Krantzberg-Godel Spongiform Encephalopathy, typically abbreviated as Krantzberg Syndrome or K-Syndrome, is a neurodegenerative disease that exclusively targets magic practitioners who engage in mental thaumaturgic calculations. The disease is caused by parasitic extradimensional entities that are attracted to the brain of the victim, literally removing microscopic quantities of gray matter from the brain as they feed. The associated symptoms are similar to Kuru or Creutzfeldt-Jakobs disease.
Krantzberg syndrome, the horrible encephalopathy that tends to afflict people who spend too much time thinking about symbolic magic. (The map is one with the territory; think too hard about the wrong theorems and you shouldn’t be surprised if extradimensional entities start chewing microscopic chunks out of your gray matter.) 
Some folks (ritual magicians) actually do the symbol-manipulation thing in their heads, risking death by Krantzberg syndrome and worse. It’s not an approach to defending the realm that scales, because you can’t take a random reasonably bright teenager and reliably turn them into a sorcerer. But you can turn some of them into computer scientists—and a whole lot more into IT support drones who can use a canned toolkit to perform a limited range of occult manipulations. - The Apocalypse Codex 
Krantzberg syndrome haunts me because it's a personal bete noire. Magic is a side effect of certain classes of mathematics, notably theorem proving. Sensible magicians-like me-use computers. But if you're smart and agile you can perform some types of invocation in your head. The trouble is, just as our symbol manipulating machines attract the attention of extradimensional agencies(demons, you might say), so do such DIY invocations. If you perform ritual magic regularly, some of things you invoke will chew microscopic holes in your gray matter. At first it's not too bad, but the dizziness, numbness and tingling in the periphery, memory problems, and lack of coordination are irreversible and frequently progressive. The end result, Krantzberg syndrome, bears a rather unpleasant resemblance to Creutzfeldt-Jakob disease - nott to mention that other happy fun prion disease, Kuru (which you can only contract by eating the brains of someone who had the condition). Krantzberg syndrome is what people like me die of if the monsters don't get us first. - The Rhesus Chart page 87
From The Rhesus Chart page 95
Now, back to the subject on hand ritual magicians who perform too many invocations in their head tend to develop a package of
symptoms after a while which eerily resembles Kuru. It starts with tremors, unsteady gait, slurred speech, and confusion. Then they become unable to walk, suffer from ataxia and uncontrollable tremors, and show signs of emotional instability. They become depressed, but
may suffer from fits of maniacal laughter. In the final stages, they lose all muscular control, can't sit, or swallow, or speak, becomeunre
sponsive to their surroundings... And from start to finish it can take between three months and three years. The best theory to account for K syndrome is that ritual magic is intentionally designed to attract extradimensional critters- and not just big ones. There seems to be some variety of microscopically small dumb eaters that materialize inside the gray mattter of the practitioner while they are in the process of carrying out some sort of invocation, take a tiny bite, and disappear again. Brain parasites from beyond spacetime, in other words. The brain is a resilient piece of squichware, but if the K-parasites chew enough tiny holes in the headmeat, it's owner will eventually succumb to a dementia-like illness. Furthermore, once the eaters have found a tasty lump of brains to chow down on they keep at it. So the disease is progressive, vile, and fatal if unchecked.
"...yes: we can stop it progressing by putting the victim inside a locked-down protective ward. This is a huge step forward over, say, Kuru or CJD. What's more, if we isolate the patient for long enough the eaters lose interest and go away; but the victim isn't going to be casting any spells ever again. (If they're lucky they'll still be able to talk coherently and tie their own shoeloaces.) It's a health and safety nightmare. -From The Rhesus Chart page 95
- “His prefrontal lobes look like Swiss cheese. It’s one of the early signs of Krantzberg Syndrome. If we can keep him isolated from work for a couple more months, then retire him to a nice quiet desk job, we might be able to stabilize him. K-Syndrome’s not like Alzheimer’s: if you remove the insult it frequently goes into remission. Mind you, he may also need a course of chemotherapy. At various times my predecessors tried electroconvulsive treatment, prefrontal lobotomy, neuroleptics, daytime television, LSD—none of them work consistently or reliably. The best treatment still seems to be bed rest followed by work therapy in a quiet, undemanding office environment.” Blue cloud spirals toward the ceiling. “But he’ll never run a great summoning again.” - From Down on the Farm