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 and a distinct odor.

Other Likely Diagnoses

• Staphylococcus aureus infection: This bacterium is commonly found in the nasal passages and can cause purulent discharge, especially in the context of repeated exposure to seawater.
• Streptococcus infection: Another common cause of sinus infections, which could be exacerbated by the introduction of seawater into the nasal passages.
• Anaerobic bacterial infection: The presence of a foul odor suggests the possibility of an anaerobic infection, which could be secondary to the introduction of seawater and subsequent bacterial overgrowth.

Do Not Miss Diagnoses

• Malignant otitis externa: Although less likely, this condition can present with foul-smelling discharge and is a serious infection that can spread to the skull base, particularly in immunocompromised individuals.
• Invasive fungal sinusitis: This is a rare but potentially life-threatening condition that could be considered, especially if the patient is immunocompromised or if the infection does not respond to conventional antibiotic therapy.

Rare Diagnoses

• Marine bacterium infection (e.g., Vibrio species): While less common, infections with marine bacteria are possible, especially given the patient's history of swimming in the Arabian Gulf.
• Parasitic infection (e.g., amoebiasis): Although rare, parasitic infections could be considered in the differential diagnosis, particularly if the patient has been swimming in contaminated water and has not responded to conventional antibiotic therapy.

Treatment should be guided by culture and sensitivity results when possible. Empirical antibiotic therapy may include coverage for Pseudomonas aeruginosa and other common pathogens. The use of antibiotics should be tailored to the specific diagnosis and susceptibility patterns. Additionally, addressing any underlying conditions that may predispose the patient to infection, such as immunocompromised states, is crucial.