You are not legally required to let them know, but:1) if you don't tell them and they let you go, you can't claim any protection from the laws prohibiting discrimination against those with disabilities;2) You can't ask for any accommodations for your condition if you don't notify your employer; and3) If you injure yourself or another because of your condition, damage or lose company property, or cost the company money (such as by costing them a sale or failing to complete a project), you would likely be solely liable.