Chapter 1 provides some important background information on the pathophysiology of impaired consciousness, as well as on the signs and symptoms that accompany it. It starts with definitions of key terms: consciousness, acutely altered states of consciousness, and subacute or chronic alterations of consciousness, as well as the definitions for stupor, coma, and other states of altered consciousness. It looks at how to approach the diagnosis of the comatose patient, and it then explains the physiology and pathophysiology of consciousness and coma. Finally, it describes three types of brain damage that may lead to altered consciousness or coma: bilateral hemispheric damage, diencephalic injury, and upper brainstem injury.