The ADA requires that an employer provide employees with disabilities equal access to whatever health insurance coverage is offered to other employees. Your employer cannot deny you coverage that is made available to other employees or impose additional costs or restrictions on you because you are disabled. Depending on the level of coverage, however, the offered coverage may or may not fully cover the medical needs you have as a result of your disability.
The ADA also does not affect clauses contained in health insurance policies about pre-existing condition even though such clauses may adversely affect employees with disabilities more than other employees.