Could one synapse be all it takes:

Terry Wallis, Mountain View, Arkansas:

Coma man wakes after 19 years
Coma man wakes after 19 years | Daily Mail Online

Rom Houben, Belgium:

'I screamed, but there was nothing to hear': Man trapped in 23-year 'coma' reveals horror of being unable to tell doctors he was conscious
Rom Houben: Patient trapped in a 23-year 'coma' was conscious all along | Daily Mail Online

Time's Top Ten Coma Cases:
Complete List - Top 1 Comas - TIME