Mood Disorders

Introduction

Mood disorders encompass a large group of related conditions that are commonly encountered by psychiatrists. The signs and symptoms vary between different age groups, and an understanding of socio-cultural backgrounds is very important when considering treatment options.

Mood disorders exist on a spectrum and include the following clinical entities:

• Dysthymia — chronic, low-grade depressed mood lasting at least 2 years
• Depression
  • Mild
  • Moderate
  • Severe
  • Severe with psychotic features
• Cyclothymia — chronic instability of mood with periods of mild depression and mild elation, lasting at least 2 years
• Hypomania — elevated mood and increased energy, distinct from normal but not as severe as mania; no psychotic features
• Mania — elevated mood of sufficient severity to cause significant impairment; may include psychotic features
• Mania with psychosis
• Bipolar Affective Disorder — alternating episodes of depression and mania or hypomania
• Recurrent Depressive Disorder — repeated episodes of depression without episodes of elevated mood

A useful conceptual framework is to think of mood disorders as lying on a continuum from chronic low-grade depression (dysthymia) through to frank mania with psychosis at the other end.

Major Depression — Classification Overview

In the past, depression was described informally as anyone experiencing hopelessness, helplessness, and worthlessness. While this description captures the subjective experience, it led to the overuse of the term — so that anyone having a bad day might say they were "depressed". Fixed diagnostic guidelines are therefore essential for accurate clinical communication and research.

Two classification systems are used currently:

1. International Classification of Diseases, 10th Revision (ICD-10) — WHO Classification, 1992. Preferred by UK psychiatrists.
2. Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) — American Psychiatric Association, 2013. Used primarily in North America and in research.

Although both classification systems have merits and limitations, psychiatrists in the UK use ICD-10 more commonly in clinical practice, and this overview is framed accordingly.

Epidemiology of Major Depression

Depression is one of the most common problems that patients present with to their GP. Lifetime prevalence rates vary between studies but appear to fall between 4–30%, with a true population value of approximately 15%. There is an approximately 2-fold higher incidence of major depression in females compared to males, a finding consistent across different cultures.

The mean age of onset is approximately 27 years, with higher rates found in divorced and unemployed populations. There is a very high co-morbidity with anxiety disorders and substance misuse. Almost 15% of medical inpatients have features of a depressive disorder. Depression is one of the leading causes of morbidity and loss of work productivity worldwide.

Aetiology of Mood Disorders

The cause of depression is multi-factorial. The following are the major contributory factors:

Genetic Causes

Mood disorders tend to run in families. Twin studies demonstrate a significantly higher concordance in monozygotic (identical) twins compared to dizygotic (non-identical) twins, confirming a genetic contribution. First-degree relatives of a patient with depression have a 2–3 times higher risk of developing depression compared to the general population.

Biological Causes

Biogenic amines: The most widely accepted neurochemical theory implicates a deficiency in monoamine neurotransmitter activity — particularly serotonin and noradrenaline — at receptor sites in the brain. This forms the basis for the pharmacological action of most antidepressants. Dopamine is also implicated, particularly in symptoms of anhedonia and reward processing.

Neuroendocrine dysregulation: A correlation between hypercortisolaemia (excessive cortisol secretion) and depression is one of the oldest theories of mood disorder. The dexamethasone suppression test — while not diagnostically specific — reflects this axis abnormality. Hypothyroidism is one of the most common medical disorders found alongside depression and should always be screened for with a TSH level.

Psychosocial Factors

Adverse life events and environmental factors can trigger depressive episodes. Events that lead to feelings of entrapment, loss, or humiliation are particularly relevant precipitants. Brown and Harris's seminal work identified vulnerability factors — such as lack of a confiding relationship, caring for young children, lack of employment, and early loss of a mother — that increase susceptibility to depression following stressful life events.

Cognitive Theory

Aaron Beck's cognitive model proposes that people with depression form characteristic cognitive distortions: negative views of the self, the world, and the future (the "negative cognitive triad"). These dysfunctional assumptions develop early in life and are activated by adverse events. Cognitive Behavioural Therapy (CBT) directly targets these cognitive distortions.

Clinical Features of Major Depression

The central features of major depression are:

Core Symptoms (ICD-10 List A)

• Depressed mood
• Loss of interest and enjoyment (anhedonia)
• Reduced energy and decreased activity

Additional Symptoms (ICD-10 List B)

• Reduced concentration and attention
• Reduced self-esteem and confidence
• Ideas of guilt and worthlessness
• Pessimistic thoughts about the future
• Ideas of self-harm or suicide
• Disturbed sleep
• Diminished appetite (or occasionally increased appetite)

Features 1–3 (the core symptoms) are particularly important. At least two of these three must be present for a minimum of two weeks to make a diagnosis of depression.

Other important features to assess: psychomotor activity (agitation or retardation), symptoms of anxiety, substance misuse, and psychotic features.

ICD-10 Diagnostic Criteria — Summary

• Mild Depression: At least 2 of the 3 core symptoms for 2 weeks, plus at least 2 additional symptoms.
• Moderate Depression: At least 2 of the 3 core symptoms for 2 weeks, plus at least 3 additional symptoms.
• Severe Depression: All 3 core symptoms for 2 weeks, plus at least 4 additional symptoms. The severity of symptoms and degree of functional impairment also guide classification.

Dysthymia

Dysthymia (classified as "persistent depressive disorder" in DSM-5) is a chronic low-grade depressed mood that persists for at least 2 years (1 year in children and adolescents). The symptoms are less severe than a full depressive episode but are persistent and debilitating. It frequently coexists with major depression ("double depression"), which worsens prognosis. Treatment involves psychotherapy (especially CBT and IPT) and antidepressants, though response rates are lower than in major depression.

Cyclothymia

Cyclothymia is characterised by chronic mood instability — numerous periods of mild depression alternating with periods of mild elation (hypomanic symptoms) — lasting at least 2 years. The mood swings are not severe enough to meet criteria for a full depressive episode or hypomania. It is considered a subsyndromal form of bipolar disorder and may progress to bipolar I or II in some patients. Treatment focuses on mood stabilisation (lithium, lamotrigine) and psychoeducation.

Hypomania

Hypomania is an elevated, expansive, or irritable mood of at least 4 consecutive days' duration. Unlike mania, hypomania does not cause severe functional impairment, does not include psychotic features, and does not require hospitalisation. Features include increased energy, reduced need for sleep, increased talkativeness, inflated self-esteem, distractibility, and goal-directed activity. Hypomania is a defining feature of Bipolar II disorder.

Mania

Mania is characterised by an elevated, expansive, or markedly irritable mood lasting at least 7 days (or any duration if hospitalisation is required). Features include marked increase in goal-directed activity, decreased need for sleep, pressure of speech, racing thoughts (flight of ideas), grandiosity, reckless behaviour, and distractibility. Psychotic features (delusions and hallucinations) occur in approximately 60% of cases of mania. Mania is a defining feature of Bipolar I disorder.

Differential Diagnosis of Depression

Depression must be differentiated from:

• Normal sadness — having a bad day does not constitute a depressive episode
• Anxiety disorders (which can present with low mood)
• Schizophrenia (negative symptoms can mimic depression)
• Substance misuse (alcohol and illicit drugs frequently cause depressed mood)
• Organic brain disorders — dementia, head injury, delirium
• Medical conditions — hypothyroidism, Addison's disease, anaemia, malignancy

Investigations

In psychiatry, investigations are usefully divided into biological, social, and psychological domains:

Biological Investigations

Basic blood investigations including full blood count, liver function tests, urea and electrolytes, thyroid function tests, and blood glucose are performed to exclude organic causes of low mood. Hypothyroidism and diabetes can both predispose to and precipitate depression. ECG is important because some antidepressants can prolong the QTc interval. Most psychiatrists would also request a CT head if the presentation follows head injury or if the patient has features of confusion or dementia.

Social Investigations

This involves exploring the patient's social support structure — family, benefits, housing, and financial situation — as well as any difficulties with dependants. Collateral information from the partner, parents, and other professionals involved in the patient's care is invaluable, and must be sought with the patient's consent.

Psychological Investigations

This includes exploring the patient's developmental history, attachment, early adversity, and bereavement. Validated mood and anxiety rating scales (e.g. PHQ-9, GAD-7, HADS) are used for baseline assessment and to monitor progress over time.