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A systemic disease is one that affects a number of organs and tissues, or affects the body as a whole. Although most medical conditions will eventually involve multiple organs in advanced stage (e.g. Multiple organ dysfunction syndrome), diseases where multiple organ involvement is at presentation[ambiguous] or in early stage are considered elsewhere.
Getting a regular eye exam may play a role in identifying the signs of some systemic diseases. "The eye is composed of many different types of tissue. This unique feature makes the eye susceptible to a wide variety of diseases as well as provides insights into many body systems. Almost any part of the eye can give important clues to the diagnosis of systemic diseases. Signs of a systemic disease may be evident on the outer surface of the eye (eyelids, conjunctiva and cornea), middle of the eye and at the back of the eye (retina)."
Since 500 B.C., some researchers have also believed that the physical condition of the fingernails and toenails can indicate various systemic diseases. Careful examination of the fingernails and toenails may provide clues to underlying systemic diseases, since some diseases have been found to cause disruptions in the nail growth process. The nail plate is the hard keratin cover of the nail. The nail plate is generated by the nail matrix located just under the cuticle. As the nail grows, the area closest to becoming exposed to the outside world (distal) produces the deeper layers of the nail plate, while the part of the nail matrix deeper inside the finger (proximal) makes the superficial layers. Any disruption in this growth process can lead to an alteration in the shape and texture.
Take pitting, for example. Pitting looks like depressions in the hard part of the nail. Pitting usually is associated with psoriasis, affecting 10 to 50 percent of patients with that disorder. Pitting also may be caused by a variety of systemic diseases, including Reiter's syndrome and other connective tissue disorders, sarcoidosis, pemphigus, alopecia areata, and incontinentia pigmenti. Because pitting is caused by defective layering of the superficial nail plate by the proximal nail matrix, any localized dermatitis (e.g., atopic dermatitis or chemical dermatitis) that disrupts orderly growth in that area also can cause pitting.