Anthony Daniels in The New Criterion has an interesting piece on how doctors have been trying to diagnose Lear’s condition for the past two centuries.
Nineteenth-century mad doctors in Britain and America said Lear’s case was just like many they saw in their asylums. Psychoanalysts perceived in Lear a case of thwarted incest (they would, wouldn’t they?). A variety of diagnoses have been offered from senile dementia to manic-depressive psychosis. (No one has suggested General Paralysis of the Insane, the last stage of syphilis.) Dr. Truskinovsky, writing in the Southern Medical Journal in 2002, makes a powerful case for mania, and suggests that Lear had been suffering from bipolar affective disorder all his life.
As Lear says, somewhere towards the end of his play:
When we are born we cry that we are come
To this great stage of fools