinus. The drainage channel may even swell shut. This tends to isolate the infected sinus, and would probably be an effective boundary, except for the fact that the infection can still spread across the bony barriers between adjacent sinuses.

Many factors control whether you get sinusitis at all and whether your sinusitis is limited to a few sinuses or spreads to all sinuses. Nasal and sinus anatomy is important. There are a number of anatomical "problems" that may predispose you to sinusitis. Your own level of immunity is also very important. A number of other questions are very relevant. Do you have nasal allergies? Do you abuse your nasal mucosa with cigarette smoke (your own or other people's)? Have you had a cold recently? Do you work with the public?

Often, as with so many health problems, the reasons underlying a particular bout of sinusitis remain mysterious. Why does my left ear get clogged with wax, but not my right? Why did my right maxillary sinus get infected, but not my left? We look for explanations based on anatomy and physiology, but we do not always find satisfying answers.