The autonomic nervous system (ANS) is distributed throughout the body and governs involuntary physiological homeostasis, operating in close coordination with the endocrine system. Understanding the ANS begins with the classic distinction proposed by John Langley in the early twentieth century, who divided it into sympathetic and parasympathetic systems. The sympathetic division coordinates patterned, integrated responses to stressors such as orthostatic challenge, thermoregulation, hypoglycemia, hemorrhage, physical exertion, and emotional stimuli. Preganglionic neurons arise in the intermediolateral columns of the spinal cord and synapse in sympathetic ganglia near the spinal cord. Postganglionic fibers release norepinephrine at effector organs to regulate vasomotor tone, cardiac output, and sweat production. The parasympathetic division, by contrast, regulates targeted reflexes affecting cranial and pelvic organs. It originates in the brainstem (cranial nerves III, VII, IX, and X) and sacral spinal cord (S2–S4), utilizes long preganglionic fibers, and is predominantly cholinergic, releasing acetylcholine at both the ganglionic and end-organ synapses. It mediates functions such as pupillary constriction, salivation, bronchoconstriction, gastrointestinal motility, and bladder contraction. Control of autonomic output is modulated by the central autonomic network (CAN), involving brain regions such as the hypothalamus, brainstem nuclei, and cerebral cortex. These centers integrate internal and environmental signals to maintain autonomic homeostasis. The peripheral autonomic nervous system encompasses the preganglionic and postganglionic fibers, autonomic ganglia, and target organs. It serves as the final common pathway through which central autonomic commands are executed, and reflex arcs are completed. A key reflex within this system is the baroreflex, which stabilizes blood pressure via afferent input from baroreceptors in the carotid sinus and aortic arch, relayed through cranial nerves IX and X to the nucleus tractus solitarius in the medulla. The resulting efferent output modulates vagal tone to the heart and sympathetic tone to the vasculature, adjusting heart rate and vascular resistance in response to acute blood pressure changes. Autonomic dysfunction refers to partial, variable, or intermittent impairment in autonomic regulation. Patients may experience fluctuating symptoms such as episodic lightheadedness, mild bladder or bowel dysregulation, or transient heat intolerance. This may represent an early or mild stage of autonomic involvement, or may occur in non-neurogenic conditions, such as medication side effects, post-viral syndromes, or metabolic imbalances. In contrast, autonomic failure reflects persistent, severe, and progressive loss of autonomic function. It manifests as sustained orthostatic hypotension, anhidrosis, fixed heart rate responses, or complete neurogenic bladder. This often indicates neurogenic pathology, such as Multiple System Atrophy, Pure Autonomic Failure, or autoimmune autonomic ganglionopathy, and typically requires targeted diagnostic testing and long-term management. A practical clinical framework begins with five essential questions: In conclusion, evaluating a patient with suspected autonomic dysfunction requires a focused, stepwise, and structured approach, grounded in a solid understanding of neuroanatomy and physiology. By systematically assessing symptom patterns, functional severity, temporal course, anatomical localization, and underlying etiology, clinicians can efficiently narrow the differential diagnosis and guide appropriate testing. References