The Hidden Danger in Your Garden: Cardiac Glycosides
Many beautiful and common garden plants harbor a potent secret: cardiac glycosides. These naturally occurring compounds are structurally and pharmacologically similar to digoxin, a medication used to treat heart failure and arrhythmias [1.3.3, 1.4.7]. When ingested, parts of these plants can lead to digitalis toxicity, a serious condition characterized by gastrointestinal distress, neurological symptoms, and life-threatening heart rhythm disturbances [1.2.3, 1.4.4]. While the pharmaceutical use of digoxin has declined, poisoning from both the medication and its plant-based cousins remains a significant concern, with U.S. poison centers managing thousands of cases annually [1.7.1]. Accidental ingestion by children, who may be attracted to colorful flowers or berries, accounts for a large percentage of plant-based exposures [1.7.3].
The Mechanism: How Plant Toxins Affect the Heart
The toxicity of these plants stems from their ability to inhibit the sodium-potassium ATPase pump (Na+/K+-ATPase) in the body's cells, particularly in the heart muscle [1.6.2, 1.6.4]. This inhibition causes an increase in intracellular sodium, which in turn leads to a buildup of intracellular calcium [1.6.5]. The elevated calcium increases the force of the heart's contractions (a positive inotropic effect) [1.6.4]. In a therapeutic setting with controlled doses of digoxin, this is beneficial for a failing heart. However, in an overdose situation from plant ingestion, this effect becomes dangerously exaggerated. The toxic effects are an extension of the therapeutic ones, leading to severe bradycardia (slow heart rate), various forms of atrioventricular (AV) block, and potentially fatal arrhythmias like ventricular fibrillation or asystole [1.3.4, 1.4.7]. The increased intracellular calcium also makes the myocardium irritable and prone to dangerous, spontaneous depolarizations [1.6.4].
Key Plants Containing Cardiac Glycosides
Several popular and widespread plants contain dangerous levels of cardiac glycosides. It's crucial for gardeners, parents, and pet owners to be able to identify them.
Foxglove (Digitalis purpurea)
Perhaps the most famous of these plants, foxglove is the original source of the medication digitalis [1.2.6]. All parts of the plant are poisonous, though the leaves, flowers, and seeds contain the highest concentration of compounds like digitoxin and digoxin [1.3.5, 1.2.7]. In its first year, the plant exists as a rosette of leaves and has been mistaken for edible plants like borage or comfrey, leading to accidental poisonings [1.3.6, 1.2.7]. The flowers, which appear in the second year, are typically purple, pink, or white and have a distinctive bell shape [1.2.7, 1.3.8]. Even small amounts can be fatal if ingested [1.2.7].
Oleander (Nerium oleander)
Commonly used as an ornamental shrub along roadsides and in parks, especially in warmer climates, all parts of the oleander plant are extremely toxic [1.3.5, 1.5.6]. It contains several cardiac glycosides, with oleandrin being a primary component [1.4.3]. Ingesting even a single leaf can be toxic to a child, and fatalities have been reported from eating food stirred with an oleander branch or inhaling smoke from the burning plant [1.4.6, 1.3.7]. The plant is recognizable by its thick, leathery, lance-shaped leaves and showy clusters of white, pink, or red flowers [1.3.5].
Lily of the Valley (Convallaria majalis)
Despite its delicate appearance and sweet fragrance, lily of the valley is highly poisonous [1.5.1]. It contains numerous cardiac glycosides, including convallatoxin [1.4.3]. All parts of the plant—leaves, stems, flowers, and red berries—are toxic, and ingestion of even a small amount can be fatal to humans and pets [1.5.1, 1.5.2]. Poisoning has even been anecdotally reported from drinking the water from a vase that held its flowers [1.3.7].
Other Notable Toxic Plants
Many other plants can cause a similar toxic effect. These include:
- Yellow Oleander / Lucky Nut (Cascabela thevetia): A major cause of self-harm poisonings in parts of Asia [1.7.5].
- Squill or Sea Onion (Drimia maritima): A bulb-producing plant containing bufadienolides, a different class of cardiac steroids [1.3.1, 1.4.3].
- Dogbane (Apocynum cannabinum): Contains cardiac glycosides and can be toxic if ingested [1.3.1].
- Milkweed (Asclepias species): Contains cardioactive steroids, and while some parts can be eaten if prepared properly, improper preparation or collection can lead to poisoning [1.3.3].
- Wallflower (Cheiranthus cheiri): Another garden ornamental that contains cardiac glycosides [1.3.7].
Comparison of Common Cardiotoxic Plants
| Plant | Scientific Name | Toxic Part(s) | Key Glycosides | Distinguishing Features |
|---|---|---|---|---|
| Foxglove | Digitalis purpurea | All parts, especially leaves and seeds [1.2.2] | Digitoxin, Digoxin [1.3.5, 1.2.7] | Tall spikes of bell-shaped, often spotted, flowers (purple, pink, white) [1.2.7]. |
| Oleander | Nerium oleander | All parts [1.3.5] | Oleandrin, Digitoxigenin [1.4.3] | Evergreen shrub with thick, leathery, lance-shaped leaves and fragrant flower clusters (white, pink, red) [1.3.5]. |
| Lily of the Valley | Convallaria majalis | All parts, including red berries [1.5.1] | Convallatoxin [1.4.3] | Low-growing plant with broad leaves and small, fragrant, white, bell-shaped flowers [1.5.3]. |
| Yellow Oleander | Cascabela thevetia | All parts, especially seeds [1.4.6] | Thevetin, Peruvoside [1.4.3] | Shrub or small tree with glossy green leaves and yellow, trumpet-shaped flowers. Seeds are known as 'lucky nuts' [1.7.3]. |
Symptoms and Treatment of Plant-Induced Digoxin Toxicity
Poisoning symptoms can appear within a few hours and are multifaceted [1.4.3].
- Gastrointestinal: Nausea, vomiting, abdominal pain, and diarrhea are very common early signs [1.4.4].
- Neurological: Confusion, drowsiness, weakness, headache, and visual disturbances like seeing yellow or green halos around lights (xanthopsia) can occur [1.3.4, 1.2.3].
- Cardiac: The most dangerous symptoms involve the heart. These range from sinus bradycardia (a very slow pulse) and heart block to life-threatening ventricular arrhythmias [1.4.7, 1.4.3]. Hyperkalemia (high serum potassium) is a key indicator of severe acute toxicity [1.4.6].
If ingestion is suspected, it is a medical emergency. Call a poison control center and seek immediate medical attention [1.2.6]. Treatment is supportive and may include gastrointestinal decontamination with activated charcoal to absorb the toxins, especially if administered soon after ingestion [1.4.2, 1.4.8]. Heart rhythm is closely monitored, and medications like atropine may be used for severe bradycardia [1.4.1]. In severe cases with life-threatening arrhythmias or hyperkalemia, the antidote is digoxin-specific antibody fragments (DigiFab), which bind to the toxins and neutralize them [1.4.1, 1.4.5].
Conclusion
While plants like foxglove, oleander, and lily of the valley are admired for their beauty, they pose a serious toxic threat due to the cardiac glycosides they contain. These compounds mimic the effects of the heart drug digoxin, and accidental ingestion can lead to severe poisoning and even death. Awareness and correct identification are the first lines of defense. By understanding which plants cause digoxin toxicity, recognizing their features, and knowing the symptoms of poisoning, you can ensure your home and garden remain both beautiful and safe.
For more information on poisonous plants, you can visit the Poison Control website.
