Idiopathic is an adjective used primarily in medicine meaning "a disease or condition the cause of which is not known or that arises spontaneously". From Greek ἴδιος, idios (one's own) + πάθος, pathos (suffering), it means approximately "a disease of its own kind". It is technically a term from nosology, the classification of disease. For some medical conditions, one or more causes are somewhat understood, but in a certain percentage of people with the condition, the cause may not be readily apparent or characterized. In these cases, the origin of the condition is said to be idiopathic.
With some medical conditions, the root cause for a large percentage of all cases has not been established—for example, focal segmental glomerulosclerosis or ankylosing spondylitis; the majority of these cases are deemed idiopathic. With other conditions, idiopathic cases account for only a small percentage (for example, pulmonary fibrosis). Regarding a particular condition or disease, as more root causes are discovered the percentage of cases designated "idiopathic" decreases.
Some congenital conditions are idiopathic, and sometimes the word "congenital" is used synonymously with "idiopathic"; but careful usage prefers to reserve the word "congenital" for conditions to which the literal sense of the word applies (that is, those whose pathophysiology has existed since the neonatal period).
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