1. Ventricular enlargement is a consequence of loss of neural tissue. Because the volume of the skull is fixed, as brain tissues decrease in volume due to a neurodegenerative process, there is a corresponding increase in ventricular volume.
2. The cerebral cortex and hippocampal formation are severely affected. By contrast, brain stem structures are not.
1. Both the dorsal column-medial lemniscal pathway and the corticospinal tract decussate within the ventral portion of the medulla. Without these decussations, the two sides of the ventral medulla become somewhat separated. CSF is present where the decussating axons should be. Axons important for coordinating eye movements normally decussate in the dorsal pons. Without this decussation, the two sides of the dorsal pons also become somewhat separate, as revealed by the presence of CSF and the formation of a shallow sulcus.
2. No, the corpus callosum is an example of a structure with intact decussating axons.
1. The proximal portion of the middle cerebral artery became occluded. This affected both deep branches to subcortical white matter and superficial branches supplying the cerebral cortex.
2. Since all descending motor control axons converge within the internal capsule, damage to this structure alone can produce the major limb and facial motor signs seen in this patient.
1. Mechanoreceptive axons ascend within the dorsal columns, including those for touch, vibration sense, and limb proprioception. It is unclear why the patient's touch sense was spared.
2. The ability to maintain an upright posture depends, in part, on lower limb proprioception. Without limb proprioception, vision can partially substitute. Therefore, when this patient closes his eyes, he is deprived of this compensating modality and, in consequence, he looses his balance.
1. Anterolateral system axons decussate just ventral to the central canal, where the syrinx originates. This is why they are damaged first. Mechanoreceptive axons are located farther dorsally, affording them some protection initially.
2. Limb motor neurons are located ventrolateral to the syrinx. As with the dorsal mechanoreceptive axons, motor neurons are initially afforded protection by their distance from the initial site of syrinx development. Eventually the syrinx expands to affect a significant portion of the ventral horn.
1. The posterior inferior cerebellar artery, or PICA, supplies the dorsolateral medulla. This was infarcted in the patient. The territory supplied by PICA receives little or no collateral circulation from other arteries, which makes this region of the medulla particularly vulnerable to ischemia. Whereas only a wedge-shaped infarcted region is revealed on the MRI, it is likely that the entire territory is affected because of the paucity of collateral circulation.