A 52-year-old man from Delhi presents to the emergency department with sudden onset severe headache, palpitations, and profuse sweating for the past 2 hours. On examination, blood pressure is 210/140 mmHg, heart rate 118/min, and he appears anxious. His pupils are dilated and he has piloerection. Urine metanephrines are markedly elevated. Which of the following best explains the constellation of sympathetic hyperactivity observed in this patient?

Explanation

## Clinical Presentation Analysis This patient presents with a classic pheochromocytoma crisis, characterized by catecholamine excess (elevated metanephrines confirm this). ## Sympathetic Receptor Physiology **Key Point:** Catecholamines (epinephrine and norepinephrine) exert their effects through α and β adrenergic receptors distributed across multiple organs. | Receptor | Location | Effect | Clinical Sign | |----------|----------|--------|----------------| | α1 | Vascular smooth muscle | Vasoconstriction | Hypertension (210/140) | | β1 | Heart | Increased HR, contractility | Tachycardia (118/min), palpitations | | α2 | Presynaptic terminals | Negative feedback | Overridden in excess catecholamine state | | β2 | Bronchial/vascular smooth muscle | Bronchodilation, vasodilation | Tremor, anxiety | ## Mechanism in This Case **High-Yield:** In pheochromocytoma, massive catecholamine release activates: 1. **α1-receptors** on arteriolar smooth muscle → sustained vasoconstriction → severe hypertension 2. **β1-receptors** on cardiac myocytes → increased heart rate, contractility, and AV nodal conduction → palpitations and tachycardia 3. **α1-receptors** on iris dilator muscles → pupillary dilation (mydriasis) 4. **Sympathetic activation** of sweat glands → profuse diaphoresis 5. **CNS sympathetic arousal** → anxiety and restlessness **Clinical Pearl:** The combination of hypertension + tachycardia + diaphoresis + anxiety is pathognomonic for catecholamine excess and distinguishes it from other hypertensive emergencies (e.g., hypertensive encephalopathy, which typically presents with bradycardia). ## Why This Is Not Parasympathetic Blockade Parasympathetic blockade (anticholinergic effect) would cause dry skin and normal or decreased sweating, not profuse diaphoresis. Sweating is a **sympathetic cholinergic** response (acetylcholine at muscarinic M3 receptors on sweat glands), which is preserved and enhanced in catecholamine excess.