George III suffered from the genetic disease porphyria, which had also tormented Mary Queen of Scots, who passed it on to her son, King James I of England.
We now know that there are at least eight types of porphyria, and that the clinical manifestation of each type is not the same. A common feature of all porphyrias is the accumulation in the body of "porphyrins" or "porphyrin precursors." These are normal body chemicals, but they do not normally accumulate. Each type of porphyria is determined by deficiency of a different enzyme. These enzyme deficiencies are usually inherited. Symptoms of the disease can include (but are not limited to) photosensitivity, strong abdominal pain, port wine-colored urine, muscle weakness or paralysis in the arms and legs, and behavioral changes including anxiety, irritability, and confusion. The interruption of nerve impulses to the brain can cause the development of psychiatric symptoms such as depression or delirium. (For more information, contact the American Porphyria Foundation .)
George III had a particularly severe form of porphyria. His first attack occurred in 1765, four years after his marriage to Queen Charlotte. Further signs of the disease showed up in 1788-1789. From 1811 to the time of his death in 1820 the royal patient became progressively insane and blind. He was nursed in isolation, and kept in straight jackets and behind bars in his private apartments at Windsor Castle.
Other members of the far-flung royal family who suffered from this hereditary disease were Queen Anne of Great Britain; Frederic the Great of Germany; George IV of Great Britain--son of George III; and George IV's daughter, Princess Charlotte, who died during childbirth of complications of the disease.