Depression

Depression is a mood disorder characterized by persistent low mood, loss of interest or pleasure in most activities, and associated cognitive, behavioral, and physical symptoms that impair daily functioning.

Definition

Epidemiology

Global lifetime prevalence is approximately 10–15%
More common in women than men
Peak onset in late adolescence and early adulthood, but can occur at any age
Often recurrent, with 50–80% experiencing multiple episodes
Comorbidities include anxiety disorders, substance use disorders, and chronic medical conditions

Etiology

Genetic predisposition and family history of depression
Neurochemical dysregulation: serotonin, norepinephrine, dopamine imbalance
Structural and functional brain changes: prefrontal cortex, amygdala, hippocampus
Psychosocial stressors: trauma, loss, chronic stress
Medical conditions: thyroid disorders, chronic pain, neurological diseases
Substance use and medications contributing to depressive symptoms

Pathophysiology

Hypoactivity of prefrontal cortex affecting emotional regulation
Hyperactivity of amygdala and limbic system contributing to negative emotions
Neurotransmitter dysregulation: serotonin, norepinephrine, dopamine
HPA axis dysregulation leading to elevated cortisol levels
Neuroinflammation and reduced neuroplasticity in chronic depression
Altered functional connectivity between brain networks
Genetic and epigenetic factors influencing stress response and mood regulation