Disorders of gastrointestinal motility are readily demonstrated radiologically. In dysautonomic states, a barium swallow may disclose a number of abnormalities, including atonic dilatation of the esophagus, gastric atony and distention, delayed gastric emptying time, and a characteristic small bowel pattern consisting of an increase in frequency and amplitude of peristaltic waves and rapid intestinal transit. A barium enema may demonstrate colonic distention and a decrease in propulsive activity. Sophisticated manometric techniques are now available for the measurement of gastrointestinal motility (see Low et al).
Nocturnal penile tumescence is recorded in some sleep laboratories and may be used as an ancillary test of sacral autonomic (parasympathetic) innervation.
Pharmacologic Tests of Autonomic Function
After examining the pupils in ambient light, bright light, and low light to determine if one has lost sympathetic or parasympathetic innervation, pharmacologic tests can be used to refine diagnosis. Part of the rationale behind these special tests is the “Cannon law,” or the phenomenon of denervation hypersensitivity, in which an effector organ, 2 to 3 wk after denervation, becomes hypersensitive to its particular neurotransmitter substance and related drugs. In clinical testing, an agent is instilled into both conjunctival sacs and the nonmiotic pupil is used as a control to compare the one suspect of being involved by Horner syndrome.
Relatively recently, the weak direct sympathetic agonist apraclonidine has been used most widely to demonstrate that miosis is due to sympathetic denervation of the pupil. It reverses miosis that is due to a central or a peripheral lesion and is easier to obtain then older agents. A positive test, reversal of miosis, depends on the denervation hypersensitivity that develops after several days or more of the presence of the Horner syndrome. If there is a negative result, no enlargement of the pupil, the miosis is probably physiologic. The drug has the additional advantage of often reversing the ptosis of Horner syndrome (see Chap. 13 and Fig. 13-10 for discussion). The drug may cause respiratory suppression in children and is avoided.
Once the presence of a genuine Horner syndrome has been established, it is possible to differentiate prefrom postganglionic (superior cervical ganglion) sympathetic denervation of the pupil by instilling 1 percent hydroxyamphetamine; its effect depends on the presence of existing norepinephrine in the end terminals of the nerves that innervate the iris. Failure to dilate indicates a postganglionic lesion.
Another test, now used mostly in children, in whom apraclonidine represents a risk, is the topical application into the conjunctival sac of a 4 to 10 percent cocaine solution that potentiates the effects of NE by preventing its reuptake. A normal response to cocaine consists of pupillary dilatation. In sympathetic denervation caused by lesions of the postor preganglionic fibers, no change in pupillary size occurs because no transmitter substance is available and the cocaine has no substrate to potentiate. The reason for lack of response in chronic preganglionic lesions is presumed to be a depletion of NE in the postganglionic fibers. In cases of central sympathetic lesions, slight mydriasis may occur.
The intracutaneous injection of 0.05 mL of 1:1,000 histamine normally causes a 1-cm wheal after 5 to 10 min. This is surrounded by a narrow red areola, which in turn, is surrounded by an erythematous flare that extends 1 to 3 cm beyond the border of the wheal. A similar “triple response” follows the release of histamine into the skin as the result of a scratch. It can be elicited in sensitive individuals by scratching the skin (dermatographia).