Generally, physicians have a legal duty—or responsibility—to inform you of all the risks involved in any given medical procedure you might undertake so that you have the opportunity to thoroughly assess the procedure and determine whether it is a procedure you want to go through or whether the potential risks outweigh the benefits. Therefore, if you consent to a procedure and a type of injury occurs that the doctor didn't inform you was a potential consequence of the procedure, the doctor maybe liable for not giving you enough information to make an informative decision. Another way of looking at it is that the consent you gave to the doctor to perform the procedure was invalid because, had you known what the doctor failed to tell you, you might not have consented to do the procedure in the first place.
The one exception to this rule involves a situation where you would have consented to the procedure regardless of not knowing a potential risk that the doctor failed to inform you of. For example, let's say you have severe lung cancer and will die shortly if an operation doesn't occur to fix the problem, but the doctor fails to tell you that you might experience some complications in breathing as a consequence of the surgery. If you do in fact experience complications in breathing, it it would be more than difficult to convince a judge and jury that you wouldn't have consented to a surgery that was necessary to save your life had you known that you might have some problems breathing as a result of it.
Doctors may also be liable for going beyond the scope of the procedure that you actually consented to. For example, lets say you give consent to the doctor to perform an operation on ligament A of your knee, but while conducting the surgery, the doctor notices something wrong with muscle B in your knee and decides to fix that up as well. If you hadn't consented to the additional surgery on muscle B, and an injury occurs as a result of that operation, the doctor may be liable for taking extra steps that you were unaware of. A real case involved a woman who, because of her religion, consented to the procedure only on the condition that no men were in the operating room. Thus, when a male nurse came into the room to part in the procedure, the hospital was liable for going beyond the scope of the consent that was given.
There is one big exception to this general rule, and that involves emergency situations that require—without your consent—the additional act of a doctor in order to save your life or prevent serious injury. The rationale behind this exception is that society would rather have a doctor go beyond the scope of their consent to prevent death or serious injury than stop short of doing so out of fear that they will be sued. After all, doctors are experts in the complicated field of medicine, and some flexibility is therefore required in order for them to do their job correctly and act upon dangerous, unexpected situations that might occur.