Other anomalies of the central nervous system

In hydrocephaly, there is too much cerebrospinal fluid. Based on the pathogenesis, hydrocephaly occurs in two major forms: communicating and non-communicating (obstructive) hydrocephaly. In communicating hydrocephaly, there is free communication between the ventricular system and the subarachnoid space. This is the case in encephalomyelocele, in congenital absence of arachnoid granulations and in Arnold-Chiari syndrome (displacement of the brain in a caudal direction and incarceration of the cerebellum in the foramen magnum). Non-communicating hydrocephaly is the form in which there is no contact between the ventricular system and the subarachnoid space. This blockage is largely the result of an obstruction of the cerebral aqueduct. However, it can also occur due to atresia of the openings of the 4th ventricle into the subarachnoid space (single median aperture and paired lateral apertures), or because of congenital processes competing for space.
Primary hypoplasia of the brain or secondary destruction of brain tissue lead to an abnormally small brain (microcephalus). Perturbations during the proliferation of both nervous and glial cells, abnormal migration of maturing neurons, and abnormal cortical organisation cause malformations of the cerebral cortex. The surface of the cortex shrinks. Gyri and sulci may fail to develop, which causes the brain surface to remain smooth (lissencephaly).
The connection between the two brain hemispheres, which normally is established by fibres of the corpus callosum, can be completely or partially absent (agenesis of the corpus callosum). This condition occurs with an abnormally enlarged 3rd ventricle. The anterior commissure may strengthen to compensate for this state.
Huntington's chorea is inherited in an autosomal dominant mode and the gene locus is well known. This disease is due to necrosis of small neurons in both the forebrain and the striatum. The symptoms are involuntary rapid and jerky movements. These generally do not appear until after the age of 35 to 40. The disease leads to dementia and ultimately to death.