Experimental research has not provided any proof that "trophic" corneal diseases are caused by nutritional deficiencies. It seems that cases in which the course is chronic, but characterized by an absence of exacerbations of great pain and failure of medical therapy, are said to be trophic. Some of these diseases are dry eye conditions. Sensory disturbances lead to corneal complications by lowering the blinking frequency, but not seem to interfere with nutrition of the tissue. Otherwise keratoplasty would be impossible, since the innervation of the graft is often impaired for months or even years.