What are the differences in diagnosis and treatment between Seasonal Affective Disorder (SAD) and Major Depressive Disorder (MDD)?

Seasonal Affective Disorder vs Major Depressive Disorder

Diagnostic Distinction

Seasonal Affective Disorder (SAD) is a subtype of Major Depressive Disorder (MDD) characterized by recurrent depressive episodes that occur at specific times of year with full remission at other times, requiring two consecutive years of seasonal episodes for diagnosis. 1, 2

Key Diagnostic Criteria Differences

SAD-Specific Requirements:

• Depressive episodes must occur at a predictable time of year (typically fall/winter onset) with full remission during other seasons (typically spring/summer) 1, 3
• Diagnosis requires two consecutive years of episodes occurring in the same season 1
• Episodes must fully remit when the specified season ends 2
• Winter-type SAD typically presents with atypical features: hypersomnia, increased appetite, weight gain, carbohydrate craving, social withdrawal, and fatigability 4, 3

MDD Diagnostic Criteria:

• Depressed mood or anhedonia plus at least 5 symptoms lasting ≥2 weeks affecting normal functioning 5, 6
• Symptoms include significant weight change, sleep disturbance, psychomotor changes, fatigue, worthlessness/guilt, concentration difficulties, and suicidal ideation 5
• No seasonal pattern required - episodes can occur at any time without predictable timing 5

Assessment Tools

For SAD:

• Seasonal Pattern Assessment Questionnaire (SPAQ) is the screening instrument of choice 7
• Structured Interview Guide for the Hamilton Depression Rating Scale, Seasonal Affective Disorders (SIGH-SAD) is used to assess severity and includes 7 SAD-specific items beyond the standard HAMD-17 4

For MDD:

• Patient Health Questionnaire-9 (PHQ-9) with cutoff ≥8 indicating clinically significant depression 6, 8
• Hamilton Depression Rating Scale (HAM-D) for severity assessment 5, 6

Treatment Differences

First-Line Treatment for SAD

Light therapy is the first-line treatment specifically for SAD, distinguishing it from standard MDD treatment. 1, 3, 9

Light Therapy Protocol:

• 2,500 to 10,000 lux intensity at eye level for 30-60 minutes daily 1, 3
• Morning administration is superior to evening treatment 3
• Response rates reach approximately 80% in selected patient populations 3
• Atypical depressive symptoms predict favorable treatment outcomes 3
• Effects are mediated exclusively through the eyes, not skin 3

Pharmacotherapy for SAD:

• Bupropion has the strongest evidence for long-term prevention of SAD episodes 4, 1
• Bupropion 150-300 mg daily initiated in autumn (September-November) before symptom onset prevents seasonal episodes in 84.3% vs 72.0% placebo 4
• Treatment continues through winter with discontinuation after 2-week taper in spring (approximately 4-6 months total) 4
• SSRIs (sertraline, fluoxetine) are effective alternatives based on double-blind, placebo-controlled trials 3

First-Line Treatment for MDD

The American College of Physicians strongly recommends either cognitive behavioral therapy (CBT) or second-generation antidepressants (SGAs) as first-line treatment for MDD, with no preference for light therapy. 5, 6

Treatment Selection Algorithm:

• Mild MDD: Start with CBT alone 6
• Moderate to severe MDD: Initiate either CBT or SGAs (SSRIs/SNRIs preferred) based on adverse effect profiles, cost, and patient preferences 5, 6
• Moderate-quality evidence shows no difference in response between CBT and SGAs (fluoxetine, fluvoxamine, paroxetine, sertraline) after 8-52 weeks 5

Treatment Duration:

• MDD: Continue treatment 4-9 months after first episode response; ≥1 year for recurrent episodes 5, 6
• SAD: Treatment typically 4-6 months (autumn through spring) with annual recurrence requiring preventive intervention 4, 1

Critical Clinical Pitfalls

For SAD:

• Failing to recognize the temporal pattern - SAD requires full remission between seasons, not just winter worsening of chronic depression 2
• Confusing SAD with dysthymia or chronic MDD with seasonal exacerbation 2
• Not initiating preventive treatment before symptom onset in autumn for patients with established SAD history 4
• Using evening light therapy instead of morning administration 3

For MDD:

• Inadequate dosing or premature discontinuation before 4-6 weeks when therapeutic effects typically emerge 6
• Discontinuing treatment before 4-9 months after response, increasing relapse risk 6
• Not monitoring for suicidality during initial treatment period 6

Epidemiology and Risk Factors

SAD-Specific Risk Factors:

• Female sex, younger age (18-30 years), living at northern latitudes, family history of depression/bipolar disorder/SAD 1, 7
• Prevalence 3-10% in temperate zones; subsyndromal SAD (S-SAD) affects 6-20% 2

MDD:

• Lifetime prevalence 16% in the United States 5
• No specific latitude or seasonal risk factors 5