In the rare event of an emergency, if you cannot obtain consent, you can provide treatment, take action or make a referral that is in the patient’s best interests and is needed to save their sight or prevent deterioration in the patient’s condition (this applies to children, young people and adults).
The exception to this is where a valid and applicable advance decision to refuse a particular treatment or healthcare more generally is in place. In these circumstances, you would need to respect the wishes of the patient which could mean not taking any action. These advance decisions are very unlikely to apply in the optical context, but for more information see the relevant incapacity legislation and its code of practice or ask your professional indemnity insurance provider or a legal adviser.