In rare instances, these infections may involve the orbit, causing orbital cellulitis, which may in turn result in blindness, or determine central nervous system complications such as meningitis, subdural empyema, brain abscess and life-threatening cavernous sinus thrombosis.Infection of the eye socket is a rare complication of ethmoid sinusitis, which may result in the loss of sight and is accompanied by fever and severe illness. Another possible complication is the infection of the bones (osteomyelitis) of the forehead and other facial bones – Potts puffy tumor. Causes
Maxillary sinusitis may also develop from problems with the teeth, and these cases were calculated to be about 40% in one study and 50% in another. The cause of this situation is usually a periapical or periodontal infection of a maxillary posterior tooth, where the inflammatory exudate has eroded through the bone superiorly to drain into the maxillary sinus.An estimated 0.5 to 2.0% of viral rhinosinusitis (VRS) will develop into bacterial infections in adults and 5 to 10% in children. Acute
Acute sinusitis is usually precipitated by an earlier upper respiratory tract infection, generally of viral origin, mostly caused by rhinoviruses (with RVA and RVC giving more severe infection than RVB), coronaviruses, and influenza viruses, others caused by adenoviruses, human parainfluenza viruses, human respiratory syncytial virus, enteroviruses other than rhinoviruses, and metapneumovirus. If the infection is of bacterial origin, the most common three causative agents are Streptococcus pneumoniae (38%), Haemophilus influenzae (36%), and Moraxella catarrhalis (16%). Until recently, H. influenzae was the most common bacterial agent to cause sinus infections.
In addition to the severity of disease, discussed below, sinusitis can be classified by the sinus cavity it affects:

Maxillary – can cause pain or pressure in the maxillary (cheek) area (e.g., toothache, or headache) (J01.0/J32.0)
Frontal – can cause pain or pressure in the frontal sinus cavity (located above the eyes), headache, particularly in the forehead (J01.1/J32.1)
Ethmoidal – can cause pain or pressure pain between/behind the eyes, the sides of the upper part of the nose (the medial canthi), and headaches (J01.2/J32.2)
Sphenoidal – can cause pain or pressure behind the eyes, but is often felt in the top of the head, over the mastoid processes, or the back of the head. Complications
Complications are thought to be rare (1 case per 10,000).The proximity of the brain to the sinuses makes the most dangerous complication of sinusitis, particularly involving the frontal and sphenoid sinuses, infection of the brain by the invasion of anaerobic bacteria through the bones or blood vessels. Abscesses, meningitis, and other life-threatening conditions may result. In extreme cases, the patient may experience mild personality changes, headache, altered consciousness, visual problems, seizures, coma, and possibly death.Sinus infection can spread through anastomosing veins or by direct extension to close structures. Orbital complications were categorized by Chandler et al. into five stages according to their severity (see table). Contiguous spread to the orbit may result in periorbital cellulitis, subperiosteal abscess, orbital cellulitis, and abscess.