What are sty symptoms and signs?
The first signs and symptoms of a sty are usually redness, tenderness, and pain in the affected area. The eye may feel irritated or "scratchy." Later signs and symptoms may include swelling, discomfort during blinking of the eye, watering of the eye, and sensitivity to light. A common sign of a sty is a small, yellowish spot at the center of the bump that represents pus rising to the surface.
How is a sty diagnosed?
A sty is diagnosed by its characteristic appearance and symptoms. No other tests are necessary to establish the diagnosis of a sty.
Who is most susceptible to the development of a sty?
Styes are very common. People of all ages can develop a sty, and men and women are equally affected. There is a slight increase in the incidence of styes during the third to fifth decades of life. People with certain chronic conditions (diabetes mellitus, chronic blepharitis [inflammation of the eyelid], seborrhea, and chronic debilitating illnesses) are more prone to develop styes than the general population. In many susceptible people, stress seems to trigger the development of a sty. Studies have shown that those who have high levels of blood lipids (fats) are more susceptible to blockages in the oil glands, including the glands of the eyelid and are, therefore, more likely to develop a sty.

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REFERENCE:
Bessette, Michael J. "Hordeolum and Stye in Emergency Medicine." Medscape.com. Feb. 24, 2010. <http://emedicine.medscape.com/article/798940-overview>.