Five to ten percent of all human exposures reported to poison centers involve plants. Probably because plants are so accessible and attractive to youngsters, in approximately 80% of these cases the individuals are younger than 6 years of age. As indoor plants have become ever more popular, the incidence of plant exposures has increased. Data compiled by the American Association of Poison Control Centers (AAPCC) give some indication of which plants are more commonly involved (Chap. 136), but these plants typically have relatively limited toxicity. More than 80% of patients reported to the AAPCC as being exposed were asymptomatic, less than 20% had minor to moderate symptomatology, and less than 7% necessitated a health care visit. The benignity of these exposures is represented by a fatality rate of less than 0.001%. This chapter addresses the toxicologic principles associated with the most potentially dangerous plants.
HISTORY AND CLASSIFICATION OF PLANT XENOBIOTICS
Aconitine, from monkshood, exemplifies the rich history of plant toxicology. It was believed by the Greeks to be the first poison—“lycotonum”—created by the goddess Hecate from foam of the river Cerebrus. Alkaloid constituents are responsible for its toxic (and therapeutic) effects. Alkaloids represent one of several classes of organic molecules found in plants as defined by the science of pharmacognosy, which is the science of medicines derived from natural sources. The pharmacognosy approach is consistent with the literature of plant efficacy and is applied here to their toxicity (Table 121–1). Unfortunately, the science of pharmacognosy is not always straightforward, and systems of classification may vary depending on the pharmacognosist. Hence our approach borrows primarily from two groups of authors to keep the classification as consistent as possible.48,120 The major groups are as follows:
Alkaloids: Molecules that react as bases and contain nitrogen, usually in a heterocyclic structure. Alkaloids typically have strong pharmacologic activity that defines many major toxidromes.
Glycosides: Organic compounds that yield a sugar or sugar derivative (the glycone) and a nonsugar moiety (the aglycone) upon hydrolysis. The aglycone is the basis of subclassification into saponin or steroidal glycosides (including steroidal cardiac glycosides, cyanogenic glycosides, anthraquinone glycosides), and others such as atractyloside and salicin.
Terpenes and resins: Assemblages of five-carbon units (isoprene unit) with many types of functional groups (eg, alcohols, phenols, ketones, and esters) attached. This is the largest group of secondary metabolites; approximately 20,000 are identified. Most essential oils are mixtures of monoterpenes, and the terpene name depends on the number of isoprene assemblages. Monoterpenes have two units (C10H16), sesquiterpenes have three isoprene units (C15), diterpenes have four isoprene units (C20), and triterpenes have six (C30). These molecules often play an active role in plant defense mechanisms.
Proteins, peptides, and lectins: Proteins consist of amino acid units with various side chains, and peptides consist of linkages among amino acids. Lectins are glycoproteins classified according to the number of protein chains linked by ...