One consultation that we often get is related to the an isolated, opacified maxillary sinus - having a "cheek sinus" that is totally (or almost totally) full but without disease in the other sinuses. In the setting of chronic sinusitis, it is certainly possible to have an opacified maxillary sinus but it is usually in the context of disease all throughout the sinuses. When the disease is localized to one sinus - for the purposes of this discussion, the maxillary sinus - the problem is less likely be a generalized or systemic condition like chronic sinusitis, which is an inflammatory condition of the sinuses that typically affects sinus on both sides and usually affects more than just one single sinus.
The descriptor "opacified" is a radiology term and essentially means "full". But this also means that when we are talking about an opacified maxillary sinus, we are talking about a finding on imaging, most commonly a sinus CT scan although it could be an MRI as well. When a patient has a single opacified maxillary sinus, the first question in our minds is to ask what is causing the opacification to occur. If a maxillary sinus is opacified, it means that there is "stuff" in it. What is that "stuff" and how did it get in there? The "stuff" could be mucus, pus, fungus, polyps, or even tumor. So the "why" of an opacified maxillary sinus is most commonly related to either infection, blockage of the sinus (preventing mucus or pus from draining) or tumor. Although imaging can give us a hint about what the "stuff" is, it's hard to make definitive statements about the cause of the maxillary sinus opacification based on imaging alone. An MRI can provide more information than a CT scan about whether the "stuff" is liquid (e.g. mucus) or soft tissue (e.g. tumor) but even then the cause may not be completely clear.
Infection as the underlying etiology for the opacified maxillary sinus is typically bacterial in nature. This could be odontogenic (dental) or it could just be the result of a sinus infection that never clears. The odontogenic cause is much more common (at least in our experience) than a sinus infection that never clears. Odontogenic maxillary sinusitis can be especially hard to treat. The bacteria in the mouth, which get seeded into the maxillary sinus for example due to infection around a tooth root or from dental work, can cause an especially inflammatory response in the sinuses, so much so that it is common for the infection to cause the opening of the maxillary sinus to swell completely shut causing the infection in the maxillary sinus to fester and drain too slowly for it to ever fully resolve. In these cases where it is a bacterial etiology, patients often complain of foul tasting, malodorous drainage from side of the affected maxillary sinus. Patients also commonly describe pain or pressure in that cheek. We also perform a nasal endoscopy to help confirm the suspicion of bacterial infection by visualizing pus draining from the area of the maxillary sinus drainage pathway. The goal of treatment when a bacterial etiology is suspected is to clear the infection and improve drainage from the maxillary sinus with aggressive medical treatment and if that doesn't work, then with endoscopic sinus surgery to open the maxillary sinus (and wash it out). Once the maxillary sinus is addressed and cleaned out, any underlying dental problem also needs to be addressed.
Fungus can also cause opacification of the maxillary sinus. This is usually in the form of a "fungal ball" (a.k.a., mycetoma). It's unclear why fungal balls form but sometimes fungus in the sinuses can begin to grow within the sinus, forming a thick ball of fungus that resembles a glob of peanut butter. This in itself is typically not a big problem since usually the fungus is typically growing within the sinus and is not invasive. The problem arises because the body responds to the overgrowth of the fungus with inflammation, which leads to chronic mucus production. Moreover, the bacteria that live in the sinuses also may use the fungal ball as a scaffold and growth media to set up a low grade, chronic bacterial infection on top of the fungal ball. Many of the patients I've seen with fungal balls will complain of a chronic post-nasal drainage from the side of the fungal ball that is due to chronic mucus production or chronic drainage of pus being caused by the fungal ball. Unfortunately, the only way to definitively take care of a fungal ball is to open the maxillary sinus with endoscopic sinus surgery and physically remove/washout the fungus. When I explain why we need to do this to my patients, I often describe the fact that these fungal balls resemble a glob of thick peanut butter and that there is virtually no chance for this to drain out through the approximately 1-2mm wide drainage pathways of the sinuses.
The maxillary sinus can also become opacified through non-microbial mechanisms for blockage of the sinus, causing build up of mucus within the sinus. Sometimes we believe, negative pressure is created in the maxillary sinus and this causes the drainage pathways of the sinus to swell shut. It's analogous to what happens when you suck too hard through a straw - it collapses. In these cases, the maxillary sinus (or its drainage pathway) may be described as atelectatic (i.e. collapsed, or sucked in). An atelectatic maxillary sinus may also be associated with a condition called silent sinus syndrome, in which there is so much negative pressure in the maxillary sinus that it pulls the roof of the maxillary (the orbital floor) downwards and eventually patients may notice that one eye sits lower than the other. Management of this typically includes using intranasal steroids to see if reduction of inflammation in the nose and sinus drainage pathways may allow the maxillary sinus to open up enough to relieve the negative pressure which is causing the problem. More often than not, however, endoscopic sinus surgery is needed to open the maxillary sinus. The good news in relation to silent sinus syndrome is that (we've shown in a past study) opening the maxillary sinus may lead to the orbital floor rising back up.
The last consideration in relation to an opacified maxillary sinus is the possibility of a tumor. As I described above, there's usually no way to definitively know based on imaging whether the opacified maxillary sinus is being caused by a tumor that's either filling the sinus or causing the sinus to be blocked. And while most tumors that grow in the sinuses are benign, many of these tumors have "malignant potential" (i.e. chance to turn into cancer if you leave it long enough). This is one of the main reasons (aside from eradicating a festering infection sitting right under the eye) that surgical exploration may be recommended for a persistently opacified maxillary sinus.
Every patient is (of course) different and a unique, patient-specific treatment plan is the key to the successful management of any sinus/nasal condition. However, I hope the above gives you some indication of general thoughts that may cross our minds when evaluating and managing an isolated, opacified maxillary sinus.