You should promptly report the incident to your supervisors. Unless you have been charged with a crime, I cannot see a need to retain an attorney just yet. If the patient is mentally ill, it's probably not the first time your superiors have dealt with a similar situation - especially if it arises because you were doing your job correctly. Your company may have attorneys on stand-by for such things but check with your company first.
This is not legal advice nor does it create an attorney-client relationship. This is for education and informational purposes only. It is always recommended that you contact an attorney with any concerns as each individual case is unique.
This is a tough question to answer with the limited amount of information you've provided. But what I will say is that if this person has filed or made an official complaint againt you then you will want to consider consulting an attorney before responding.
As the other attorneys have stated, it is not clear that this has even been reported outside of your organization. Whether it has or not, many Seattle criminal defense attorneys will offer a free consultation. You might talk to a couple, and then you could provide more facts than you could put in an open forum like this one. Many people are mandatory reporters who would be required to report an assault. It is usually not up to them to decide if it happened or not. That would be a job for the police/prosecutor/attorney general depending on circumstances. Don't say anything more on here without at least consulting with an attorney.