Equine metabolic syndrome (EMS) is the most common cause of laminitis.

Laminitis occurs when the pedal bone attachments to the hoof wall are inflammed.  Normally, pedal bone is suspended by strong attachments within the hoof capsule. 

With laminitis, these attachments fail and cause the pedal bone to rotate and sink which is very painful for your horse or pony.  Laminitis can range from acute and severe to mild, chronic and insidious.