Phrenitis means an inflammation of the brain, or of the meninges of the brain, attended with acute fever and delirium. Symptoms varie widely in severity, from shortlived, relatively slight effects of headache, drowsiness, and fever to paralysis, coma, and death.
History teaches that the ancient phrenitis concept has been used until the 19th century. After that time the concept was replaced by the word delirium. By their epigonic character the detailed descriptions of phrenitis by Gerard van Swieten mark only the end of an uncritical use of the term. The epoch-making work of Morgagni, based on clinical-anatomical observations, provides a definitive insight into the location of the condition and into many pathologic features. Pinel is the last author who mentions phrenitis in a classification of diseases.