Understanding Depression: Causes, Symptoms and Clinical Forms

What Is Depression?

Depression, or major depressive disorder, is a mental illness characterized by persistent sadness, loss of interest or pleasure in usual activities, and a set of cognitive, emotional and physical symptoms that significantly impair daily functioning. Unlike a passing case of the blues, depression lasts at least two weeks and does not simply go away through willpower alone.

According to the World Health Organization (WHO), depression is the leading cause of disability worldwide. In France, approximately 20% of the population will experience a depressive episode during their lifetime. Despite this high prevalence, depression remains frequently underdiagnosed and stigmatized.

Neurobiological Mechanisms

The Role of Neurotransmitters

Depression involves an imbalance in several neurotransmitter systems. Serotonin, often called the "happiness hormone," plays a central role in regulating mood, sleep and appetite. Reduced serotonin levels are associated with depressive symptoms. Norepinephrine and dopamine are also involved, respectively explaining fatigue, lack of motivation and anhedonia (inability to feel pleasure).

The Stress Axis (HPA)

The hypothalamic-pituitary-adrenal (HPA) axis is frequently hyperactive in depressed individuals. This hyperactivity leads to excessive cortisol production, the stress hormone, which over time can damage the hippocampus — a brain region essential for memory and emotional regulation. Neuroimaging studies have shown reduced hippocampal volume in patients with chronic depression (Sheline et al., 2003).

Chronic Inflammation

Recent research has highlighted the role of systemic inflammation in depression. Elevated levels of pro-inflammatory cytokines (IL-6, TNF-alpha, CRP) are found in many depressed patients. This discovery opens promising therapeutic avenues, particularly through anti-inflammatory diets and gut microbiome modulation.

Risk Factors

Biological Factors

• Genetics: the risk of depression is multiplied by 2 to 3 when a first-degree relative is affected. Studies on monozygotic twins show a concordance rate of 40 to 50%.
• Hormones: hormonal fluctuations (puberty, pregnancy, menopause) increase vulnerability, explaining the higher prevalence in women (2:1 ratio).
• Chronic diseases: diabetes, cardiovascular diseases, chronic pain and thyroid disorders are often associated with depression.

Psychological Factors

• Childhood trauma: abuse, neglect or early loss of a parent.
• Cognitive style: tendency toward perfectionism, rumination, negative automatic thoughts.
• Low self-esteem: feelings of helplessness and worthlessness.

Environmental Factors

• Stressful life events: bereavement, divorce, job loss, relocation.
• Social isolation: lack of support network.
• Economic precarity: financial insecurity and difficult living conditions.
• Toxic work environment: harassment, overload, loss of meaning.

Symptoms of Depression

Emotional Symptoms

• Deep and persistent sadness
• Loss of interest or pleasure (anhedonia)
• Feelings of emptiness, hopelessness
• Excessive or inappropriate guilt
• Increased irritability

Cognitive Symptoms

• Difficulty concentrating and with memory
• Indecisiveness
• Recurring negative thoughts
• Dark thoughts, suicidal ideation in severe cases

Physical Symptoms

• Intense and persistent fatigue
• Sleep disturbances (insomnia or hypersomnia)
• Appetite changes (loss or increase)
• Psychomotor retardation or agitation
• Physical pain without identified medical cause (headaches, back pain, digestive issues)

Different Forms of Depression

Major Depressive Episode (MDE)

The most classic form, diagnosed when at least five symptoms are present for at least two weeks, necessarily including depressed mood or loss of interest.

Persistent Depressive Disorder (Dysthymia)

A chronic but less intense form lasting at least two years. The person functions daily but with a permanent feeling of gloominess and demotivation.

Seasonal Affective Disorder (SAD)

Linked to lack of natural light, it typically occurs in autumn-winter. Light therapy is a first-line treatment validated by numerous studies.

Postpartum Depression

Occurs in the weeks following childbirth, affecting approximately 15% of mothers. It is distinguished from "baby blues" by its intensity and duration. Early screening is essential.

Bipolar Depression

Depressive phases alternating with manic or hypomanic episodes. Management differs significantly from unipolar depression.

Masked Depression

Physical symptoms predominate (pain, fatigue, digestive issues), masking the emotional picture. Common in men and elderly individuals.

Diagnosis and When to Seek Help

Diagnosis is based on DSM-5 criteria (Diagnostic and Statistical Manual of Mental Disorders). It is made by a physician, psychiatrist or clinical psychologist after a thorough clinical interview.

Consult a healthcare professional if:

• You have felt persistent sadness or emptiness for more than two weeks
• You have lost interest in activities you used to enjoy
• Your sleep, appetite or energy are significantly disrupted
• You have dark or suicidal thoughts (call your local crisis hotline immediately)

Depression is not a character weakness. It is an illness that can be treated. Seeking help is an act of courage, not an admission of weakness.