Occasionally, as a doctor, I am asked to see a patient in ED who is completely mute. They sit motionless, staring around the room.

I lift up their arm and it stays in that position. Someone takes a blood test, and they don’t even wince. They haven’t eaten or drunk anything for a day or two.

Questions start running through your mind. What’s wrong with them? Would they respond to someone else? Do they have a brain injury? Are they putting it on? And — hardest of all — how am I to know what’s going on if they can’t tell me?

I’m a psychiatrist and a researcher specialising in a rare condition known as catatonia: a severe form of mental illness where people have problems with movement and speech.