Clinical Vignette
Patient: "Ms. R," 34-year-old woman, referred by her primary care physician after failing two adequate trials of SSRIs (sertraline 200mg × 12 weeks; escitalopram 20mg × 10 weeks).
Chief Complaint: "Nothing works. I've been depressed for as long as I can remember."
History of Present Illness: Ms. R reports persistent low mood, anhedonia, hypersomnia (10-12 hours/night with difficulty waking), increased appetite with 15-pound weight gain over 6 months, psychomotor retardation, difficulty concentrating, and feelings of worthlessness. She denies suicidal ideation but describes passive death wishes ("I wouldn't mind not waking up").
Psychiatric History: First episode at age 16. She reports 4-5 distinct depressive episodes. Between episodes, she describes periods lasting 2-4 weeks of "feeling great," during which she sleeps 4-5 hours without fatigue, takes on multiple projects, talks more than usual, and makes impulsive purchases. She does not perceive these periods as problematic: "That's just when I feel normal."
Substance Use: Social alcohol (2-3 drinks/week). Denies cannabis, stimulants, or other substance use.
Medical History: Hypothyroidism (treated, TSH within normal limits). No other significant medical conditions.
Family History: Mother has Bipolar I Disorder; maternal uncle completed suicide at age 45.
Mental Status Exam: Cooperative. Psychomotor retardation observed. Affect flat with congruent mood. Speech normal rate and rhythm. Thought process linear. No perceptual disturbances. Insight limited; judgment fair.
Step 1: Medical and Substance Rule-Outs
DSM-5-TR requires exclusion of medical conditions and substance effects before diagnosing a primary mood disorder. This hierarchical exclusion is the mandatory first step in psychiatric differential diagnosis.
Medical Etiologies
Ms. R has treated hypothyroidism. Hypothyroidism can produce depressive symptoms (fatigue, weight gain, cognitive slowing, depressed mood). However, her TSH is within normal limits on replacement therapy. A free T4 level should be confirmed, but the clinician can provisionally proceed with the differential while awaiting lab confirmation.
Other medical conditions to screen for in treatment-resistant presentations:
- Anemia (CBC)
- Vitamin B12 and folate deficiency
- Sleep apnea (given hypersomnia)
- Cushing syndrome (weight gain, mood disturbance)
Substance-Induced Etiology
Alcohol consumption at 2-3 drinks/week is below the threshold typically associated with substance-induced depressive disorder. No other substance use is reported. This etiology is provisionally ruled out.
Step 1 Outcome
Medical and substance etiologies are provisionally cleared. Proceed to primary mood disorder differential.
Step 2: Bipolar II Disorder Evaluation
This is the critical diagnostic pivot in this case. Treatment-resistant depression is one of the most common presentations of undiagnosed Bipolar II Disorder. Antidepressant monotherapy in Bipolar II can worsen outcomes and increase cycling frequency.
DSM-5-TR Criteria for Hypomanic Episode (Required for Bipolar II)
Criterion A: Distinct period of abnormally elevated, expansive, or irritable mood AND increased energy/activity, lasting at least 4 consecutive days.
Evidence: Ms. R describes 2-4 week periods of "feeling great" with significantly decreased sleep need (4-5 hours), increased project initiation, pressured speech, and impulsive purchasing. These represent a distinct change from her baseline depressive presentation. Duration exceeds the 4-day minimum. MET.
Criterion B: During the mood disturbance, three or more of the following (four if mood is irritable):
- Inflated self-esteem or grandiosity — Not directly endorsed but implicit in taking on "multiple projects" with confidence. Possibly met.
- Decreased need for sleep — Sleeping 4-5 hours without fatigue, compared to baseline hypersomnia of 10-12 hours. This is a dramatic shift. MET.
- More talkative than usual — "Talks more than usual." MET.
- Flight of ideas — Not directly reported. Needs clarification.
- Distractibility — Not directly reported during these periods. Needs clarification.
- Increase in goal-directed activity — Takes on multiple projects. MET.
- Excessive involvement in activities with high potential for painful consequences — Impulsive purchases. MET.
At minimum 4 symptoms are met (decreased sleep, talkativeness, goal-directed activity, impulsive spending). Criterion B is MET.
Criterion C: Episode associated with unequivocal change in functioning uncharacteristic of the individual when not symptomatic.
Evidence: The shift from hypersomnia/anhedonia to reduced sleep/high productivity represents an unambiguous functional change. MET.
Criterion D: Disturbance in mood and change in functioning observable by others.
Needs verification: Collateral information from family, partner, or close friends is essential. This is a common clinical gap.
Criterion E: Episode not severe enough to cause marked social/occupational impairment or necessitate hospitalization. No psychotic features.
Evidence: Ms. R does not describe hospitalizations or fully impaired functioning during these periods. She perceives them positively. MET (consistent with hypomania, not mania).
Critical Diagnostic Consideration
Ms. R's perception that her hypomanic periods represent "feeling normal" is a classic presentation. Patients with Bipolar II frequently identify depressed states as their illness and hypomanic states as their baseline. This perceptual bias leads to significant underreporting of hypomanic symptoms and is a primary driver of Bipolar II misdiagnosis. The Mood Disorder Questionnaire (MDQ) or Hypomania Checklist (HCL-32) can supplement clinical interviewing.
Step 3: Major Depressive Disorder (Recurrent) Evaluation
If the Bipolar II hypothesis is ultimately rejected (e.g., hypomanic episodes do not meet duration or severity thresholds upon further investigation), the differential proceeds to MDD.
Ms. R meets DSM-5-TR criteria for a Major Depressive Episode:
- Depressed mood (most of the day, nearly every day)
- Anhedonia
- Weight gain / increased appetite
- Hypersomnia
- Psychomotor retardation (observed on MSE)
- Difficulty concentrating
- Feelings of worthlessness
Seven of nine criteria are met; five are required. The depressive episode component is unambiguous regardless of whether the final diagnosis is MDD or Bipolar II.
The presence of atypical features (hypersomnia, increased appetite, leaden paralysis, rejection sensitivity) should be specified. Atypical features are statistically more common in Bipolar II than in unipolar MDD, which adds further weight to the bipolar hypothesis.
Step 4: Persistent Depressive Disorder (Dysthymia) Evaluation
Ms. R states she has been depressed "for as long as I can remember," with onset at age 16. This raises the possibility of Persistent Depressive Disorder (PDD), defined by depressed mood for most of the day, more days than not, for at least 2 years (1 year in adolescents).
Key Differentiating Questions
- Has there been a symptom-free interval of 2+ months? If yes, PDD is excluded for that period.
- Are the hypomanic episodes symptom-free intervals or distinct mood states? If they represent distinct mood elevation (as the evidence suggests), a Bipolar II diagnosis supersedes PDD.
- Does the severity of the current episode exceed PDD criteria? Ms. R meets full MDE criteria, which is consistent with PDD with persistent major depressive episode (the DSM-5-TR allows this specification).
PDD remains a consideration if Bipolar II is ruled out. It would be specified with "persistent major depressive episode" and "with atypical features."
Step 5: Borderline Personality Disorder Evaluation
Mood instability is a feature of Borderline Personality Disorder (BPD), and some clinicians consider BPD in treatment-resistant depression presentations. Key distinguishing features:
- Mood reactivity pattern: BPD mood shifts are typically brief (hours to days), triggered by interpersonal events, and involve rapid oscillation between depression, anxiety, and anger. Ms. R's mood states are sustained (weeks to months) and do not appear triggered by interpersonal events. This pattern favors a mood disorder.
- Interpersonal instability: BPD involves a pattern of unstable relationships, fear of abandonment, and identity disturbance. These features are not described in the vignette.
- Self-harm / suicidality: BPD frequently involves recurrent self-harm behaviors. Ms. R denies these.
- Chronic emptiness: Ms. R reports anhedonia during depressive episodes, but this is better accounted for by a mood disorder.
BPD Assessment
Insufficient evidence for BPD. The mood pattern (sustained episodes with clear onset/offset rather than rapid reactive shifts) is more consistent with a primary mood disorder. BPD is not excluded definitively but is a lower-probability consideration.
Diagnostic Formulation
Primary Diagnostic Consideration
Bipolar II Disorder, current episode depressed, with atypical features
The convergence of multiple lines of evidence supports this diagnosis: (1) treatment resistance to SSRI monotherapy, (2) distinct hypomanic episodes meeting DSM-5-TR criteria for duration and symptom count, (3) atypical depressive features (hypersomnia, weight gain), (4) family history of Bipolar I Disorder, and (5) early onset (age 16).
Recommended Next Steps
- Obtain collateral information from family members or partner regarding behavior during "good periods" (Criterion D verification).
- Administer the Mood Disorder Questionnaire (MDQ) or Hypomania Checklist (HCL-32) as structured screening instruments.
- Confirm thyroid panel (free T4, TSH) to rule out subclinical hypothyroidism.
- Screen for sleep apnea given hypersomnia and weight gain.
- Construct a life chart (retrospective timeline of mood episodes, treatments, and life events) to visualize the cycling pattern.
- If Bipolar II is confirmed: Taper SSRI and initiate mood stabilizer (e.g., lithium, lamotrigine) per current evidence-based guidelines.
Teaching Points
- Treatment-resistant depression warrants systematic re-evaluation of the diagnosis before adding or switching antidepressants.
- Bipolar II is misdiagnosed as unipolar MDD frequently. The average delay from onset to correct diagnosis exceeds 10 years.
- Patients rarely report hypomania spontaneously because they experience it as normal or desirable functioning. Direct, behaviorally specific questioning is required: "Have you ever had periods of several days where you needed much less sleep than usual and didn't feel tired?"
- Atypical features (hypersomnia, hyperphagia, leaden paralysis, rejection sensitivity) are present more commonly in Bipolar II depressive episodes than in unipolar MDD episodes.
- Family history of Bipolar I or II significantly increases the prior probability of a bipolar diagnosis in the proband.