Phenylephrine eyedrops as a diagnostic test in equine grass sickness

The effect of an ocular administration of the alpha -1 adrenergic agonist p henylephrine was studied in 23 cases of grass sickness and 12 control horse s. In the horses with grass sickness there was a significantly greater mean increase in the size of the palpelbral fissure, as measured by the change in the angle of the eyelashes to the head observed from a frontal view.