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: In 3 patients a pre - existing neuralgia increased to excruciating intensity and in 2 of these cases myoclonus occurred simultaneously .
### Output Text:
In 3 patients a pre - existing neuralgia increased to excruciating intensity and in 2 of these cases myoclonus occurred simultaneously . 