The "glass delusion" is an extraordinary psychiatric phenomenon in which people believe themselves to be made of glass and thus liable to shatter. It peaked centuries ago but there are still isolated cases today, writes Victoria Shepherd.
The late medieval French King Charles VI was one of the most notable sufferers of glass delusion. He was reported to have wrapped himself in blankets to prevent his buttocks from breaking.
Instances of the delusion cropped up in medical encyclopaedias from across Europe. There were references in fiction - most notably Cervantes' short story The Glass Graduate of 1613, in which the hero is poisoned by a quince intended as an aphrodisiac but which instead triggers a glass delusion.
Sufferers were seen to be normal in all ways bar the belief that they had turned to glass, and so could function, albeit anxious that other people shouldn't come too close and risk shattering fragile limbs.
But in the 1830s, cases disappear from the records.
It's easy to assume society and culture are so changed that mentally ill people would no longer manifest this particular delusion.
But Andy Lameijn, a psychiatrist from Leiden in the Netherlands, has uncovered contemporary cases. One case cropped up in his own hospital, offering him the chance to probe the meaning of this enigmatic delusion with a living patient. "It was an authentic case - it was unmistakable that it was a glass delusion."
Lameijn's research while director of the Endegeest Psychiatric Hospital in Leiden had led him to lost cases recorded after the 1830s. A lecture of 1883 from the archives in a Edinburgh mental hospital cited the symptoms of 300 female patients, one of whom thought her legs were made of glass.
Another case from the 1880s turned up in the footnotes to an edition of Cervantes' The Glass Graduate, referring to a contemporaneous case having occurred in an asylum in Paris, but with no further details.
Lameijn wrote and lectured on the subject, and was approached by a fellow psychiatrist who had found a case in the archives of his own Dutch hospital which dated back to the 1930s. The woman had been admitted to a psychiatric hospital believing that her legs and back were made of glass. Such was her fear of personal contact, the notes showed, that the nurses couldn't get near her to change her clothes or help her. She had apparently recovered after treatment. Another doctor brought him a case from a different hospital, from 1964.
But then a young man turned up at the University Clinic in Leiden, claiming to be made of glass. "I really dropped everything," Lameijn recalls, "I didn't want to miss this." He was to have the opportunity to speak to the only contemporary person to present with glass delusion for decades.
Lameijn talked for several hours to the man, who confirmed that he felt that he was made of glass. Lameijn asked what this feeling meant to him, not wanting to distort the conversation by suggesting ideas of fragility or transparency, and after initial reticence, the patient began to open up.
He pointed to the window in the consulting room and asked Lameijn what he could see. Lameijn replied that he could see a street, some cars, more buildings, people walking past, and waited.
The patient said: "Ah! You've missed the glass in the window. You didn't see it. But it is there." He leaned forward, and said: "That's me. I'm there, and I'm not there. Like the glass in the window."
The conversation continued and the patient expanded on his feeling of being made of glass, claiming that he was able to turn this feeling of being "there" and "not there" on and off at will, like a switch in his own mind - he could "disappear" and "reappear".
It transpired that the patient had recently had an accident, and Lameijn began to formulate a theory as to why a modern person might present with glass delusion. His conclusion was that the man in question was using this glass delusion as a sort of distance-regulator - following the accident, his family had become over-protective, and the glass delusion was an attempt to regain privacy and hide from overbearing family.
There are reasons why someone with mental illness in the Middle Ages - or indeed the 17th Century - might manifest glass delusion. That was a time when clear glass was a new material on the scene, seen as magical, alchemical even.
Why might glass delusion reappear at a time when glass is no longer new? What contemporary psychological resonance might it have?
Psychoanalyst Adam Phillips argues that the glass delusion has powerful contemporary resonance in a society in which anxieties about fragility, transparency and personal space are pertinent to many people's experience of, and anxieties about, living in the modern world.
The feeling of being made of glass could be a useful way of understanding how we negotiate society, a society that is increasingly crowded, in which modern technological advances isolate us and offer apparently boundary-less communication.
Novelist Ali Shaw, author of The Girl with Glass Feet, suggests that glass delusion might simply be at the extreme end of a scale of social anxiety which many of us experience to a lesser extent. The fear of tripping and breaking is really an exaggerated fear of social humiliation.
Prof Edward Shorter, a historian of psychiatry from the University of Toronto, suggests that it is the relative newness of clear glass as a material in 17th Century Europe which holds the key to understanding the disorder. Throughout history, Shorter argues, the inventive unconscious mind has pegged its delusions on to new materials and the technological advances of the age.
In the 19th Century cement delusions appeared at a time when cement emerged as a new building material, just as common delusions of recent decades include the false belief that the CIA or other security services can download thoughts through micro-transmitters, that people could "read your mind".
Glass is not new, but it certainly still has the power to captivate, and could reflect aspects of people's fears and wishes as they negotiate the modern world.
The Glass Delusion is broadcast on BBC Radio 4 at 11:00 BST on Friday 8 May 2015 - or catch up on iPlayer
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