Blind person property registration procedure by sub registerer

Hi Experts, Can anyone please clarify, what steps to be taken by sub registerer while old & blind  person executing sale deed / gift deed of immovable property? this blindness occurred due to old age & mentioned in the registration document as well. on top of that, executant put thumb impression on registration document instead of signature. Can legal heirs challenge the registered document? Are there any supreme court guide lines?