Acute sinusitis is inflammation of the paranasal sinus mucosal lining lasting less than 4 weeks. It most commonly develops as bacterial superinfection following a viral upper respiratory tract infection; Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis are the most frequent pathogens. The maxillary and ethmoid sinuses are most commonly affected. Imaging characteristically shows air-fluid levels, mucosal thickening, and sinus opacification. Imaging is generally not required for uncomplicated cases; however, CT and MRI are indicated when orbital or intracranial complications are suspected.
Age Range
5-80
Peak Age
35
Gender
Equal
Prevalence
Very Common
Acute sinusitis typically begins with a viral upper respiratory infection; the viral infection causes mucosal edema and ciliary dysfunction, leading to obstruction of the ostiomeatal complex. Ostiomeatal obstruction impairs sinus ventilation and mucociliary clearance, creating negative pressure within the sinus and facilitating secretion accumulation. This stasis environment creates ideal conditions for bacterial colonization and superinfection. Bacterial infection causes intense neutrophilic infiltration and vascular congestion in the mucosa, further increasing mucosal thickening and leading to purulent material accumulation. On imaging, the air-fluid level reflects gravitational pooling of purulent secretions at the sinus floor; mucosal thickening is a direct consequence of inflammatory edema and vascular congestion.
A sharp transition between air and fluid in the form of a straight line within the paranasal sinus is the most characteristic and specific finding of acute sinusitis. This finding reflects gravitational pooling of purulent or serous secretions at the sinus floor. It is best visualized on upright lateral radiography or coronal CT. In chronic sinusitis, the sinus is usually completely opacified so an air-fluid level is not expected. Air-fluid levels can rarely also be seen in hemosinus due to barotrauma or sinus fracture — clinical correlation is required.
On non-contrast CT, an air-fluid level is seen within the sinus as a straight horizontal line. The fluid component typically measures 10-25 HU suggesting serous secretions, while 25-40 HU suggests denser purulent content. On upright or prone positioning, gravitational effect causes fluid to pool at the sinus floor. It is most commonly and easily recognized in the maxillary sinus. The air-fluid level is a strong indicator of an acute process and plays an important role in differentiating from chronic sinusitis.
Report Sentence
An air-fluid level is noted in the left maxillary sinus, consistent with acute sinusitis.
Smooth, concentric mucosal thickening lining the sinus walls is seen. Normal sinus mucosa is too thin to be visible on CT (<1 mm), and thickening greater than 3 mm is considered pathologic. In acute sinusitis, thickening is typically smooth and homogeneous; polypoid or irregular thickening is more suggestive of chronic processes. The thickened mucosa appears at soft tissue density (30-60 HU) and shows marked enhancement on contrast-enhanced studies. Sometimes mucosal edema is so prominent that it can completely opacify the sinus.
Report Sentence
Smooth mucosal thickening is noted in bilateral maxillary sinuses, consistent with inflammatory/infectious sinusitis.
Contrast-enhanced CT evaluates orbital complications. According to the Chandler classification: Stage I preseptal cellulitis (periorbital soft tissue thickening), Stage II orbital cellulitis (haziness and streaky increased density in intraorbital fat planes), Stage III subperiosteal abscess (biconvex, rim-enhancing collection between the medial orbital wall and periorbita), Stage IV orbital abscess (intraconal or extraconal abscess formation), Stage V cavernous sinus thrombosis (filling defect and expansion of the cavernous sinus). It most commonly develops as a complication of ethmoid sinusitis because the lamina papyracea is a very thin bony barrier.
Report Sentence
A subperiosteal abscess formation is noted along the left medial orbital wall with lamina papyracea defect, secondary to left ethmoid sinusitis.
On T2-weighted MRI, acute sinusitis secretions demonstrate markedly hyperintense signal. Serous secretions show very bright T2 signal due to high water content, while purulent secretions display slightly lower but still hyperintense signal as protein content increases. Mucosal thickening also appears hyperintense on T2 and is easily distinguished from normal bone marrow. Inflammatory mucosal changes and retention cysts may show similar signal characteristics. T2 imaging is complementary to CT, particularly in evaluating orbital and intracranial complications.
Report Sentence
Fluid retention and mucosal thickening showing hyperintense signal on T2-weighted sequences is noted in the right maxillary sinus, consistent with acute sinusitis.
On contrast-enhanced T1-weighted MRI, inflamed mucosa demonstrates marked and homogeneous enhancement. Enhancement is seen as a smooth band along the sinus wall and can be clearly distinguished from non-enhancing secretions in the sinus lumen. This pattern is called the 'tramline sign' — enhancing mucosa externally and non-enhancing secretion internally form two parallel layers. In complicated cases, perimucosal spread, enhancement in orbital fat planes, and meningeal enhancement should be investigated. Diffusion-weighted imaging (DWI) shows restricted diffusion in purulent collections and aids in abscess diagnosis.
Report Sentence
Mucosal enhancement is noted in bilateral ethmoid sinuses on contrast-enhanced T1-weighted sequences, with non-enhancing secretions in the sinus lumen.
Ultrasonography plays a limited role in paranasal sinus evaluation but can be used as a radiation-free alternative, particularly in pediatric patients and pregnant women. In normal pneumatized sinuses, a strong echo from the anterior wall with air-bone reflection is seen and the posterior wall cannot be visualized (total reflection). When the sinus fills with fluid, an acoustic window is created and the posterior wall echo becomes visible — this is known as the 'sinus ultrasonography finding'. Mucosal thickening >4 mm can appear as an anechoic or hypoechoic band behind the sinus anterior wall. Air-fluid levels can also sometimes be detected on B-mode imaging.
Report Sentence
Posterior wall echo is visible on right maxillary sinus ultrasonography, suggesting intrasinusoidal fluid accumulation.
On non-contrast CT coronal sections, ostiomeatal complex (OMC) obstruction is evaluated. The OMC consists of the maxillary sinus ostium, infundibulum, uncinate process, hiatus semilunaris, and middle meatus — a narrow anatomic region. In acute sinusitis, obstruction due to mucosal edema and secretion accumulation is seen in these structures. Infundibular pattern (single sinus obstruction) or OMC pattern (ipsilateral maxillary, anterior ethmoid, and frontal sinuses involved together) may be observed. Thin-section (≤1 mm) coronal CT is the gold standard imaging modality for evaluating infundibular narrowing. Anatomic variants (concha bullosa, Haller cell, septal deviation, paradoxical middle turbinate) are predisposing factors for OMC obstruction.
Report Sentence
The right ostiomeatal complex is obstructed due to mucosal edema, with secretion accumulation in the ipsilateral maxillary and anterior ethmoid sinuses.
Complete opacification of the sinus with soft tissue density indicates advanced sinusitis. Complete opacification results from the combination of secretion accumulation and marked mucosal thickening. Density measurements provide clues about etiology: low density (10-25 HU) suggests serous, moderate density (25-40 HU) purulent, and high density (>60 HU) fungal mucin or desiccated secretions. Complete opacification alone does not differentiate acute from chronic sinusitis; clinical duration and bone changes (sclerosis, remodeling) are helpful in this distinction. Bilateral widespread opacification suggests polyposis or fungal sinusitis, while single sinus opacification is more consistent with acute bacterial sinusitis or mucocele.
Report Sentence
The left maxillary sinus is completely opacified with homogeneous soft tissue density content; acute sinusitis is the primary consideration.
Criteria
Symptoms <10 days, nasal congestion, clear-mucoid rhinorrhea, mild facial pain. Usually self-limiting.
Distinct Features
Mild mucosal thickening on imaging, usually no air-fluid level. Mild-moderate sinus opacification may be present. CT is not indicated — clinical diagnosis is sufficient.
Criteria
Symptoms ≥10 days or worsening (double-worsening), purulent nasal discharge, facial pain/pressure, fever may be present. Requires antibiotic treatment.
Distinct Features
Air-fluid level, marked mucosal thickening, sinus opacification on CT. Purulent secretion density is typically 25-40 HU. On contrast-enhanced study, mucosa shows intense enhancement.
Criteria
Chandler classification: Stage I preseptal cellulitis, Stage II orbital cellulitis, Stage III subperiosteal abscess, Stage IV orbital abscess, Stage V cavernous sinus thrombosis. Ethmoid sinusitis is the most common source.
Distinct Features
On contrast CT: orbital fat plane haziness, lamina papyracea defect, rim-enhancing collections, extraocular muscle thickening, proptosis. MRI is superior for evaluating intracranial extension. DWI shows restricted diffusion in purulent collections.
Criteria
Epidural abscess, subdural abscess (empyema), brain abscess, meningitis, cavernous/sagittal sinus thrombosis. Frontal sinusitis is the most common source (Pott's puffy tumor). Mortality 5-10%.
Distinct Features
On contrast MRI: meningeal enhancement, epidural/subdural rim-enhancing collections, DWI restriction in brain parenchyma (abscess). Contrast MR venography shows sinus thrombosis. CT may show frontal bone osteomyelitis and Pott's puffy tumor (subperiosteal abscess in the forehead region).
Criteria
Unilateral maxillary sinusitis originating from dental infection (periapical abscess, periodontal disease, dental implant, oroantral fistula after tooth extraction). Accounts for 10-12% of all maxillary sinusitis cases.
Distinct Features
Unilateral maxillary sinus opacification + dental pathology adjacent to the sinus floor (periapical lesion, root canal widening, implant penetration) on CT. Focal bone defect or periosteal reaction in the sinus floor may be seen. Antibiotic treatment differs due to mixed anaerobic flora.
Distinguishing Feature
Chronic sinusitis shows bony wall sclerosis, osteoneogenesis, and mucosal polypoid changes; air-fluid level is usually absent. Clinical duration >12 weeks and bony wall thickening on CT are distinguishing features of chronic inflammation.
Distinguishing Feature
Retention cyst is a smooth-bordered, dome-shaped, mucosal-based lesion originating from the sinus wall but not completely filling the sinus. Usually incidental and does not show air-fluid level. Acute sinusitis shows diffuse mucosal thickening and possible air-fluid level.
Distinguishing Feature
Mucocele completely fills the sinus AND expands/remodels the sinus walls. In acute sinusitis, sinus walls maintain their normal size and shape. Mucocele is usually due to chronic obstruction and bony expansion is pathognomonic.
Distinguishing Feature
Sinonasal malignancies show bone destruction (erosion, lysis) — bone structures are intact in acute sinusitis. Tumoral mass shows heterogeneous enhancement and invasion to adjacent structures (orbit, pterygopalatine fossa, intracranial) may be seen. Unilateral mass + bone destruction = suspect malignancy.
Distinguishing Feature
Allergic fungal sinusitis shows hyperdense mucin (>60 HU) within sinuses and expansion/remodeling of multiple sinuses. T2 hypointense mucin on MRI is characteristic. Acute sinusitis shows low-moderate density secretions without bone remodeling.
Urgency
urgentManagement
medicalBiopsy
Not NeededFollow-up
Komplike olmayan akut bakteriyel sinüzit antibiyotik tedavisi ile 7-10 günde iyileşir. Tedaviye yanıtsız veya komplikasyon gelişen olgularda BT çekilmelidir. Orbital semptomlar (proptozis, oftalmopleji, görme kaybı) veya intrakranyal semptomlar (başağrısı, bilinç değişikliği, fokal nörolojik defisit) acil görüntüleme ve cerrahi konsültasyon gerektirir.Uncomplicated acute sinusitis generally resolves with medical treatment (amoxicillin-clavulanate first line). Imaging is indicated only when complications are suspected, in cases unresponsive to medical therapy, or before surgical planning. Orbital complications require urgent CT and ophthalmology/ENT consultation. Intracranial complications (subdural empyema, brain abscess, sinus thrombosis) may require emergent neurosurgical intervention. Recurrent acute sinusitis (≥4 episodes/year) is a candidate for endoscopic sinus surgery.
Acute sinusitis is typically treated with antibiotics. Complications (orbital cellulitis, intracranial abscess, cavernous sinus thrombosis) are rare but require emergency intervention. In recurrent episodes, underlying anatomical predisposing factors should be investigated.