## Diagnosis: Prolactinoma with Mass Effect **Key Point:** This patient has a macroprolactinoma (>1 cm, prolactin >200 ng/mL) with visual field defect from optic chiasm compression—a classic presentation of symptomatic prolactinoma. **High-Yield:** Even with mass effect and visual symptoms, dopamine agonists are first-line therapy for prolactinomas because: 1. They shrink the tumor in 80–90% of cases 2. Visual field defects improve in most patients within weeks to months 3. Surgery is reserved for dopamine-resistant tumors or acute apoplexy **Clinical Pearl:** Bitemporal hemianopia indicates compression of the inferior nasal fibers of the optic chiasm as the tumor grows superiorly—a hallmark of sellar/suprasellar pathology. ## Management Algorithm ```mermaid flowchart TD A[Prolactinoma diagnosed]:::outcome --> B{Macroprolactinoma with mass effect?}:::decision B -->|Yes| C[Start dopamine agonist]:::action C --> D[Repeat MRI in 3-6 months]:::action D --> E{Tumor shrunk + symptoms improved?}:::decision E -->|Yes| F[Continue medical therapy]:::action E -->|No| G[Consider surgery or higher dose]:::action B -->|No microprolactinoma| H[Medical therapy ± monitoring]:::action ``` **Mnemonic:** **DOPAMINE FIRST** — Dopamine agonists are first-line even for large prolactinomas with mass effect, unless there is acute apoplexy or visual loss refractory to medical therapy. ## Why This Works - **Cabergoline** (preferred) or **bromocriptine** bind D2 receptors on lactotroph cells, inhibiting prolactin secretion and causing tumor shrinkage - Typical response: prolactin normalization in 2–4 weeks; tumor shrinkage in 3–6 months - Visual field defects often improve before radiological shrinkage is evident **Tip:** Surgery is indicated only if: - Dopamine agonist resistance (rare, <5%) - Intolerance to medical therapy - Acute apoplexy with severe vision loss or pituitary apoplexy [cite:Harrison 21e Ch 375]
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