Wells versed in neurology here and thought I could share some facts to answer some common questions about comas.
1) Dreams. The dreams you remember when you come out of a coma mean nothing. It is most likely that those dreams and associated memories were formed either seconds before waking up or as you were waking up. You swear your grandma was talking to you the whole time? False, your mind filled in the gaps of some memories as you awoke. The fact is that it doesn't take time to experience those triggered memories. Once the right spots were triggered your mind experiences the entire memory at once, not like the storyline we normally associate dreams with. So when you wake up and your mind quickly puts some pieces together you might feel as if those thoughts had developed and come together over some period of time. Tl;dr you don't dream in comas and we can prove it with imaging
2) Length of time in coma. When a patient goes into a coma you can usually expect something to happen by around 2 weeks. At this time they usually wake up, go into vegetative state, or go brain dead
3) Vegetative state. The state of being awake but unaware. This is not the same as a coma. This person may show eye movements, reflex responses, and may even reflexively scratch an itch. However, they are not aware/conscious. This person can sometimes react to a physical stimulus such as a pinprick because the components of the nervous system for delivering, processing, and motor response to that stimulus are still intact. However the components that make you aware of the stimulus are gone.
4) how do you get a coma? Comas usually require extensive bilateral damage, as in it has to be on both sides of your brain. The smallest lesion though, and the site of most lesions that cause a coma, is at your midbrain-pons junction of your brain stem. Here you have your Ascending Reticular Activating System, or ARAS. ARAS, along with a few other sub cortical nuclei, are responsible for mediating consciousness. So a lesion at ARAS is certainly going to prevent consciousness