Difficulty: Intermediate | Prerequisites: Parts 1 and 2 of these notes.
This final set of notes covers differential diagnosis for the remaining major DSM categories: neurodevelopmental disorders, feeding and eating disorders, sleep-wake disorders, substance-related conditions, somatic symptom disorders, impulse control disorders, and several other diagnostic groups. Many of these share overlapping features with the mood, anxiety, and psychotic disorders covered in Parts 1 and 2.
Neurodevelopmental disorder
A group of conditions with onset in the developmental period (typically before school age) that produce deficits in personal, social, academic, or occupational functioning. In simple terms, these are conditions the person grows up with, not conditions that develop later in life.
Eating disorder
A persistent disturbance in eating behaviour or related behaviour (e.g. purging) that results in altered consumption or absorption of food and significantly impairs physical health or psychosocial functioning. Think of it as: the relationship with food or body image has become the central problem.
Substance use disorder
A pattern of substance use leading to clinically significant impairment or distress, assessed across 11 criteria (tolerance, withdrawal, loss of control, etc.). The number of criteria met determines severity: mild (2 to 3), moderate (4 to 5), severe (6 or more).
Somatic symptom disorder
One or more somatic (physical) symptoms that are distressing or result in significant disruption of daily life, accompanied by excessive thoughts, feelings, or behaviours related to the symptoms. In simple terms, the person has real physical symptoms, and the problem is the disproportionate psychological response to them.
Impulse control disorder
A class of disorders characterised by the failure to resist an impulse, drive, or temptation to perform an act that is harmful to the person or others. Examples include kleptomania, pyromania, and intermittent explosive disorder.
Paraphilic disorder
A paraphilia (an intense, persistent sexual interest in atypical objects, situations, or individuals) that causes distress, impairment, or involves non-consenting persons. The distinction between a paraphilia and a paraphilic disorder is important: the disorder requires distress, impairment, or harm to others.
Differentials: ODD, DMDD, ASD, Reactive Attachment Disorder, Personality Disorders, Depressive Disorders, Bipolar Disorder, Substance Use, Specific Learning Disorder, Intellectual Disability, Anxiety Disorders, Psychotic Disorders, Medication-Induced Symptoms, Neurocognitive Disorders
ADHD requires onset before age 12 and symptoms present in two or more settings. ODD involves defiance; ADHD involves inattention and hyperactivity without intentional defiance. Bipolar disorder is episodic; ADHD is persistent. Anxiety can mimic inattention (the student is distracted by worry, not by ADHD). Always ask whether a medication (e.g. bronchodilators, anticonvulsants) could be producing the symptoms.
Differentials: Rett Syndrome, Selective Mutism, Intellectual Disability without ASD, ADHD, Language Disorder, Social Pragmatic Communication Disorder, Stereotypic Movement Disorder, Schizophrenia
ASD requires deficits in social communication and interaction plus restricted, repetitive patterns of behaviour. Social Pragmatic Communication Disorder involves the social communication deficits without the restricted/repetitive behaviours. Intellectual disability alone does not include the restricted interest pattern. Schizophrenia has a later onset and involves psychotic symptoms.
Differentials: Major/Mild Neurocognitive Disorders, Communication Disorders, Specific Learning Disorder, ASD
Onset during the developmental period with deficits in intellectual and adaptive functioning. Neurocognitive disorders involve a decline from a prior level of functioning, whereas intellectual disability reflects a developmental deficit. Specific learning disorder involves difficulties in a circumscribed academic area with otherwise normal intellectual functioning.
Differentials: Abnormal Movements Due to Other Medical Conditions, Stereotypic Movement Disorder, Substance-Induced Paroxysmal Dyskinesias, Myoclonus, OCD-Related Disorders
Tourette's requires both motor and vocal tics present for more than one year. Stereotypic movements are rhythmic and patterned (rocking, hand flapping); tics are sudden, rapid, and non-rhythmic. OCD compulsions are preceded by obsessions; tics are preceded by premonitory urges.
Differentials: Bulimia Nervosa, Body Dysmorphic Disorder, ARFID, Substance Use, OCD
Restriction of energy intake leading to significantly low body weight, with intense fear of gaining weight and disturbance in self-perceived body shape. Bulimia involves recurrent binge eating and compensatory behaviours at a normal or above-normal weight. BDD concerns appearance but not weight. ARFID involves food restriction without the body image disturbance. OCD rituals around food exist but are driven by contamination or other obsessional content, not weight and shape.
Differentials: Binge Eating Disorder, Borderline Personality, Kleine-Levin Syndrome, Anorexia Nervosa (binge-eating/purging type), MDD with atypical features
Recurrent binge eating followed by compensatory behaviours (purging, fasting, excessive exercise), with self-evaluation unduly influenced by body shape and weight. Binge eating disorder lacks the compensatory behaviours. Anorexia (binge-eating/purging type) involves the same behaviours but at significantly low body weight.
Differentials: Bulimia Nervosa, Bipolar Disorders, Depressive Disorders, Borderline Personality, Obesity
Recurrent binge eating without compensatory behaviours. Unlike bulimia, there is no purging, fasting, or excessive exercise to offset binges. Obesity is a medical condition, not a mental disorder, though the two can co-occur.
Differentials: ASD, Specific Phobia, Social Anxiety, Anxiety Disorders, OCD, Anorexia Nervosa, Other Medical Conditions, Neurological/Neuromuscular Conditions, MDD, Schizophrenia Spectrum, Factitious Disorder
Food avoidance or restriction based on sensory characteristics, fear of aversive consequences (choking, vomiting), or apparent lack of interest in eating, without the body image disturbance of anorexia. The absence of weight and shape concern is the key distinguishing feature from anorexia.
Differentials: Factitious Disorder, Anorexia Nervosa, Nonsuicidal Self-Injury in Personality Disorders
Persistent eating of non-nutritive, non-food substances. The behaviour must be inappropriate to the developmental level and not part of a culturally sanctioned practice.
Narcolepsy differs from other hypersomnias, ADHD, seizures, depression, and sleep deprivation. The hallmark features are cataplexy (sudden loss of muscle tone triggered by emotion) and REM intrusion symptoms (hypnagogic hallucinations, sleep paralysis).
Nightmare Disorder must be distinguished from Sleep Terror Disorder (the person wakes from NREM sleep, is confused and inconsolable, and has no dream recall, unlike nightmares which produce vivid recall), REM Sleep Behaviour Disorder (the person acts out dreams physically), and nocturnal panic attacks.
Circadian Rhythm Sleep-Wake Disorders must be separated from normative sleep variation in shift workers, insomnia disorder, and mood disorders that disrupt sleep.
Somatic Symptom Disorder differs from Panic Disorder, GAD, Illness Anxiety, Conversion Disorder, Delusional Disorder, BDD, Depressive Disorders, and OCD. The somatic symptoms are real (not fabricated), and the problem is the excessive cognitive, emotional, or behavioural response to them.
Illness Anxiety Disorder differs from Somatic Symptom Disorder (minimal or no somatic symptoms, whereas somatic symptom disorder has prominent physical symptoms), Anxiety Disorders, OCD, Psychotic Disorders, and medical conditions.
Conversion Disorder (Functional Neurological Symptom Disorder) must be distinguished from Factitious Disorder/Malingering (intentional production of symptoms), Somatic Symptom Disorder, BDD, Dissociative Disorders, Neurological Disease, Panic Disorder, and Depression.
The DSM lists substance-induced variants for nearly every major diagnostic category (depressive, bipolar, anxiety, psychotic, OCD, sleep). The core differential question is always: did the symptoms emerge during or shortly after intoxication or withdrawal, and do they resolve with sobriety? If yes, the substance-induced diagnosis applies. If symptoms persist well beyond the expected physiological effects, a primary disorder is more likely.
Caffeine Intoxication mimics Panic Disorder, GAD, and Manic Episode. Cannabis Use Disorder overlaps with GAD, MDD, and Psychotic Disorders. Stimulant Intoxication can produce symptoms indistinguishable from Schizophrenia (Paranoid Type) or Mania.
Intermittent Explosive Disorder must be distinguished from DMDD, ODD, Conduct Disorder, ADHD, ASD, ASPD, BPD, Delirium, and substance intoxication/withdrawal. The outbursts are impulsive (not premeditated) and grossly out of proportion to the provocation.
Kleptomania differs from ordinary theft (deliberate, for gain), ASPD, Conduct Disorder, Mania, and Malingering. The impulse to steal is driven by tension and relief, not by need or profit.
Pyromania must be distinguished from Conduct Disorder, ASPD, Manic Episodes, and fire-setting in the context of delusions, substance intoxication, or intellectual disability.
Differentials: Psychotic Disorders, Bipolar/Depressive Disorders with Psychotic Features, Acute Stress Disorder, Malingering/Factitious Disorder, Other Neurocognitive Disorders, Depersonalisation/Derealisation
Acute disturbance in attention and awareness that develops over hours to days and fluctuates in severity. The acute onset and fluctuating course distinguish it from dementia (Major Neurocognitive Disorder), which has a gradual onset and stable or progressive course. Delirium can co-occur with dementia.
Differentials: Normal Cognition, Delirium, MDD, Specific Learning Disorder, Other Neurodevelopmental Disorders
Cognitive decline from a prior level of functioning in one or more cognitive domains (memory, attention, executive function, language, etc.). MDD can produce "pseudodementia" with cognitive slowing, but it resolves with treatment of the depression. Delirium fluctuates acutely; neurocognitive disorders are stable or progressive.
Differentials: Borderline Personality, Somatic Symptom Disorder, Conversion Disorder, Malingering
Intentional falsification of medical or psychological symptoms in the absence of external incentives. Malingering also involves intentional fabrication, but for external gain (avoiding work, obtaining compensation). Conversion disorder involves genuine distress with neurological-type symptoms that are not intentionally produced.
Differentials: BDD, Transvestic Disorder, Nonconformity to Gender Roles, Schizophrenia/Psychotic Disorders
Marked incongruence between experienced gender and assigned gender, with associated distress. Nonconformity to gender roles (e.g. a girl who prefers stereotypically masculine activities) does not in itself constitute gender dysphoria. BDD involves dissatisfaction with a perceived physical defect, not gender identity. In rare cases, psychotic delusions may involve gender, but the broader psychotic presentation distinguishes this.
Differentials: Non-disordered Gambling, Manic Episode, Personality Disorders, Other Medical Conditions
Persistent and recurrent problematic gambling leading to clinically significant distress or impairment. Excessive gambling during a manic episode does not warrant a separate gambling disorder diagnosis if it occurs exclusively during mania.
Students often think ADHD cannot co-exist with ASD. It can, and the DSM-5 removed the previous exclusion rule. Both diagnoses can be given when criteria for each are independently met.
Students sometimes assume that any food restriction equals an eating disorder. ARFID involves restriction without body image concerns; medical conditions, OCD, and anxiety can all produce food avoidance for non-eating-disorder reasons.
Students frequently believe that delirium and dementia are interchangeable terms for confusion in older adults. Delirium is acute, fluctuating, and often reversible. Dementia (major neurocognitive disorder) is gradual, stable or progressive, and typically irreversible.
Students may think that factitious disorder and malingering are the same because both involve fabricating symptoms. The distinction is motivation: factitious disorder has no external incentive (the goal is to assume the sick role), while malingering is driven by external gain (financial compensation, avoiding legal consequences).
⚠️ ADHD onset must be before age 12. Any exam vignette with cognitive symptoms beginning in adulthood should prompt consideration of other diagnoses (neurocognitive disorder, depression, substance use, medical condition).
⚠️ ASD vs. Social Pragmatic Communication Disorder: the differentiator is whether restricted/repetitive behaviours are present (ASD) or absent (SPCD).
⚠️ Anorexia vs. ARFID: the presence of body image disturbance and fear of weight gain points to anorexia. ARFID involves food restriction for other reasons (sensory aversion, fear of choking, lack of appetite).
⚠️ Bulimia vs. Binge Eating Disorder: compensatory behaviours (purging, fasting, excessive exercise) are present in bulimia and absent in binge eating disorder.
⚠️ Delirium vs. Dementia: acute and fluctuating (delirium) vs. gradual and stable/progressive (dementia). This is one of the most commonly tested distinctions in both psychology and medical exams.
⚠️ Substance-induced disorders: always consider whether symptoms appeared during intoxication or withdrawal and whether they resolve with sobriety. If yes, the substance-induced diagnosis applies.
True or False: ADHD can now be co-diagnosed with ASD in the DSM-5. (True.)
Fill in the blank: ARFID differs from Anorexia Nervosa because ARFID lacks ___. (Body image disturbance / fear of weight gain.)
True or False: Delirium typically has a gradual onset over months. (False. Delirium develops over hours to days.)
Fill in the blank: In factitious disorder, symptoms are fabricated without ___; in malingering, they are fabricated for ___. (External incentives; external gain.)
True or False: Binge Eating Disorder includes compensatory behaviours such as purging. (False. That is Bulimia Nervosa.)
Q: A 7-year-old child has difficulty sustaining attention in class, frequently loses belongings, and cannot sit still. His teacher reports that these behaviours have been present since kindergarten and occur both at school and at home. What is the most likely diagnosis, and what must you rule out?
A: ADHD (combined presentation). Rule out anxiety disorder (worry can mimic inattention), specific learning disorder (academic struggles may look like inattention), intellectual disability, and medication side effects.
Q: A 16-year-old girl has restricted her diet to only three foods due to fear of vomiting. She is significantly underweight. She denies any concern about body shape or desire to be thinner. What diagnosis fits best?
A: ARFID. The food restriction is driven by fear of aversive consequences (vomiting), not by body image disturbance, which rules out Anorexia Nervosa.
Q: An 82-year-old man is brought to the emergency department confused and agitated. His family says he was fine yesterday. He is unable to focus, keeps picking at his hospital gown, and his alertness fluctuates over the hour. What is the most likely diagnosis?
A: Delirium. The acute onset (fine yesterday), fluctuating course, and attentional disturbance are characteristic. Dementia is ruled out by the acuity of onset.
Q: A woman presents with weakness in her left arm. Neurological examination finds no organic cause, and the pattern of weakness is inconsistent with known neurological pathways. She appears distressed by the symptom. What is the most likely diagnosis, and what should be ruled out?
A: Conversion Disorder (Functional Neurological Symptom Disorder). Rule out true neurological disease (always re-evaluate), factitious disorder (symptoms are intentionally produced), and malingering (symptoms produced for external gain). Her genuine distress argues against factitious disorder and malingering.
Q: A patient reports recurrent urges to steal small items from shops. He does not need the items and could afford them. He describes mounting tension before stealing and relief afterward. What is the diagnosis, and what must you differentiate it from?
A: Kleptomania. Differentiate from ordinary theft (planned, for gain), ASPD (broader pattern of disregard for others), Conduct Disorder (in adolescents), and theft during a manic episode (other manic symptoms would be present).
Substance-induced disorders are relevant to every category covered in Parts 1 and 2: substance-induced anxiety, depression, psychosis, and OCD variants must always be considered before assigning a primary diagnosis. The neurodevelopmental disorders covered here (ADHD, ASD) appear as differentials for mood disorders, personality disorders, and psychotic disorders in Part 2.
The somatic symptom and factitious disorder material connects to the dissociative and conversion disorder content in Part 1, since all involve physical or neurological-type symptoms that require careful differential assessment.
Related Terms / Search Tags
Differential diagnosis, DSM-5, DSM-5-TR, ADHD, attention deficit hyperactivity disorder, autism spectrum disorder, ASD, intellectual disability, Tourette's, tic disorder, anorexia nervosa, bulimia nervosa, binge eating disorder, ARFID, avoidant restrictive food intake disorder, pica, narcolepsy, nightmare disorder, circadian rhythm, somatic symptom disorder, illness anxiety disorder, conversion disorder, functional neurological symptom disorder, factitious disorder, malingering, delirium, dementia, neurocognitive disorder, gender dysphoria, gambling disorder, kleptomania, pyromania, intermittent explosive disorder, substance use disorder, substance-induced disorders, caffeine intoxication, cannabis use disorder, stimulant intoxication, paraphilic disorders, impulse control, abnormal psychology, psychopathology, clinical psychology exam