Giant breed dogs (Great Danes) are prone to grow cysts in bones when young. The cysts can lead to bone fracture and lameness. The vet will diagnose this disease by X-ray and biopsy. If your Dane suffers from bone cysts, surgery is required to remove them.    Cysts can also develop in Dane's iris which can cause vision loss. It mostly occurs in old Danes. So, consult a canine eye specialist to treat this eye condition.