The diagnostic manual published by the Guild of Barber-Surgeons lists Planets as an occupational disease of those working in high-magic environments. Therefore wizards are particularly prone to this, although it is not exclusively a wizardly ailment. Thaumaturgists and other magical technicians, witches, and in certain circumstances conjurors, may also suffer from Planets. The current Bursar of Unseen University has been witnessed to manifest the symptoms, and in many ways his is the classic case study. High magical environments can cause a breakdown in the inhibitory circuits that prevent most people over the age of five from believing that they are the centre of the known universe and that everything revolves around them.
The primary observed symptom is that the sufferer will manifest small planets which will duly orbit around their head. Advanced sufferers will notice some of these planets even have moons and in extreme cases will see comets flash in and out of existence. One unfortunate wizard called Roraty Williams suffered for many years and developed the most extreme case ever seen.
Cerebral manifestation gives a rather large clue to there being a high level of magic. During the events of The Last Continent, when the Faculty of Unseen University approaches Fourecks where the background magical radiation is over a million thaums, the Bursar develops Planets. They are insubstantial and go through him and through each other, but they are nevertheless there.
Mugroop's Syndrome is a related concept. Mugroop was a wizard who reasoned that Planets only ever seem to manifest at magical temperatures registering over a million Thaums. Therefore a sufferer might usefully be employed as the magical equivalent of a canary in a coal mine, an alert to other wizards that the thaumic levels are getting dangerously high.
The medical manual is called Household Medicine, Hair-Care and Simple Surgery, and is published by Goatberger and Cropper, at $AM2.00 per copy. It lists this and many other interesting disorders.