Clinical trial of cetirizine in seasonal headshaking horses. Can we predict the effectiveness using population parameters?

Abstract

Idiopathic headshaking is a disorder caused by a lower stimulus threshold of the trigeminal nerve. Headshaking is characterized by uncontrollable shaking and tossing of the head and can be seasonal or non-seasonal. Because an allergic component may play a role in seasonal headshaking the therapeutic effect of the antihistamine cetirizine was tested. This paper focusses on population parameters. Can they help in predicting whether cetirizine treatment will be effective? 83 owners of headshaking horses filled out a questionnaire regarding the use of the horse, the way of housing, the history of the horse and questions about the shaking behaviour and possible triggers for shaking. A double blind placebo controlled clinical trial was conducted using 30 horses. The horses were treated with cetirizine (0.4 mg/kg twice daily) and a placebo, both for a week with a washout week in between. At the start of the experiment and at the end of every treatment week a standardised movie was made to assess the amount of headshaking. When counting the headshaking in these movies, there was no significant difference between the cetirizine and placebo treatment. However, four of the 30 horses showed a clinically significant improvement with cetirizine based on (blinded) owner opinion. Population parameters were analysed in order to compare these 4 horses to the non-responders. The four horses in the effective group all showed signs of respiratory disease beside their headshaking and this proportion was significantly different from that of the non-responding horses. Although the group of horses responding to therapy was small, this significant finding may be of clinical interest and warrants further research.