@Amperage,
There are a number of such neurological deficiencies by which a person can lose their memory functions from injury onward, or from injury retroactively. It must be kept in mind that there are a number of types of memory, and memory 'lays' and 'routes,' so the autobiographical memory can be even one which is not registered by a direct state of consciousness, but, nevertheless, can be identified as a memory based on information obtained through testing.
Anosognosia in Alzheimer's is one such case, were autobiographical memory causes the patient to not keep up with (in the sense of being aware of through active, up-to-date acknowledged cognition in the state of consciousness) his or her 'selfness.' In such cases as these, and others, the former 'self,' to the person, evidences a lack--
and thus we can basically say that the present, up-to-date person is not 'them.'
This is, although, just a philosophical distinction, and is best understood as meaning that that patient is not who they would have been if no brain damage had occurred...and that's all, otherwise, they are still just themselves, in a less 'conscious' state. (meaning the threshold of consciousness has been damaged, and thus reduced, so that it is not as full in content and context as before the damage)