What does an air‑fluid level in the maxillary sinus on imaging signify in a patient with recent upper respiratory tract infection symptoms (facial pain, nasal congestion, purulent discharge, or hyposmia), and how should it be managed?

Air-Fluid Level in Maxillary Sinus: Clinical Significance and Management

An air-fluid level in the maxillary sinus on imaging in a patient with recent upper respiratory tract infection symptoms is a specific radiologic sign of acute bacterial rhinosinusitis (ABRS), but imaging should not be routinely obtained for uncomplicated cases, as the diagnosis is clinical and treatment decisions should be based on symptom duration and pattern rather than radiographic findings. 1, 2

Clinical Context and Diagnostic Approach

When Imaging is NOT Indicated

• Uncomplicated acute rhinosinusitis (<4 weeks duration) should be diagnosed clinically without any imaging, as recommended by both the American College of Radiology and the American Academy of Otolaryngology-Head and Neck Surgery 1, 2
• Up to 40% of asymptomatic adults show abnormalities on sinus CT scans, and over 80% with minor upper respiratory tract infections have imaging findings 1
• The decision to treat with antibiotics should be made on clinical grounds alone, not based on radiographic extent of disease 1

Clinical Diagnosis of ABRS

ABRS is diagnosed when the patient has:

• Purulent nasal discharge with nasal obstruction, facial pain, or pressure 1
• Either: Symptoms persisting ≥10 days without improvement, or symptoms that initially improve then worsen within 10 days (double-sickening pattern) 1
• Duration <4 weeks 1

Radiologic Significance of Air-Fluid Levels

Specificity of the Finding

• Air-fluid levels are highly specific for acute bacterial sinusitis but are only seen in approximately 60% of cases 3
• Air-fluid levels, along with complete sinus opacification, are more specific findings than simple mucosal thickening 1, 3
• The "Fizz sign" (air mixed with fluid appearing as bubbles, resembling carbonated cola) has been described as a CT finding in acute bacterial sinusitis 4
• Air density measurements can help differentiate acute from chronic rhinosinusitis: mean air density is significantly higher in acutely inflamed sinuses (-846.6 HU) compared to chronic sinusitis (-980 HU) 5

Limitations and Pitfalls

• There is significant discrepancy between radiologic diagnosis of sinusitis and microbiologic confirmation on antral puncture 1
• Mild-to-moderate mucosal thickening is nonspecific and does not reliably indicate bacterial infection 1
• Standard radiography has limited sensitivity (88% with five views) and cannot adequately visualize ethmoid sinuses 1

When Imaging IS Indicated

Appropriate Clinical Scenarios

Imaging should be obtained in:

• Persistent sinusitis despite adequate medical therapy 2
• Recurrent acute rhinosinusitis (≥4 episodes per year) 1, 2
• Suspected complications: orbital involvement (proptosis, diplopia, periorbital swelling), intracranial extension (altered consciousness, severe headache, seizures), or sepsis 1, 6, 2
• Preoperative evaluation for functional endoscopic sinus surgery 1
• Special populations: ICU patients with nasotracheal tubes or nasogastric tubes, immunocompromised patients, or those with head trauma 1, 6, 2

Imaging Modality Selection

• Non-contrast CT is the gold standard when imaging is indicated, providing superior anatomic detail compared to plain radiography 2, 3
• CT with IV contrast is reserved for suspected complications (orbital or intracranial) 2
• MRI is indicated only for suspected fungal sinusitis, neoplasm, or intracranial complications 1, 2
• Ultrasound has poor sensitivity (39-61%) and specificity (42-53%) and is not recommended, though it may have limited utility in pregnant women to avoid radiation 1, 7

Management Based on Clinical Presentation

Uncomplicated ABRS (No Imaging Needed)

Treatment algorithm:

1. Symptoms 5-10 days without worsening: Likely viral—supportive care, no antibiotics 1
2. Symptoms ≥10 days without improvement OR double-sickening pattern: ABRS—consider antibiotics 1
   • First-line: Amoxicillin 1500 mg three times daily for 10 days 1
   • Alternative: Azithromycin 500 mg once daily for 3 days 1, 8
   • Amoxicillin-clavulanate 500/125 mg three times daily for 10 days for treatment failures 1

When Air-Fluid Level is Documented on Imaging

If imaging was obtained (appropriately or inappropriately) and shows air-fluid level:

• This confirms acute inflammation but does not change management if the clinical picture is uncomplicated ABRS 1
• Treat based on clinical criteria (symptom duration and pattern) as outlined above 1
• Do not use imaging findings to decide between viral versus bacterial etiology 2

Critical Care/Sepsis Context

In ICU patients or those with sepsis:

• Suspect nosocomial sinusitis in any patient with nasotracheal tube, nasogastric tube, or head injury 1, 6
• Obtain maxillary sinus radiograph first; if negative, proceed to CT 1
• If fluid is demonstrated, perform antral puncture for culture and therapeutic drainage BEFORE starting antibiotics 1, 6
• Predominant pathogens are gram-negative enterobacteria and gram-positive cocci 6

Key Pitfalls to Avoid

• Do not order imaging for routine uncomplicated acute rhinosinusitis—this leads to overdiagnosis and unnecessary antibiotic use 1, 2
• Do not assume all radiographic abnormalities represent bacterial infection requiring antibiotics—correlation with clinical duration and pattern is essential 1
• Do not rely on nasal swabs or aspirates for diagnosis—they cannot differentiate colonization from true sinus infection 6
• Do not miss dental pathology—up to 20% of maxillary sinusitis originates from dental disease 1
• Do not dismiss subtle signs in immunocompromised patients—maintain lower threshold for imaging and consider invasive fungal infection 2, 3