Microcephaly can have a variety of etiologies, depending on when you discover it. What I mean by that is some people are born microcephalic because they may have a chromosomal syndrome, or Cornelia de Lange or fetal alcohol, or one of the STORCH, meaning syphilis, toxo, rubella, and all that type of syndromes. On the other hand, it can become an acquired process if you have perinatal asphyxia. So someone is born with the proper head circumference but let’s say they have severe meconium aspiration, get very sick, the neonatal course is very stormy, and then you find that at nine-months-of-age they have not gained 10 centimeters because they acquired the damage at that time. So that’s a different situation.
Macrocephaly is kids with big heads. Here we have again a big differential. It can be metabolic disease, like Sturge, Tay-Sachs, Hurler’s, some of those. Leukodystrophies like Alexander and Canavan are known to create progressively large head. You can see it sometimes in neurocutaneous syndromes, or bone disease. There is a syndrome called Sotos syndrome which is truly macrencephaly. There is no hydrocephalus. The brain looks normal although it’s rather large. Kids present with a big head, mildly hypotonic, and mental acuity varies and a lot of them tend to be kind of dull. They are not only cerebrally large, they are just big kids. They are kind of macrosomic. You will notice that when I talked about macrocephaly I didn’t really say anything about hydrocephalus, because that will come under CSF circulation and that’s not really big encephalus, but hydrocephalus.

Obstructive hydrocephalus can be due to a congenital problem, like aqueductal stenosis. We are referring to the aqueduct of Sylvius that connects the third ventricle to the fourth. It can also be acquired due to midline brain tumors that compress the aqueduct. It may be a congenital anomaly, such as Dandy-Walker syndrome which has atresia of the foramina of Magendie and Luschka with compensatory dilatation of the fourth ventricle and cerebellar hypoplasia. Later when we talk about ataxia I’m going to show you a picture of Dandy-Walker. Another reason kids sometimes present with obstructive hydrocephalus is they have a vein of Galen’s aneurysm. It’s often called an aneurysm but that’s a misnomer. It’s not an aneurysm, it’s really an AVM. The difference being, an aneurysm is an abnormal swelling due to weakness of the wall of an artery. AVM is really anomalous arteriovenous channels. It’s a high conductance, low resistance channel. There are inferior fossa hematoma, like after trauma you could develop obstructive hydrocephalus. The key features of obstructive hydrocephalus, you will notice, that we are focusing on obstruction in the vicinity either due to a mass lesion or due to a congenital lesion, in such a manner that the flow from third to fourth ventricle is affected. That’s the key. What we call communicating hydrocephalus, where there still may be a problem with obstruction but it is not proximal to the fourth ventricle. This could happen partly because you have a problem with arachnoid granulations on the convexities. This could happen because the child had meningitis as a neonate and there was a lot of pus. Because the meninges got fibrosed and the absorptive surfaces are damaged. It could be post CMV or toxo, maybe a sequelae of large subarachnoid hemorrhages again. Rarely you can get communicating hydrocephalus due to excessive production of CSF, such as a choroid plexus papilloma. These tend to occur quite frequently between the second and third ventricles, near the foramen of Monroe. Then there can be obstruction downstream from the fourth ventricle. We already saw the anatomy of the Chiari and I did indicate to you then that sometimes they will gradually develop headaches and hydrocephalus, so that could be another etiology.

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