Biochemical Assessment of Sympathoadrenal Activity

This chapter discusses the major biochemical techniques for the assessment of sympathoadrenal system (SAS) activity. Because of the complex pattern, no single measurement of catecholamine or its metabolites can provide full assessment of SAS activity at any given time. Full assessment of SAS activation requires integration of information from several diverse analytic approaches that are generally more complementary than redundant. Plasma norepinephrine (NE) remains the most durable and widely used biochemical index to assess physiologic and pathologic increases in sympathetic activity. Total or fractionated urinary catecholamines are used occasionally in the determination of 24-hour sympathetic nervous activity and are still quite commonly used in the diagnosis of pheochromocytoma. Under usual conditions, the bulk of urinary NE is derived from circulatory rather than renal sources. Normal total urinary catecholamine values are usually 100 mcg or less per day. Metanephrines, the primary O-methylated metabolites of catecholamines, have limited usefulness in the physiologic assessment of sympathetic tone.