Parasympatholytics act on the parasympathetic nervous system that regulates internal organs like the digestive tract and heart. They make this aspect of the nervous system less active, and can be used in the treatment of a variety of conditions. The same compounds are present in some toxins; these work by damaging the parasympathetic nervous system to disrupt key internal functions. Some medications in this family have a very narrow therapeutic index, meaning that the difference between a medically useful and dangerous dose can be very small.
There are a wide range of uses for parasympatholytics in medicine. One is in eye examinations, where medications like atropine can dilate the eye. The medication blocks smooth muscle contractions so the eye can’t focus and the pupil widens instead of contracting. This allows a vision specialist to look inside the eye and clearly see its internal structures, which may be important for diagnosis and treatment of disease.
Some of these compounds can help in the treatment of asthma and heart conditions by forcing muscles to relax. Smooth muscle contraction can constrict the airways and cause heart rate problems, so parasympatholytics may be used to interrupt these contractions. They are also useful in the management of some gastrointestinal conditions where the patient experiences frequent uncontrolled smooth muscle contractions, and in the treatment of painful bladder spasms.
Salivary and tear glands can also be controlled by parasympatholytics, because these secretions are regulated by the parasympathetic nervous system, as are the sweat glands. People with severe sweating problems, for instance, may be given these drugs as therapy to keep sweat production down. Care is required to make sure the patient is still able to safely regulate internal temperature, a concern when sweating is suppressed; while excessive sweating is not desired, some sweat is necessary to control temperature.
Another clinical indication for parasympatholytics can be treatment of certain kinds of poisoning. Atropine is a useful antidote to nerve gas, for instance, and members of the military may be provided with instruction in how to use it in an emergency. Hospitals may maintain a small stock of parasympatholytics for this purpose so they are prepared for patients exposed to nerve toxins.
Concerns about the safety of parasympatholytics may lead some medical facilities to keep them in a carefully controlled area. Before using them, health care providers have to demonstrate that the medication and dosage are correct, and may need to follow a specific protocol before administering any drugs to patients. This ensures that patients are not accidentally given dangerous medications or excessive doses of prescribed drugs.