It takes extensive testing to assess levels of wakefulness and awareness before a disorder of consciousness can be confirmed.
This may involve tests such as brain scans, but is largely based on the specific features a person displays, such as whether they can respond to commands.
Doctors can score a person's level of consciousness using a tool called the Glasgow Coma Scale.
This assesses 3 things:
A lower score indicates a more severely impaired consciousness, such as a coma, although this level will be monitored regularly to look for any changes.
The brain injury association Headway has more detailed information about the Glasgow Coma Scale.
There are also more specific scoring systems based on more detailed observations of a person's behaviour.
One example is known as the JFK Coma Recovery Scale-Revised (CRS-R).
This system uses 23 different items, each with individual scales to assess how a person is responding.
You can read more detailed information about the CRS-R on the Center for Outcome Measurement in Brain Injury (COMBI) website.
Brain scans are used to help assess the level of brain damage in someone with impaired consciousness.
They can also check for signs of any complications, such as hydrocephalus, a build-up of fluid on the brain.
There are several types of scans that can assess brain structure.
There are separate scans that can show areas of brain activity as well as brain damage.
These are only used in research settings at the moment, but may be useful if a person can't move or speak.
One example is a functional magnetic resonance (fMRI) scan.
An fMRI scan is able to show changes if the brain is responding to light and sounds.
But they don't necessarily show awareness, as the brain is able to respond to stimulation even without the person actually being aware of it.
Research is being carried out into ways brain scans can be used to show true awareness.
A vegetative state is when a person is awake but showing no signs of awareness.
Doctors are particularly careful when diagnosing a permanent vegetative state, as there's a risk of misdiagnosis.
A confident diagnosis can only be made if the following criteria have been met:
For a permanent vegetative state to be confirmed, the above criteria must apply and either: