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: PURPOSE : To determine the natural history of visual field defects in a group of patients known to have Vigabatrin - associated changes who elected to continue the medication because of good seizure control .
### Output Text:
PURPOSE : To determine the natural history of visual field defects in a group of patients known to have <span class="chemical">Vigabatrin</span> - associated changes who elected to continue the medication because of good seizure control . 