Usually the problem worsens while exercising but can be seen at anytime.
Diagnosis is difficult and treatment frequently unrewarding.
Recently sunlight was implicated as one of the causes, resembling photic sneezing in humans. This has provided a successful way to treat some of theses horses. This article deals with the diagnosis and treatment of this frustrating problem. Certainly the first thing to try is to see if working out of the sunlight helps. If not attempt blindfolding to determine if the problem has some relationship with the brightnesss of light.