Causes
There are three main causes of PVS (persistent vegetative state):

Acute traumatic brain injury
Non-traumatic: neurodegenerative disorder or metabolic disorder of the brain
Severe congenital abnormality of the central nervous systemPotential causes of PVS are:
Bacterial, viral, or fungal infection, including meningitis
Increased intracranial pressure, such as a tumor or abscess
Vascular pressure which causes intracranial hemorrhaging or stroke
Hypoxic ischemic injury (hypotension, cardiac arrest, arrhythmia, near-drowning)
Toxins such as uremia, ethanol, atropine, opiates, lead, dimethylmercury, endrin, parathion, and colloidal silver
Physical trauma: Concussion, contusion, etc. Seizure, both nonconvulsive status epilepticus and postconvulsive state (postictal state)
Electrolyte imbalance, which involves hyponatremia, hypernatremia, hypomagnesemia, hypoglycemia, hyperglycemia, hypercalcemia, and hypocalcemia
Postinfectious: Acute disseminated encephalomyelitis (ADEM)
Endocrine disorders such as adrenal insufficiency and thyroid disorders
Degenerative and metabolic diseases including urea cycle disorders, Reye syndrome, and mitochondrial disease
Systemic infection and sepsis
Hepatic encephalopathyIn addition, these authors claim that doctors sometimes use the mnemonic device AEIOU-TIPS to recall portions of the differential diagnosis: Alcohol ingestion and acidosis, Epilepsy and encephalopathy, Infection, Opiates, Uremia, Trauma, Insulin overdose or inflammatory disorders, Poisoning and psychogenic causes, and Shock. Diagnosis
Despite converging agreement about the definition of persistent vegetative state, recent reports have raised concerns about the accuracy of diagnosis in some patients, and the extent to which, in a selection of cases, residual cognitive functions may remain undetected and patients are diagnosed as being in a persistent vegetative state. Objective assessment of residual cognitive function can be extremely difficult as motor responses may be minimal, inconsistent, and difficult to document in many patients, or may be undetectable in others because no cognitive output is possible.
A 1994 report found that of those who were in a vegetative state a month after a trauma, 54% had regained consciousness by a year after the trauma, whereas 28% had died and 18% were still in the vegetative state. For non-traumatic injuries such as strokes, only 14% had recovered consciousness at one year, 47% had died, and 39% were still vegetative. Patients who were vegetative six months after the initial event were much less likely to have recovered consciousness a year after the event than in the case of those who were simply reported vegetative at one month. A New Scientist article from 2000 gives a pair of graphs showing changes of patient status during the first 12 months after head injury and after incidents depriving the brain of oxygen. After a year, the chances that a PVS patient will regain consciousness are very low and most patients who do recover consciousness experience significant disability. The longer a patient is in a PVS, the more severe the resulting disabilities are likely to be. Rehabilitation can contribute to recovery, but many patients never progress to the point of being able to take care of themselves. The medical literature also includes case reports of the recovery of a small number of patients following the removal of assisted respiration with cold oxygen. The researchers found that in many nursing homes and hospitals unheated oxygen is given to non-responsive patients via tracheal intubation.