Failing to inform your insurer of any pre-existing medical conditions means that you would not be covered in the event of a claim related to said medical condition and in some cases, your policy may be considered null and void. In the event that a claim arises, you would be expected to cover the costs of any associated expenses.
To avoid this, you should make sure that all of your medical conditions are disclosed before purchasing your policy. This applies to all travellers with a pre-existing medical condition.