Tests of Vasomotor Reactions
These generally test sympathetic cholinergic function. Measurement of the skin temperature is a rough but useful index of vasomotor function. Vasomotor paralysis results in vasodilatation of skin vessels and a rise in skin temperature; vasoconstriction lowers the temperature. With a skin thermometer, one may compare affected and normal areas under standard conditions. The normal skin temperature is 31°C (87.8°F) to 33°C (91.4°F) when the room temperature is 26°C (78.8°F) to 27°C (80.6°F). Vasoconstrictor tone may also be tested by measuring the reduction in skin temperature at a distant site before and after immersing one or both hands in cold water (see the discussion of the cold pressor test later).
The integrity of the sympathetic reflex arc, which includes baroreceptors in the aorta and carotid sinus, their afferent pathways, the vasomotor centers, and the sympathetic and parasympathetic outflow can be tested in a general way by combining the cold pressor test, grip test, mental arithmetic test, and Valsalva maneuver, as described below.
Vasoconstriction induces an elevation of the blood pressure. This is the basis of the cold pressor test. In normal persons, immersing one hand in ice water for 1 to 5 min raises the systolic pressure by 15 to 20 mm Hg and the diastolic pressure by 10 to 15 mm Hg. Similarly, the sustained isometric contraction of a group of muscles (e.g., those of the forearm in handgrip) for 5 min normally increases the heart rate and the systolic and diastolic pressures by at least 15 mm Hg. The response in both of these tests is reduced or absent with lesions of the sympathetic reflex arc, particularly of the efferent limb, but neither of these tests has been well quantitated or validated. The stress involved in doing mental arithmetic in noisy and distracting surroundings will also stimulate a mild but measurable increase in pulse rate and blood pressure. Obviously this response does not depend on the afferent limb of the sympathetic reflex arc and must be mediated by cortical–hypothalamic mechanisms.
If the response to the Valsalva maneuver is abnormal and the response to the cold pressor test is normal, the lesion is probably in the baroreceptors or their afferent nerves; such a defect has been found in diabetic and tabetic patients and is common in many neuropathies. A failure of the heart rate and blood pressure to rise during mental arithmetic coupled with an abnormal Valsalva maneuver suggests a defect in the central or peripheral efferent sympathetic pathways.
Tests of Sudomotor Function
The integrity of sympathetic efferent pathways can be assessed further by tests of sudomotor activity. There are several of these, all used mainly in specialized autonomic testing laboratories. The most rudimentary tests involve weighing sweat after it is absorbed by small squares of filter paper. Also, powdered charcoal dusted on the skin will cling to moist areas and not to dry ones. The starch iodine (Minor) is a qualitative test, which uses color changes of cornstarch dusted on the skin covered with tincture iodine. Minor’s tests can be used to detect hypoor hyperhidrosis. Previously described quinizarin (gray when dry, purple when wet) uses similar principle. The methods described above reflect postganglionic sudomotor function.