As reported in the journal, The Lancet, a man has been found who had a small brain, but a normal life. The article is subscription only so I am not even going to link to it. But it is still noteworthy.
There is a fair summary in the online version of Der Spiegel, and it is even in English. It describes the case of a 44-year-old man employed in a tax office. He was married, father of two, holding down a job, seemingly fine. In 2003 he noticed some weakness in his left leg. He ultimately was seen by Dr Lionel Feuillet of the Universite de la Mediterranee in Marseille, France.
Upon evaluation, he did reveal that, as a child, he had been told he had hydrocephalus. A shunt was placed to drain the fluid. He was doing fine, and at age 14 the shunt was removed. That would have been around 1973, so CAT scans and MRIs would not have been available.
Of course, by 2003, detailed brain imagining studies had become routine. The physicians were astonished at what they saw.
With more evaluation, he was found to have an IQ of 75. Significantly below average, but adequate for what he had to do.
Below is a drawing (from Gray's Anatomy) showing what the ventricular system in the brain is supposed to look like. Notice in particular the size of the lateral ventricles. It is not terribly clear in this line drawing, but the ventricles are fluid-filled cavities in the middle of the brain.
The three-dimentional view is a bit easier to grasp with the MRI images...
This is copied from Der Spiegel. The patient is looking to the left. The eye socket is the white circle on the left. The LV label indicates the lateral ventricle.
For comparison, here are three different views of a normal brain:
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