how do eye issues work with HMO

I am currently on a PPO but employer is thinking of switching to HMO.  I recently had a regular eye exam and the doctor referred me to a retina specialist for macular degeneration.

How would this have worked on an HMO?  Could I have seen the opthamologist without a referrel from my PCP?  Could I go to the retina specialist without a referrel from my PCP?