<s>[INST] <<SYS>> This is a medical question of type : causes <</SYS>> What causes Cholesteatoma ? [/INST] What causes cholesteatoma? A cholesteatoma usually occurs because of poor eustachian tube function in conjunction with infection in the middle ear. Negative pressure within the middle ear pulls a part of the eardrum the wrong way, creating a sac or cyst that fills with old skin cells and other waste material. As the cyst gets bigger, some of the middle ear bones break down, affecting hearing. A rare congenital form of cholesteatoma (one present at birth) can occur in the middle ear and elsewhere, such as in the nearby skull bones. </s>