An idiopathy is any disease with unknown pathogenesis or apparently spontaneous origin. From Greek ἴδιος idios "one's own" and πάθος pathos "suffering", idiopathy means approximately "a disease of its own kind". 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 other 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).
Advances in medical science improve etiology (the study of causes of diseases) and nosology (the classification of diseases); thus, regarding any particular condition or disease, as more root causes are discovered, and as events that seemed spontaneous have their origins revealed, the percentage of cases designated as idiopathic decreases.
The word essential is sometimes synonymous with idiopathic (as in essential proteinuria or essential thrombocythemia), and the same is true of primary (as in primary biliary cirrhosis or primary amenorrhea), with the latter term being used in such cases to contrast with secondary in the sense of "secondary to [i.e., caused by] some other condition." Another, less common synonym is agnogenic (agno-, "unknown" + -gen, "cause" + -ic). The word cryptogenic (crypto-, "hidden" + -gen, "cause" + -ic) has a sense that is synonymous with idiopathic and a sense that is contradistinguished from it.
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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