Malignant epithelial cell erosion into bone, often caused by TM rupture at its margin with the canal.  Can also be congenital or arise spontaneously.  Otoscopy reveals a usually pearly-white growth in the attic [uppermost part of the TM] contiguous with the canal.  Commonly leads to deafness; in past, could lead to fatal brain infections.  Acute vertigo plus cholesteatoma is worrisome.