Methylxanthines are alkaloids that occur naturally in plants and are found in tea, coffee beans, cola beans, and cocoa beans.
The methylxanthines in chocolate include caffeine and theobromine. Some methylxanthines are used therapeutically as bronchodilators
including theophylline and aminophylline. Chocolate toxicity is a very common, year round problem with peak times or "Chocolate
Season" beginning with Halloween then Christmas through Valentine's Day and ending with Easter.
Methylxanthines are rapidly absorbed by mouth and metabolized in the liver. Elimination is via the bile and/or urine and
varies with the age and species of the patient. Methylxanthines undergo enterohepatic recirculation and can be reabsorbed
via the bladder wall. These characteristics will guide decontamination recommendations (see below).
The elimination half-life of methylxanthines varies widely by species. For theophylline, the elimination half-life is 5.7
h in dogs, 7.8 h in cats and 20-36 h in humans. For theobromine, the elimination half-life is 17.5 h in dogs and 6-10 h in
humans. For caffeine, the elimination half-life is 4.5 h in dogs, 3-6 h in humans (longer in infants and for overdoses in
adults) and 1.5 - 2 h in mice and rats. The toxic doses for dogs are theobromine LD50 250-500mg/kg, caffeine MLD 140-150 mg/kg and theophylline LD50 250 mg/kg. For cats, theobromine LD50 200 mg/kg, caffeine MLD 100-150 mg/kg and theophylline LD50 800 mg/kg.
Methylxanthines work by blocking adenosine receptors (adenosine is a bronchoconstrictor, anticonvulsant, and regulates heart
rhythm), by increased calcium movement into cells and by increasing cAMP via inhibition of phosphodiesterase. These actions
show clinically as stimulation of the CNS and cardiovascular systems.
In dogs, the listed chocolate dosages are expected to show the following clinical effects. 20 mg/kg: mild signs (vomiting,
hyperactivity, PD). 40-50 mg/kg: moderate to severe signs (cardiotoxicity). 60 mg/kg: seizures possible. And 100 mg/kg
is the minimum lethal dosage.
Clinical signs usually develop within 6-12 hours of ingestion and can include polydipsia, vomiting, diarrhea, bloating, restlessness,
hyperactivity (progressing to ataxia, tremors and seizures), cardiac effects (PVCs, tachycardia), tachypnea, hypertension,
hyperthermia and coma. Pancreatitis is also possible since chocolates have high fat and sugar content and may contain nuts.
Calculating the dosage of methylxanthines exposure is an important step in managing these cases. It is often difficult to
identify the amounts and types of chocolate used with products from a bakery or a prepackaged mix and the same problem can
arise with assorted filled candies and chocolate-covered nuts. It is therefore best to assume the worst case scenario and
calculate the dosage as if the product was solid chocolate. First determine the amount of methylxanthines in the type of
chocolate using the table below then calculate the dosage of total methylxanthines per kg of patient body weight.
An example dosage calculation follows: A 9 # dog eats 5 oz dark chocolate. Dark chocolate contains approximately 130 mg theobromine
and 20 mg caffeine per ounce. To calculate dose based on total methylxanthines: 5 oz X 150 mg/oz = 750 mg and 750 mg divided
by 9 kg equals 83.3 mg/kg as the total methylxanthine exposure in the dog.
Calculating the dosage for the newer gourmet chocolate bars on the market is a little different. To calculate the dosage
in those cases follow this example. The % cocoa actually refers to the % of chocolate liquor in the bar. Chocolate liquor
is cooled into blocks and used as unsweetened baking chocolate. So the % cocoa listed on the label should be multiplied by
the value for unsweetened baking chocolate (~400 mg/oz). A 70% bar would have (0.7 X 400) = 280 mg/oz and a 9 kg dog would
only have to eat 2 oz = 60 mg/kg.
Management of chocolate exposures includes stabilizing the patient then deciding on decontamination. Emesis may be effective
even hours after ingestion since chocolate can form a bezoar in the stomach. Lavage can be considered in cases where emesis
is contraindicated, not possible or has been unsuccessful. For example, lavage is an option if the patient is agitated, seizing
or recumbent or has other health concerns. Lavage is not likely to be as effective as emesis and is associated with potential
risks so should not be decided on haphazardly. Activated charcoal is also used frequently in the management of chocolate
exposures. Repeated doses may be used due to the extensive enterohepatic recirculation (q 4-6 hr X up to 3 treatments).
It is important to monitor serum sodium levels when using activated charcoal due to the risk of hypernatremia developing.
The patient should be monitored including heart rate and rhythm, blood gases and baseline pancreatic enzymes and repeat as
needed. Supportive care may include thermoregulation, diazepam, barbiturate, methocarbamol for tremors/seizures and oxygen
as needed for cardiopulmonary compromise. Treat arrhythmias as needed (propranolol). IV fluids are used to enhance excretion
and provide CV support. A urinary catheter can be placed to decrease reabsorption across bladder wall or the patient should
be walked frequently.