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Answer: In this instance, you should code the patient's symptom. The "Symptoms, Signs, and Ill-Defined Conditions" section of ICD-9 (780-799), specifically the category for symptoms involving nervous and musculoskeletal systems, directs you to the Disorders of the Eye and Adnexa section (360-379). There, under the category Visual Disturbances (368) you will find subcategory Subjective Visual Disturbances, with code 368.13 listed as visual discomfort, which includes asthenopia, among others. You could also potentially use diagnosis code 379.91 (Pain in or around eye).
You should list the circumstance that brought this on, the fireworks in this case, using an E code: E923.0 (Accident caused by explosive material; fireworks).