Entropion is a medical condition in which the eyelid (usually the lower lid) folds inward. It is very uncomfortable, as the eyelashes constantly rub against the cornea. Entropion is usually caused by genetic factors and may be congenital. The upper or lower eyelid can be involved, and one or both eyes may be affected. Surgery may be recommended. If so, the procedure is usually a small adjustment turing the eyelid back to its ideal position – at which eyelashes do not rub against the eyeball.
Classification

• Involutional entropion – the most common form of entropion, this is due to the changes in the tissue of the eyelid(s) associated with the normal ageing process. For example, the small muscles controlling the affected eyelid may have weakened. In this case either or both eyes may be affected. This is also known as senile entropion.
• Cicatricial entropion – related to scarring e.g. due to accident burns or surgery (recall that cicatrix is the medical term for a scar). Either one or both eyes may be affected.
• Spastic entropion – of the lower-eyelid may be due to (possibly temporary) spasm of the orbicularis oculus muscle, e.g. following surgery.
• Congenital entropion – that is, entropion present since birth.
  This is very rare but when present may be due to hypertrophy, i.e. increased size (due to enlargement of the cells) of the skin of the affected area or the orbicularis oculi muscles responsible for opening/closing the eyes. Either one, or both, eyes may be affected.