In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The current study investigated the visual fields and visual electrophysiology of eight patients with known vigabatrin - attributed visual field loss , three of whom were reported previously .
### Output Text:
The current study investigated the visual fields and visual electrophysiology of eight patients with known <span class="chemical">vigabatrin</span> - attributed visual field loss , three of whom were reported previously . 