Subcortical U-fibres

A.Prof Frank Gaillard et al.

Subcortical U-fibres, also known as short association fibres, represent connections between adjacent gyri of the brain, located within the cortex or immediately deep to it in the very outer parts of the subcortical white matter 1

They are among the last parts of the brain to myelinate, as late as the third or fourth decade and have very slow myelin turnover. The exception is the perirolandic and occipital poles, which are the same parts which myelinate earliest in infancy (you have to be able to move and see pretty early on I guess) 4.

Whether or not they are involved in diseases that primarily affect the white matter depends on the underlying relationship of the pathology to myelin metabolism and oligodendroglial cells. 

In diseases in which the abnormality is one of myelin metabolism/turnover as these fibres have very slow metabolism, they tend to be affected last. Note: often the word 'spared' is used, which is correct in so far as at the time one is imaging these patients there is little involvement of these fibres, however, this should not be taken to mean that they cannot be involved; in fact, in many instances they are involved eventually

In contrast, diseases in which the damage is direct to oligodendroglial cells, then no such relative sparing will be evident. 

As if this wasn't potentially confusing enough, superimposed on these metabolic differences between subcortical U-fibres and deeper white matter, is that merely by their location, subcortical U-fibres are relatively protected from chronic small vessel ischaemic change 5. As the hemispheric white matter receives dual blood supply, superficially from penetrating arteries that pass through the cortex as well as deeper vessels that ascend from the perforators, it stands to reason that the white matter closest to either source of blood should be least affected. Indeed this is what is observed, with chronic small vessel ischaemic change relatively sparing both the periventricular and subcortical white matter 5. It is important to note that this does not apply for large vessel occlusion with will involve the cortex and underlying subcortical white matter. 

Examples of early and late subcortical involvement include 3,4

 

Neuroanatomy
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Article information

rID: 32173
Synonyms or Alternate Spellings:
  • Short association fibers
  • Subcortical U-fiber
  • Subcortical U-fibers
  • Subcortical U-fibre

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Cases and figures

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    Figure 1
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    Case 1: spared methotrexate toxic leukoencephalopathy
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    Case 2: spared in adrenoleukodystrophy
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