Clonidine is a commonly prescribed biochemical derivative of imidazoline with a variety of clinical uses. It was originally developed as a nasal decongestant, but its main use has been as an anti-hypertensive agent. In recent years it has gained popularity in pediatric patients for the treatment of attention-deficit hyperactivity disorder (ADHD), Tourette syndrome, and sleep disturbances. It has also been shown to be useful in treating several different drug withdrawal syndromes. Clonidine’s effect on the body stems from its action on alpha-2 receptors, imidazoline receptors, and the functional overlap of alpha-2 receptors on mu receptors. Overdose results in a toxidrome not easily identified.
Clonidine toxicity may occur from over-placement of transdermal patches or accidental or deliberate overdose of the tablet form.
Over the past few decades, as more clinical indications for clonidine have been discovered, prescribing rates have increased. Consequently, misuse and accidental ingestion of clonidine have increased. According to an analysis looking at data from the National Poison Data System, the number of unintentional pediatric exposures to alpha-2 agonists increased 5.9% per year between 2000 and 2011. Clonidine was the most commonly reported culprit agent and with the highest percentage of “moderate to major outcomes as compared to other commonly prescribed A-2 agonists, specifically, guanfacine and tizanidine.
As mentioned, clonidine is an alpha-2 receptor and imidazoline-1 receptor agonist. These receptors have been found to work in tandem to produce the hypotensive effects of clonidine. By activating the presynaptic alpha-2 adrenergic receptors of the central nervous system, specifically in the rostral ventrolateral medulla and the nucleus tractus solitarii, release of norepinephrine (NE) into neuronal synapses is inhibited. This inhibition then reduces sympathetic tone throughout the body, particularly decreasing the amount of vasoconstriction causing hypotension and bradycardia. A-2 receptors are also found in the locus coeruleus (LC), which plays a large role in autonomic function and states of arousal. Clonidine’s inhibitory effect on the LC causes disinhibition of the ventrolateral preoptic nucleus which is responsible for the sedative effect of the drug. The inhibition of the LC also contributes to the decrease in circulating norepinephrine and indirectly contributes to the hypotension and bradycardic effects.
In tandem with this, Imidazoline-1 receptors in the rostral ventrolateral medulla, which have a high affinity for clonidine, compound the inhibition of sympathetic neurotransmission when activated by clonidine. I-1 receptors are also found peripherally in the plasma membranes of the heart and kidneys and cause a decrease in peripheral vascular resistance, heart rate, and cardiac contractility. Clonidine’s effect on nitric oxide production in the endothelial cells and the subsequent vasodilation has also been discussed. Additionally, clonidine has been shown to affect GABA levels by several different mechanisms, which clearly play a role in sympathetic/parasympathetic regulation.
Clonidine is rapidly absorbed, with 100% bioavailability when given by mouth. Its peak concentration in the blood occurs 2 to 4 hours post-ingestion, and its half-life is between 5 and 13 hours. Half the administered dose is excreted unchanged in the urine. Toxic exposure via clonidine transdermal patches, while less common, is more likely to produce significant symptoms in those exposed. Patches can contain up to 9 mg of clonidine and if ingested may have a prolonged release in the gastrointestinal (GI) tract.
Poisoning with clonidine classically presents with somnolence, respiratory depression, hypotension, bradycardia, and miosis. Alteration in level of consciousness usually presents within the first one and a half hours, preceding any cardiovascular effects which usually present within 4 hours. Of note, patients who ingest large amounts may initially have early hypertension which then progresses to hypotension. This is poorly described in the literature, but it is likely related to peak serum concentrations and baseline sympathetic tone prior to administration of clonidine. Hypothermia is also observed, which can help in differentiation from opioid toxicity as this is not observed in those cases.
Many sources report dose-dependent and age-dependent response, although neither is well established. It is suggested that cardiovascular and respiratory effects are limited to ingestions of greater than 0.01 mg/kg to- 0.02 mg/kg while other sources document toxicity after ingestion of only 0.2 mg. One study done on adults showed the severity of bradycardia to be consistently dose-dependent, while hypotension was much more variable in severity, time of onset and duration. Age affects the sensitivity of presynaptic A-2 receptors and decreases the reuptake of NE, so elderly patients will have an overall increase in NE concentration at baseline as compared to younger patients. This can attenuate the effects of clonidine and the doses required to cause toxicity. A study done on pediatric overdoses shows the highest incidence of clinical effects in the 0-4 years age group when compared to five to eight year and nine to 12 year age groups.
Clonidine drugs levels do not correlate with toxicity, and blood should not be drawn routinely. Blood should be evaluated for electrolyte and glucose levels and to screen for anion gap acidosis or hypoglycemia.
Other studies to consider include:
The majority of clonidine overdoses in adults and children have not been shown to cause significant symptoms requiring major intervention. Death or the need for invasive intervention such as intubation or use of vasopressors are rare and are reported only in extreme cases, for example with very young age or extremely large dose/patch ingestion. In a study done on adults, bradycardia was the most consistent finding, lasting about 24 to 48 hours post ingestion.
As severe toxicity requiring advanced resuscitation is rare, supportive care is the basis of managing clonidine overdoses. Hypotension should be addressed first with fluid resuscitation. In refractory cases, pressors are recommended. Dopamine and NE have both been cited as the agent of choice. However, there have been no studies to support the use of one over the other. Dopamine may also improve associated bradycardia. One case report demonstrated worsening hypotension with the prehospital administration of epinephrine. Rarely, intubation is needed for patients with severe respiratory depression. No specific recommendations for RSI medications are found.
The use of naloxone as an antidote to clonidine has been debated for many years. Its efficacy in this situation is thought to stem from the reversal of endogenous opioids released in the body secondary to clonidine toxicity. This, however, is dependent on several factors including the patient’s baseline blood pressure and sympathetic tone before ingestion. Several published studies and case reports show differing outcomes, highlighting the likely role of unknown cofactors. Despite its questionable utility, sources recommend judicious use of naloxone in cases where the patient is not protecting their airway, as the incidence of adverse effects is rare. Escalating doses of 0.1 mg to 10 mg is suggested.
Atropine for bradycardia has also been suggested although it too has inconsistent results when studied. One study reported eight adults treated with atropine to have had transient improvement of heart rate of about an hour duration. In a bradycardic patient who becomes hemodynamically unstable, the transcutaneous or transvenous pacing is suggested but unsubstantiated.
The consensus from multiple sources is that a patient who does not show any clinical signs of toxicity within several hours of ingestion may be discharged home following an observation period. Four to 8 hours has been suggested as the appropriate time frame. In adults without co-ingestion, intensive care unit (ICU) stay is not indicated as significant delayed effects are rare. Adults with isolated bradycardia may even be discharged in some situations. Pediatric ICU admission is recommended for pediatric patients with any cardiovascular or central nervous system symptoms.
Clonidine toxicity can impact many organ systems, and it is important to have a multidisciplinary team looking after the patient. Patients who are stable can be discharged and need to be educated on how to properly store the medication, away from the reach of children. For those with intentional overdose, a mental health consult is necessary prior to discharge. 
For the majority of patients with clonidine toxicity, the prognosis is good with treatment. However, those who delay seeking treatment may suffer adverse cardiac or neurological problems. Deaths from clonidine toxicity are very rare.