August 2018 guidelines say about patients in vegetative states or unresponsive wakefulness syndrome, even after 3 or 12 months:
"a substantial minority ... will recover consciousness... While most of these patients will be left with severe disability... some will regain the ability to communicate reliably, perform self-care activities, and interact socially" (p.6)
They say doctors should stop talking about permanent states and tell people the "prognosis is not universally poor," though after 3 months for non-traumatic injuries (loss of oxygen to the brain) and 12 months for accident injuries (trauma), they should emphasize "the likelihood of permanent severe disability and the need for long-term assistive care" (p.4).
The guidelines also recommend transfer to a specialized center.
In 2018 a man was about to have his life support removed after 3 weeks in a diabetic coma, when his doctor father arranged to have him air-lifted to a specialized hospital, where he woke up in 4 days.
Videos include the following, and you can find many more. Several of these people were aware of what was happening when they were in comas. Most were not treated in specialized centers. A wise commenter said, "I hope they read to the patient." Talking books and music can fill a thirsty mind. Knowing the patient may be conscious makes everyone more helpful, considerate and respectful.
This page gives a very brief summary of the 2018 guidelines. Families with someone in a coma need to read the guidelines, the studies they cite, and any newer studies, to find what is relevant to their situation. Most doctors and hospitals are not experts in this area, which is why the guidelines recommend transfer to a specialized setting, and why the doctor above did so for his son.
A 2017 book for lay people is Into the Gray Zone ($2). A 2014 textbook for doctors, which some family members may want, is The Comatose Patient, $100 or available in a list of university libraries.
Researchers in Cambridge and Liege found that 5 of 54 patients in persistent vegetative states could respond to yes/no questions through MRI. "5 were able to willfully modulate their brain activity. In three of these patients, additional bedside testing revealed some sign of awareness, but in the other two patients, no voluntary behavior could be detected by means of clinical assessment. One patient was able to use our technique to answer yes or no to questions during functional MRI; however, it remained impossible to establish any form of communication at the bedside." They used functional MRI to measure responses. NEJM 2010, Monti et al.
Researchers in Paris found that 2 of 22 patients in persistent vegetative states could recognize patterns of sounds in ways that only conscious people can. "Interestingly, these 2 patients showed unequivocal clinical signs of consciousness within the 3 to 4 days following the experiments." The experiments measured responses with "high density scalp EEG," and they show a picture of the network of EEG sensors on a patient's head. Neurology, 2011, Faugeras et al.
In 2010 there were 300,000 US patients in a non-responsive state.
People with a severe disability are usually happy in their life, and say they have a good quality of life, so they want to continue. Their unhappiness, if any, comes from pain, fatigue, lack of control or purpose, and isolation. These can usually, not always, be helped by pain specialists, good care, assistive technologies which even respond to eye movement or breaths, and social connections. People who cannot move and are locked in their bodies without motion, can still spell or speak through assistive devices, use social media, listen to books, magazines, podcasts, radio, and maybe select music from a service like Pandora or Slacker, which adjusts itself to their choices.
The "disability paradox" is that people without a disability cannot imagine how life with a disability can be fun. Family members rate quality of life much lower than a disabled person herself does. Geriatricians say, "it is vanishingly rare that a patient reports to us a preference to be dead," no matter how badly disabled the patient is.