Aetiology

Childhood depression is likely to be caused by both genetic and environmental factors, and by their interactions. It is estimated that up to 40% of variance can be explained by genetic factors, and the remaining variances can be explained by environmental factors and by the interactions between genetic factors and the environment.[8][9][10]

Pathophysiology

Exactly how genetic and environmental factors lead to the clinical manifestation of a depressive disorder is not fully understood. It is suspected that the process is complex and multifactorial. The pathophysiology of childhood depression is less well understood than that of adult depression. Fewer studies have been conducted in the paediatric population, and some of the adult findings have not been demonstrated in childhood depression. For example, cortisol hypersecretion, a consistent finding among depressed adults, was not replicated in depressed children.[11] However, there is evidence indicating the dysregulation of the central serotonergic systems in childhood depression. Pre-pubertal depressed children were found to have attenuated cortisol but increased prolactin secretion responding to L-5-hydroxytryptophan challenge.[12] It is suggested that dysregulation of the central serotonergic system could lead to an impaired stress and emotional response, decreased impulse control, and emotional dysregulation.[13][14]

Studies in adults with depression have indicated several sleep abnormalities through polysomnographic studies, including reduced sleep continuity, reduced slow-wave sleep, shortened rapid eye movement (REM) latency, and increased REM density. However, sleep studies in children and adolescents with depression have been inconsistent. Some studies indicate that depressed children and adolescent inpatients have sleep continuity disturbances and an increase in REM pressure (shortened REM latency and increased REM sleep %), but not disturbances in slow-wave sleep.[15][16][17][18] The results in outpatients have been mixed.[19][20][21]

The disruption in the motivation-and-reward neurological pathway has also been indicated in paediatric depression.[22] There is evidence of the involvement of the glutamatergic system in the pathophysiology of depressive disorders.[23] Increased morning cortisol in youth at risk for depression predicts onset of depression.[24] Imaging studies have found volumetric and functional disruptions in multiple brain regions and pathways (e.g., in the amygdala, anterior cingulate, prefrontal cortex) that are important in emotional regulation, stress response, motivation, behavioural inhibition, and the manifestation of depressive symptoms.[25][26][27][28]

Classification

Diagnostic and statistical manual of mental disorders, fifth edition (DSM-5): classification of depressive disorders[1]

Categorises depressive disorders in children into the following categories: major depressive disorder (MDD), persistent depressive disorder (dysthymia), disruptive mood dysregulation disorder (DMDD), premenstrual dysphoric disorder, substance/medication-induced depressive disorder, depressive disorder due to another medical condition, other specified depressive disorder, and unspecified depressive disorder. This topic focuses on MDD and persistent depressive disorder (dysthymia).

MDD

  • Similar to adults, characterised by at least 5 depressive symptoms, occurring over the same 2-week period, although children and adolescents may have irritability instead of depression as one of the primary mood symptoms.

  • A child needs to have at least one of the key symptoms (sad or irritable mood, anhedonia), associated with a significant impairment in functioning in multiple areas.

Persistent depressive disorder (dysthymia)

  • Includes DSM-IV-defined chronic MDD and dysthymic disorder.

  • A more chronic form of depressive disorder than MDD.

  • Characterised by a chronic sad or irritable mood that lasts for at least 1 year (must be 2 years in adults). Must also have at least 2 of the following symptoms: disturbed appetite, insomnia or hypersomnia, decreased energy level and self-esteem, poor concentration, and feelings of hopelessness.

  • During the 1-year period, a child never has been without the minimum 3 symptoms for more than 2 consecutive months.

DMDD

  • A new category of depressive disorders for children 6 to 18 years of age, with age of onset before 10 years of age.

  • Characterised by severe and persistent irritability or angry mood nearly every day, and severe and recurrent temper outbursts at least 3 times a week.

  • Symptoms are inconsistent with developmental level, present in at least 2 out of 3 settings (i.e., home, school, and with peers) and are severe in at least one setting.

  • Symptoms have occurred for at least 12 months and during this time the child must not have gone 3 or more consecutive months without symptoms.

  • There have never been symptoms lasting for more than 1 day that meet the criteria for a manic or hypomanic episode except for duration.

  • Symptoms do not occur during an MDD episode and are not better accounted for by another mental disorder, and are not due to a substance/medication or other medical condition.

Premenstrual dysphoric disorder

  • An independent category of depressive disorders in DSM-5.

  • Characterised by at least 5 mood symptoms that have been present a week before the onset of the majority of menstrual cycles in the preceding year. Symptoms become minimal or absent within a few days after the onset of menses.

  • At least one symptom is one of the following symptoms: marked affective lability (mood swing), marked irritability or anger or interpersonal conflict, marked depression, and marked anxiety or tension.

  • At least one symptom is one of the following symptoms: anhedonia, poor concentration, fatigue, change in appetite, insomnia or hypersomnia, sense of feeling out of control/overwhelmed, and physical symptoms (e.g., aches and pains, sensation of bloating).

  • Symptoms cause significant distress or interference with functioning and are not part of, or an exacerbation of, another disorder.

  • Symptoms are not due to a substance/medication or another medical condition.

Substance/medication-induced depressive disorder

  • Marked, persistent, and function-impairing depressed mood or anhedonia caused by a substance or medication.

  • Symptoms developed during or soon after exposure to a substance or medication known to cause the types of depressive symptoms.

  • Symptoms are not better accounted for by an independent, non-substance/medication-induced depressive disorder, and do not occur exclusively during the course of a delirium.

Depressive disorder due to another medical condition

  • Marked, persistent, and function-impairing depressed mood or anhedonia caused by another medical condition.

  • Symptoms are not better accounted for by another mental disorder, and do not occur exclusively during the course of a delirium.

Other specified depressive disorder and unspecified depressive disorder

  • Formerly known (in DSM-IV) as depressive disorder not otherwise specified.

  • Symptoms do not meet criteria for any of the other depressive disorder categories.

  • Characterised by fewer depressive symptoms, or shorter duration, than the other types of depressive disorders.

  • Examples of the other specified depressive disorder include recurrent brief depressive disorder and short-duration depressive episode.

  • The unspecified depressive disorder does not specify the reason for not meeting criteria for other depressive disorder categories.

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