 The most common congenital anomaly that we encounter in the inner ear is the vestibular aqueduct enlargement, sometimes called the large endolymphatic sac anomaly. This is the most common abnormality and as long as you are looking for it you'll have no difficulty finding it. Then there is a spectrum of abnormalities that affect the cochlea and vestibule. And we like to give names to specific points along this spectrum, but it's important to understand that it's a continuous spectrum of abnormalities. At one end is the Michelle anomaly, the most severe version where there is nothing, it's just all white where the cochlea and vestibule should be. And then a little bit less severe is the common cavity where there's just one big hypodense blob where the cochlea and vestibule should be. And then there is an extensive area that we just call cochlea vestibular anomalies and we give it that vague term. Until we reach Mondini's triad which has three very specific abnormalities of the upper turns of the cochlea, the lateral semi-circular canalum, the vestibular aqueduct. And then at the very other end of the spectrum, the most mildest abnormalities, the incomplete partition where the bony septa between the upper turns of the cochlea are missing. And some of those patients are asymptomatic, it's so mild. Once we've talked about our cochlear malformations, then we will talk briefly about cochlear nerve aplasia, a very important diagnosis in children who are being considered for cochlear implantation. So first are vestibular aqueduct enlargement. This is the vestibular aqueduct running from the back of the vestibule to the posterior fossa. Normally, the vestibular aqueduct should be as small as the semi-circular canal. This posterior semi-circular canal is a good reference point unless the semi-circular canals are abnormal. So first convince yourself that the semi-circular canals are normal and then you can use them as a comparison to the vestibular aqueduct. You can see that this one's many times larger than the semi-circular canal. As is often the case, this is a bilateral abnormality in this patient. 90% of the time, bilateral abnormality. This is a common cavity malformation. You can see that there is just a big malformed blob where the cochlear vestibule ought to be. There's nothing functional in that area, just a big blob. Occasionally there's still a nerve there and cochlear implantation may actually work in some of these patients. Next we have cochlear vestibular anomalies. Now it's not just a big blob. You can tell where the cochlear might have formed and you can tell where the vestibule and semi-circular canals might have formed. You can even make out a vestibular aqueduct there, but there's no small parts. There's no fine detail here. So there's a whole spectrum of abnormalities here that we call cochlear vestibular anomalies where it's not quite as bad as a common cavity and not quite as good as Mondini's triad. So here's a patient where the upper turns of the cochlea. We should have pancakes here, right? But the upper turns of the cochlea are fused together. There's no separation of the upper turns of the cochlea. This is incomplete partition, but in addition we see enlargement of the vestibular aqueduct and I'm not showing it, but on another cut. The lateral semi-circular canal is expanded and abnormal and those three things together are called Mondini's triad. Now, let us turn our attention to the cochlear nerve. And the best way to evaluate the cochlear nerve is, of course, with MRI. I'm showing you two sides of a single patient. These are sagittal oblique images oriented perpendicular to the internal auditory canal, anterior, posterior, superior, inferior. And this is a cross-section of the internal auditory canal. We're supposed to see four nerves, right? We're supposed to see the facial nerve up top and in front. We're supposed to see the cochlear nerve in front and the bottom. And we're supposed to see the two vestibular nerves in the back, superior and inferior vestibular nerves. There's our four nerves on this side. But if we go to the other side, we're down to three nerves. Facial nerve and two vestibular nerves are still there. The cochlear nerve that should be anterior and inferior is absent. This is cochlear nerve aplasia. And this child will not benefit from a cochlear implant. So that's such a critical thing before they undergo that surgery.