General approach to the diagnosis of autonomic disorders

      Autonomic disorders typically manifest with autonomic failure or less commonly autonomic hyperactivity affecting the sympathetic, parasympathetic, or enteric nervous system either selectively or in various combinations. Their evaluation requires a complete medical and drug history, neurologic and general physical examination, general laboratory tests, studies tailored to specific autonomic syndromes, and autonomic laboratory evaluation. Thermoregulatory function is tested with the thermoregulatory sweat test and sudomotor axon reflex tests. The heart rate responses to deep breathing and the Valsalva ratio assess cardiovagal function, and the beat-to-beat blood pressure responses to the Valsalva maneuver and head-up tilt assess sympathetic vasomotor function. Disorders causing autonomic failure are classified according to their temporal profile and the presence or absence of associated neurologic manifestations. Subacute isolated autonomic failure typically occurs in autoimmune ganglionopathies. Their evaluation includes testing for ganglionic nicotinic receptor and paraneoplastic antibodies. In selected cases, cerebrospinal fluid examination (CSF), electromyogram (EMG) and imaging to detect underlying malignancy are indicated. Autonomic failure may be a major manifestation of sensorimotor peripheral neuropathies such as seen in diabetes or amyloidosis, ganglionopathies, or distal painful neuropathies. Evaluation includes EMG, immunoelectrophoresis with immunofixation in plasma and urine, HIV and Sjögren disease serology, and sometimes tissue biopsy. Progressive autonomic failure occurs in synucleinopathies, including multiple system atrophy and Lewy body disorders. Evaluation includes brain imaging, urodynamic studies, and polysmonogram. Autonomic hyperactivity may occur in autoimmune limbic encephalitis. Evaluation includes testing for autoantibodies, particularly those against the voltage gated potassium channel complex or NMDA receptors in serum and CSF.
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