Atopic dermatitis is one of the most common causes of pruritus in dogs. It is diagnosed based on a dog's history and clinical
signs and the elimination of other causes of pruritus.
The pathogenesis of pruritus in dogs is still not well understood. To further complicate matters, some clinicians speculate
that certain dogs have a combination of sensory irritation (true itch) and repetitive behaviors (self-directed itch), though
these dogs are hard to differentiate from dogs with purely sensory irritation. But the dogs that are most suspected of having
this combination are those that clients describe as becoming itchy when stressed.
Treatment options for atopy vary depending on a variety of factors, including the severity of signs, whether the signs are
year-round or seasonal, the dog's response to medical treatment, and the owner's financial constraints. Dogs receiving immunotherapy
often also receive concurrent antipruritic therapy on either a long-term or intermittent basis.
In a recent study, oral dextromethorphan hydrobromide was evaluated in 14 dogs with atopic dermatitis to determine whether
the drug had any effect on repetitive behaviors associated with or suggestive of pruritus (e.g. self-licking, self-chewing, self-biting).1 A veterinary dermatologist diagnosed atopic dermatitis in the dogs, and all of the dogs were free of other complicating causes
of pruritus (e.g. mites, yeast). After a three-week washout period from any drugs the dogs may have been receiving before the study, the dogs
were randomly enrolled in a four-week, placebo-controlled, double blind crossover study. The dogs received 2 mg/kg dextromethorphan
in a gelatin capsule every 12 hours and a placebo for two weeks each. During the study, oatmeal soaks were allowed, but no
other therapies. Twelve dogs completed the study; two dropped out because of adverse effects (sedation, diarrhea). Because
of a lack of compliance with data collection, the investigators were only able to evaluate data from 10 dogs. An important
component of the study was the owners' scoring of their dogs' pruritus. The owners were required to record how much time they
spent with their dogs and the amount of time the dogs exhibited certain itch behaviors. In addition, a dermatologist examined
the dogs at various points in the study.
The investigators used a calculation called the itch percent to evaluate the data. The itch percent was calculated based on the time the owners spent with their dogs and how much time
the dogs were observed to be involved in itch-scratch behaviors during these periods. Dogs receiving the placebo were observed
to exhibit itch behaviors 8.7% of the time; dogs receiving dextromethorphan exhibited itch behaviors 6% of the time. The difference
(2.7%) was statistically significant and represents a 31% decrease in observed itch behaviors. A global dermatology score
was calculated for all the dogs, which consisted of three components of the dermatologist's assessment (a pruritus score,
an inflammation score, and an overall score). The dermatologist's global assessment was more favorable after the active treatment
phase in 11 of the 12 dogs, and the pruritus score during dextromethorphan treatment was significantly less compared with
the placebo and baseline. The investigators concluded from these findings that dextromethorphan was beneficial in reducing
the time the dogs spent licking, chewing, and self-biting.
The findings in this study suggest that dextromethorphan may be clinically useful in atopic dogs with habituated pruritic
behaviors. However, a placebo effect may still have been a factor. Moreover, the investigators stated that the oatmeal soaks
might have contributed to some of the overall gross improvement in the skin. And the 31% decrease in pruritus represents 23
minutes of itch-scratch behavior when the dogs received the placebo vs. 17 minutes when they received dextromethorphan. I'm
not sure that the average client would find a decrease of six minutes of itching to be a great improvement. Finally, in a
recent study on the pharmacokinetics of dextromethorphan after intravenous and oral administration in healthy dogs, the investigators
found that the drug had a short half-life, was rapidly cleared, and had poor bioavailability.2 The authors of this pharmacokinetics study concluded that the drug's erratic absorption, short elimination half-life, and
rapid clearance limit its potential usefulness when administered orally long-term.2