An idiopathic disease or condition is one whose cause is not known or one that arises spontaneously. From Greek ἴδιος idios "one's own" and 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.
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). 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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