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Anencephaly is relatively common. Itís slightly less than 1:1,000 in the United States and the recurrence rate is 3-5% anencephaly, ancephaly, absent brain. Anencephaly can be monitored with alpha fetoprotein. Then there are encephaloceles. These are herniations of meninges with or without brain parenchyma, usually through a bony defect. Three out of four are 

There is an association with hypertelorism and short stature, and frontal encephaloceles. This illustration is there in your handout on the right side, and what it is, is to schematically show that there are two types. One is called a meningocele where the meninges are herniating. The space would be of course CSF filled and in a meningo and encephalocele there is actually some extrusion or herniation of 

Neural tube closure defects will continue by briefly mentioning that there is a condition called diastematomyelia. As you know, anything with myelia pertains to spinal cord. And this is a failure of fusion with some persistence of mesodermal elements where the cord actually looks like a doublet. Itís a split cord. Then there is hydromyelia which is an enlarged central canal. Then there are syringomyelia and syringobulbia. Syrinx in Greek means tube. So these are somewhat like hydromyelia in the sense that it is a central cavity, however this can occur either in the spinal cord per se or more rosterally in the brain stem, in which case we call it syringobulbia. And it can be due to tumor. It may be due to tumor following resection, it can be from an infarction, and it can present with spasticity, depending on its size, and there can be very distinct sensory findings that I will demonstrate to you momentarily. But the combination; sometimes these patients are mistaken to have multiple sclerosis or some other disease. In this day and age it is not at

The sensory findings have to do with the anatomy. We told you in the morning, neurologists tend to be very interested in anatomic correlations. These are patients in whom you will get a very distinctive sensory examination. You will find that they have some deficiency in sensation to pain or temperature and yet seem to be capable of perception of vibratory sense and, in an old enough and mature enough child, you may be able to test them for graphesthesia and so on. And you may wonder, how can this be possible? They can barely feel the hot or cold tuning fork, and yet they know the vibration. It has to do with the fact that this is the dorsal root ganglion on this side where pain temperature fibers enter the segment and they cross and go to the lateral spinal thalamic track. And then they ascend. So when there is a syrinx, which is shown in red here, you have an interruption of that, and yet when you look at the posterior column sensory findings, if you look at vibratory sense and those, again the dorsal root

Now we are going to disorders of cell division and migration. And here, before I go too far, let me just remind you of something from embryology. As the brain develops in early fetal stage, after the ventricular cavities are formed as you know there is a layer of cells there that are capable of cell division. These are the neuroblasts that divide in a highly orchestrated, controlled manner.