This will really depend on the what your POA states. You may want to have an attorney look into this issue.
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Without knowing the extent of your mother's dementia it is difficult to give you a real good answer. Depending on whether the POA is in the chart and what it says, it may not be illegal, but the question is should they have done it without your consent... probably not. What is not included in your fact pattern is whether any issues or problems arose as a result of the procedure. If not, the best advice I can give you is to discuss this with her primary care doctor and make sure they flag the chart before any other procedures are done without you being notified.
This question might be better addressed by a health care attorney.
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I agree that if your powers extend to medical procedures you should have been consulted for consent. However, if in the end this is a necessary procedure there is no physical harm and it appears that you have set the record straight and hopefully this will not happen again. Your mother is lucky to have you. God bless.
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