Mental Disorders: Classification, Presentation, and Treatment

This lecture covers the classification and clinical features of common mental disorders seen in medical practice, with a focus on diagnostic frameworks, the diathesis-stress model, and evidence-based psychological treatments.

CLASSIFICATION SYSTEMS

The two dominant classification systems are the DSM-5 (Diagnostic and Statistical Manual of Mental Disorders, 5th edition, American Psychiatric Association) and the ICD-11 (International Classification of Diseases, 11th edition, WHO). Both use categorical diagnoses based on symptom criteria, duration, and functional impairment. Neither relies on biomarkers. The DSM-5 abandoned the multiaxial system of DSM-IV, integrating psychosocial and medical information into a single axis. The ICD-11 incorporates a dimensional component for personality disorders.

DIATHESIS-STRESS MODEL

This foundational model proposes that mental disorders arise from the interaction between a pre-existing vulnerability (diathesis — genetic, biological, or psychological) and environmental stressors. A person with a high diathesis requires less stress to develop disorder; a person with low diathesis may only develop disorder under extreme stress. The model has been refined to include resilience factors (protective factors that raise the threshold for disorder) and gene-environment interactions.

ANXIETY DISORDERS

Generalised Anxiety Disorder (GAD): excessive, uncontrollable worry across multiple domains, present most days for at least 6 months, accompanied by physical symptoms (muscle tension, fatigue, concentration difficulty, sleep disturbance). Prevalence ~5%. Treatment: CBT (first line), pharmacotherapy (SSRIs/SNRIs, short-term benzodiazepines in acute crises only).

Panic Disorder: recurrent unexpected panic attacks (sudden surge of intense fear with somatic symptoms: palpitations, sweating, trembling, shortness of breath, chest pain, dizziness), followed by persistent concern about further attacks or avoidance behaviour. Agoraphobia frequently co-occurs. CBT with interoceptive exposure is highly effective.

DEPRESSION

Major Depressive Disorder (MDD): depressed mood and/or anhedonia (loss of pleasure) present most of the day, nearly every day, for at least 2 weeks, accompanied by at least 4 of: weight/appetite change, insomnia or hypersomnia, psychomotor agitation or retardation, fatigue, worthlessness or guilt, concentration difficulties, recurrent thoughts of death or suicidal ideation. Must cause functional impairment. Lifetime prevalence ~15%. Risk factors: female sex, adverse childhood experiences, chronic illness, social isolation.

Neurobiological theories: monoamine hypothesis (deficiency of serotonin, noradrenaline, dopamine); however, this is an oversimplification — neuroendocrine dysregulation (HPA axis hyperactivity), neuroinflammation, and disrupted neuroplasticity (reduced BDNF) are also implicated.

Treatment: mild-moderate — CBT or behavioural activation; moderate-severe — SSRIs (first line pharmacotherapy). SNRIs, TCAs, and MAOIs are alternatives. Combination of psychotherapy and pharmacotherapy superior to either alone for moderate-severe depression. Electroconvulsive therapy (ECT) for treatment-resistant or psychotic depression.

EATING DISORDERS

Anorexia Nervosa: restriction of energy intake leading to significantly low body weight, intense fear of gaining weight, disturbed body image. Highest mortality of any psychiatric disorder (~5-10% per decade, combining suicide and medical complications). Complications: electrolyte disturbances (hypokalaemia, hypophosphataemia — refeeding syndrome risk), cardiac arrhythmias, osteoporosis, amenorrhoea.

Bulimia Nervosa: recurrent binge eating followed by compensatory behaviours (vomiting, laxatives, excessive exercise). Normal or elevated body weight. Dental erosion (parotid hypertrophy, Russell's sign — calluses on dorsal hand from induced vomiting). CBT-BN is first-line.

PSYCHOTIC SPECTRUM DISORDERS

Schizophrenia: positive symptoms (hallucinations — usually auditory, delusions, disorganised thinking/behaviour), negative symptoms (flat affect, alogia, avolition, anhedonia, social withdrawal), and cognitive symptoms. Onset typically late teens to early twenties. Dopamine hypothesis: mesolimbic dopamine overactivity underlies positive symptoms; mesocortical underactivity underlies negative symptoms. All antipsychotics block D2 receptors. Clozapine is reserved for treatment-resistant schizophrenia (agranulocytosis risk requires regular FBC monitoring).

PERSONALITY DISORDERS

Enduring, inflexible patterns of inner experience and behaviour that deviate markedly from cultural expectations, are pervasive across contexts, and cause impairment. ICD-11 uses a dimensional model (mild-moderate-severe) with five trait domains (negative affectivity, detachment, dissociality, disinhibition, anankastia) plus a borderline specifier. Borderline Personality Disorder (BPD): characterised by emotional instability, impulsivity, unstable relationships, identity disturbance, and self-harm. Dialectical Behaviour Therapy (DBT) is the evidence-based treatment.

PSYCHOLOGICAL TREATMENTS

Cognitive Behavioural Therapy (CBT): structured, time-limited therapy targeting the relationship between thoughts, feelings, and behaviours. Identifies cognitive distortions (catastrophising, black-and-white thinking, mind-reading, fortune-telling) and challenges them via Socratic questioning and behavioural experiments. Strong evidence for depression, anxiety disorders, OCD, PTSD, and eating disorders.

Acceptance and Commitment Therapy (ACT): third-wave CBT; emphasises psychological flexibility, acceptance of difficult thoughts and emotions, and commitment to values-based action rather than symptom elimination.

Interpersonal Therapy (IPT): focuses on improving interpersonal functioning and communication skills in four problem areas: grief, role disputes, role transitions, and interpersonal deficits. Effective for depression.

ADOLESCENCE AND MENTAL HEALTH

Adolescence (approximately 10-25 years) is the peak onset period for mental disorders — 75% of lifetime mental disorders begin before age 25. Brain development continues through the mid-twenties, with prefrontal cortex maturation lagging limbic development, explaining increased risk-taking. Sleep architecture shifts (delayed circadian phase) are normal but conflict with school start times. Identity formation (Erikson's identity vs. role confusion stage) is a central task.