Question: Whenever I get a checkup with my eye doctor, they have me sign a consent-to-treat form. I never gave it a second thought. This last visit, though, the front desk attendant was distracted when I arrived. They knew I was there waiting, but forgot to have me sign. They called me for my appointment, I finished, paid, checked out with the same attendant at the desk, and left.

Now I'm wondering. Does it matter that they forgot to have me sign? Obviously I consented to being treated because I'm an adult and I willingly went with the doctor, so what's the need to have me sign? Is there some reason that an optometrist in the US would have me sign for my consent to treat at every visit when my implied consent is so obviously given? Is there some liability that they reduce by getting my physical signature?

Answer: 
If they do a treatment which you didn't consent to and harm you they risk being sued. As such, they get you to sign a consent form to prove you agreed to the treatment and were informed about what was going on and any material risks.

They had implied verbal consent from you which is enough legally, but there's a risk that if it went wrong you could claim you didn't consent, they lied about what treatment you had, and they illegally did it.

As an example, they often drip something into your eye which can sometimes cause blurry vision for a while. If you had blurry vision after and crashed your car you could theoretically sue them because you say you didn't consent to that and they have no proof you did.
One vital word is missing from here: **informed** consent.

This means the health care provider has given you the relevant information about risks etc., which is precisely the sort of thing that you *wouldn't* otherwise know, so that when you decide, you know what you're actually choosing to consent to. Such consent should be properly documented, particularly what/how/when information was given.

Uninformed consent is legally invalid.

See for example, positions of the [AMA](https://www.ama-assn.org/delivering-care/ethics/informed-consent), [NHS](https://www.nhs.uk/conditions/consent-to-treatment/), [CMPA](https://www.cmpa-acpm.ca/en/advice-publications/handbooks/consent-a-guide-for-canadian-physicians), [ACSQHC](https://www.safetyandquality.gov.au/our-work/partnering-consumers/informed-consent), [NZMOH](https://www.health.govt.nz/your-health/services-and-support/your-rights). This is universal, per [WHO](https://www.who.int/news-room/fact-sheets/detail/human-rights-and-health).

The documentation serves multiple purposes, including at least:

1. Protection in the case the patient later decides to claim that they didn't consent (or their expressed consent was invalid).
2. Proper medical records for future reference, either by the same provider or a different one.
3. Following procedural rules that most, if not all, physician regulators require as part of professional codes of conduct.

The necessity of having written paperwork is proportional to the risks involved in the procedure. Taking blood pressure? Nobody is going to care\*. Putting stuff in someone's eye that does have possible side effects and limitation on driving? Yep.

\*: And it's not like that's risk-free. You can turn a blood pressure cuff into a [tourniquet](https://en.wikipedia.org/wiki/Tourniquet) and cause damage, but at that point you just sue the device manufacturer for defect and/or provider for blatant malpractice, regardless of consent.