Differential Diagnosis for Purulent Nose and Sinuses Discharge

Single Most Likely Diagnosis

• Pseudomonas aeruginosa infection: This is likely due to the exposure to seawater, which can be contaminated with Pseudomonas species. The characteristic "dead fish" smell is consistent with infections caused by this bacterium, which is known to produce a blue-green pigment (pyocyanin) and a sweet, corn tortilla-like or fishy odor due to its metabolic byproducts.

Other Likely Diagnoses

• Staphylococcus aureus infection: Common in skin and soft tissue infections, S. aureus can also cause sinusitis, especially in the context of compromised nasal mucosa from seawater exposure.
• Vibrio species infection: Since the patient was swimming in the Arabian Gulf, exposure to Vibrio species (found in marine environments) could lead to infections, though these are more commonly associated with wound infections or gastrointestinal symptoms.
• Mixed bacterial infection: Given the chronic nature of the symptoms and the exposure to seawater, a polymicrobial infection involving both aerobic and anaerobic bacteria is possible.

Do Not Miss Diagnoses

• Mucormycosis: Although rare, this fungal infection can cause severe sinusitis, especially in immunocompromised individuals. It's crucial to consider this diagnosis due to its high mortality rate if left untreated.
• Nocardia infection: Nocardia species can cause sinusitis, and while less common, this diagnosis should not be missed due to its potential for severe disease, especially in immunocompromised patients.

Rare Diagnoses

• Marine fungal infections (e.g., Alternaria, Aspergillus): Fungal infections from marine exposure are rare but could be considered in the differential diagnosis, especially if the patient is immunocompromised or if other treatments fail.
• Leptospirosis: Although primarily known for causing a systemic illness, leptospirosis could potentially cause sinusitis symptoms in rare cases, especially if the patient was exposed to contaminated water.

Treatment and Empiric Antibiotic Therapy

Given the likely bacterial cause, empiric antibiotic therapy could include:

• Ciprofloxacin or Levofloxacin for their coverage of Pseudomonas aeruginosa and other potential bacterial pathogens.
• Trimethoprim-Sulfamethoxazole as an alternative, considering local resistance patterns.
• Clindamycin or Amoxicillin-Clavulanate for broader coverage, including anaerobes and Staphylococcus aureus.

It's essential to culture the discharge to guide targeted antibiotic therapy and to consider consulting an infectious disease specialist or an otolaryngologist for further management.