Fuchs' dystrophy is a relatively common corneal dystrophy. To understand what it is, you must first understand what the cornea is. The cornea is the clear "window" covering the iris and the pupil of the eye. The cornea must be clear in order for the vision to be clear. The cornea can be thought of in three layers. The bulk of the cornea is called the stroma. The stroma is covered externally by an epithelium which is several cell layers thick. The stroma is lined internally by the endothelium which is one single layer thick.
The endothelium of the cornea is the primary area of pathology in Fuchs' corneal dystrophy. When the endothelium is unhealthy, the cells die, do not regenerate and they leave dots on the inside of the cornea which we refer to as guttata. Guttata is often the first sign of Fuchs' dystrophy. Guttata can be visually significant and cause some glare and reduce quality of the vision, however, guttata do not in themselves cause serious visual loss.
In late stages of Fuchs' dystrophy, the corneal endothelial cells lose their ability to perform their primary duty. The chief job of the endothelial cells is to pump fluid from the stroma. As the endothelial cells die and this ability to pump fluid from the stroma is lost, the stroma accumulates fluid, referred to as corneal stromal edema. As this corneal stromal edema develops, the cornea becomes thicker because of the fluid. It also becomes more hazy, like a steamy window. This results in fairly significant decreased vision. When corneal stromal edema develops as results of Fuchs' corneal dystrophy, many patients require penetrating keratoplasty (corneal transplantation) in order to recover their vision.
Without penetrating keratoplasty, patients with corneal stromal edema can develop epithelial edema. Epithelial edema can lead to microscopic blisters on the cornea and occasionally large blisters on the cornea which can become painful and cause recurrent erosions of the eye. In this situation, corneal transplantation is helpful as it can relieve the pain the patient is suffering.