seasonal affective disorder

**Definition (DSM-5):** Not a standalone diagnosis; it is the "with seasonal pattern" specifier for recurrent MDD (or bipolar I/II depressive episodes). Criteria: regular temporal relationship between onset and time of year, full remission (or switch to mania/hypomania) at a characteristic time of year, pattern present for at least 2 consecutive years with no non-seasonal episodes in that period, and seasonal episodes substantially outnumber non-seasonal episodes over the patient's lifetime.

**Epidemiology/risk factors:** Prevalence increases with distance from the equator (reduced winter daylight). More common in women (~3-4:1), onset typically in young adulthood (20s-30s). Family history of mood disorders or SAD; the pattern is more common in bipolar II than in unipolar MDD. A summer-pattern variant also exists (less common): insomnia, decreased appetite, weight loss, agitation.

**Pathophysiology (proposed):** - Reduced light exposure disrupts circadian rhythms (phase delay hypothesis) and alters melatonin secretion (prolonged nocturnal melatonin duration in winter). - Decreased serotonergic tone: reduced serotonin transporter (SERT) availability changes in winter (higher SERT binding leads to lower synaptic serotonin). Light therapy and SSRIs both enhance serotonergic signaling. - Reduced vitamin D has been hypothesized, but evidence for supplementation is weak. - Light is detected by melanopsin-containing intrinsically photosensitive retinal ganglion cells, which signal to the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN regulates pineal melatonin release.

**Clinical features:** Depressed mood, anhedonia, low energy, and impaired concentration for at least 2 weeks, with "atypical" or reverse vegetative features in winter type: hypersomnia, hyperphagia with carbohydrate craving, weight gain, leaden paralysis, and interpersonal rejection sensitivity. Suicidality can occur and must be assessed.

**Diagnosis:** Clinical. Rule out hypothyroidism (check TSH), substance use, bipolar disorder (screen for past mania/hypomania, because light therapy and antidepressants can precipitate a switch), and other medical causes.

**Management:** - **Bright light therapy** (first line, mild-moderate): 10,000 lux white light box for about 30 minutes each morning, started in early fall before symptoms begin. Response typically occurs within 1-2 weeks. Adverse effects: headache, eye strain, nausea, insomnia, and hypomania/mania induction. Use caution with retinal disease or photosensitizing drugs; UV-filtered boxes are preferred. - **Pharmacotherapy:** SSRIs (e.g., fluoxetine, sertraline) are effective. **Bupropion XL** is FDA-approved for prevention of SAD when started in autumn (before symptoms) and continued through winter until early spring. - **CBT** adapted for SAD (CBT-SAD) shows durable benefit and may have lower relapse rates than light therapy at 1-2 years. - Others: dawn simulation, outdoor time, exercise. Melatonin timing strategies have limited evidence.

**High-yield pearls:** - Depression with seasonal pattern = atypical features (reverse neurovegetative symptoms) in winter type. - Bupropion XL is the only agent FDA-approved for SAD prevention. - Always screen for bipolar disorder before starting light therapy or an antidepressant. - Differentiate from adjustment disorder or situational stressors that happen to be seasonal (e.g., an annual school schedule or the anniversary of a loss). These do not count toward the seasonal pattern. - Persistent depressive disorder and atypical depression are distinct diagnoses, though they can overlap in features.