Chapter 18: Disorders of Thought, Emotion, and Memory

The etiology of these disorders is multifactorial, encompassing biological, psychological, and environmental contributions that frequently involve disruptions in neurochemical balance and neurotransmitter regulation. Epigenetics offers a framework for understanding how heritable modifications to gene expression occur without alterations to the underlying DNA sequence, influencing predisposition to psychiatric illness. Endophenotypes represent intermediate phenotypic traits that are genetically linked to psychiatric disorders and help bridge the gap between genetic risk and observable symptoms. The pathophysiology of many mental disorders involves dysregulation of neuroplasticity, the nervous system's remarkable ability to reorganize and adapt in response to experience and environmental demands. Memory formation and retention depend on interconnected biochemical and neuronal networks that are fundamentally organized around emotional, cognitive, and sensory experiences. The chapter introduces Neurocognitive Disorders, the DSM-5 terminology replacing dementia, which emphasizes that severe cognitive impairment is a clinical condition rather than a normal consequence of aging. Neurocognitive Disorders impact essential cognitive domains including language processing, memory retention, and executive function. Alzheimer Disease, a major subtype of Neurocognitive Disorders, is characterized by genetic abnormalities on chromosome 21, which overlaps with genetic factors in Down syndrome. Although Alzheimer Disease is diagnosed through clinical assessment during life, definitive pathological confirmation requires postmortem neuropathological examination. The chapter provides systematic methods for screening potentially reversible causes of cognitive decline and memory impairment using established diagnostic frameworks and mnemonics.