Simon JH, Jacobs LD, Campion MK, Rudick RA, Cookfair DL, Herndon RM, Richert JR, Salazar AM, Fischer JS, Goodkin DE, Simonian N, Lajaunie M, Miller DE, Wende K, Martens-Davidson A, Kinkel RP, Munschauer FE 3rd, Brownscheidle CM.A longitudinal study of brain atrophy in relapsing multiple sclerosis. The Multiple Sclerosis Collaborative Research Group (MSCRG).Neurology. 1999;53:139-48.
OBJECTIVE:To determine if progressive brain atrophy could be detected over 1- and 2-year intervals in relapsing MS, based on annual MR studies from the Multiple Sclerosis Collaborative Research Group (MSCRG) trial of interferon beta-1a (Avonex).
METHODS:All subjects had mild to moderate disability, with baseline expanded disability status scores ranging from 1.0 to 3.5, and at least two relapses in the 3 years before study entry. Atrophy measures included third and lateral ventricle width, brain width, and corpus callosum area.
RESULTS:Significant increases were detected in third ventricle width at year 2 and lateral ventricle width at 1 and 2 years. Significant decreases in corpus callosum area and brain width were also observed at 1 and 2 years. Multiple regression analyses suggested that the number of gadolinium-enhancing lesions at baseline was the single significant contributor to change in third ventricle width. Atrophy over 1 and 2 years as indicated by enlargement of the third and lateral ventricle and shrinkage of the corpus callosum was greater for patients entering the trial with enhancing lesions. Greater disability increments over 1 and 2 years were associated with more severe third ventricle enlargement.
CONCLUSION: In patients with relapsing MS and only mild to moderate disability, significant cerebral atrophy is already developing that can be measured over periods of only 1 to 2 years. The course of cerebral atrophy in MS appears to be influenced by prior inflammatory disease activity as indicated by the presence of enhancing lesions. Brain atrophy measures are important markers of MS disease progression because they likely reflect destructive and irreversible pathologic processes.
You can see lateral ventricle getting bigger and more crenulated as the brain shrinks.
The above is a brain from Huntingtons disease.
You said is it just the third ventricle that enlarges as the brain shrinks, well as can be seen from this post it occurs in other ventricles and can be seen in the picture below as the lateral ventricles (left and right) are enlarging