H/O left sided hemiparesis at the age of 2 years with mental regression. Patient (12 years of age) also has seizures.
Findings:
There is hemiatrophy of the right cerebral hemisphere. There are
areas of cystic encephalomalacia in the right fronto-parietal lobes. There is
ex-vacuo dilatation of the right lateral ventricle. There is prominence of the
right Sylvian fissure and right perimesencephalic cistern. Also seen is thinning
of the body of the corpus callosum. There is a shift of the midline to the right
(pull). There is thickening of the skull vault in the right
fronto-temporo-parietal region. The left cerebellar hemisphere appears smaller
in size and this would represent crossed cerebellar diaschisis. There is blunting
of the right cerebral peduncle. Subtle hyperintense signal on the T2 Weighted
images was seen in the right cerebral peduncle and in the pons to the right of
the midline suggesting Wallerian degeneration. Also seen is slight flattening
of the pyramid of the medulla, on the right.
Cerebral hemiatrophy
Patients with cerebral hemiatrophy or Dyke-Davidoff-Masson syndrome usually
have seizures, facial asymmetry, contralateral hemiplegia or
hemiparesis, mental retardation and learning difficulties. It is due to cerebral
injury that may occur early in life or in utero. Insult to immature brain
results in neuronal loss and impaired brain growth.
Prenatal - congenital malformation, infection and vascular insult
Perinatal - birth trauma, anoxia/hypoxia and intracranial
hemorrhage
Postnatal - trauma, tumor, infection and prolonged febrile
seizures
In addition to showing the generalized decrease in volume of the cerebral hemisphere, three patterns of cerebral hemiatrophy have been described: pattern I, diffuse cortical and subcortical atrophy: pattern II, diffuse cortical atrophy associated with expanded porencephalic cysts: pattern III, old infarction with necrosis in the territory of the middle cerebral artery. Pattern I and pattern II may result from hypoxic-ischemic insults. They usually show Wallerian degeneration with unilateral atrophy of the cerebral peduncle as far as the pyramid of the medulla oblongata. Structural changes such as thickness of the calvarium, hyperpneumatization of the paranasal sinuses and mastoid cells, elevation of the petrous ridge and reduced size of the middle cranial fossa may be seen.
Crossed Cerebellar Diaschisis
Diaschisis is reduced function of a region of the brain due to the interruption at a remote site of an afferent pathway which normally supplies background excitation to the neurons in that part, keeping them in a state of low activity.
Diaschisis Corticospinalis - progression of functional
depression of the spinal cord following an injury to the motor cortex
Diaschisis Commisuralis - functional depression of the
contralateral cerebral cortex after injury to cortex of one hemisphere
Diaschisis Associativa - depression of function in intact
cortical areas adjacent to the site of a cortical injury
Crossed Cerebral Cerebellar Diaschisis - following injury (usually
vascular) to the motor cortex of one cerebral hemisphere
The cerebellar hemisphere is connected to the contralateral cerebral cortex through feed-back circuits, needed for smooth execution of motor function. MR reveals the structural changes induced in the cerebellum, as a result of such changes in vascularity and metabolism, secondary to injury to the contralateral cerebral hemisphere.
Diaschisis (manifested by hypoperfusion) presents itself as a different form (ipsilateral or contralateral) depending upon the age of cerebral insult. Early insults would be likely to produce ipsilateral and the later ones to produce a crossed cerebellar diaschisis. There appears to be some correlation between the degree of morphologic damage to the cerebrum and the volume loss in the opposite cerebellum. Naturally, there is difference in cerebellar functional reorganization pattern following contralateral cerebral injury between developing and developed brains.
At present it has mainly been described in cerebrovascular pathology, and been classified according to the connecting fibers involved. When the connecting fibers are intra-hemispheric, the phenomenon of ipsilateral thalamic or subcortical-cortical diaschisis may be seen; when they are interhemispheric, there is transcallosal diaschisis, and if they are cerebellar, the diaschisis is of the contralateral cerebellum or crossed cerebellar diaschisis. Ipsilateral thalamic and crossed cerebellar diaschisis are phenomena which are frequently observed, but have no clinical significance.
Wallerian Degeneration
Degeneration of the myelin sheath and axon distal to the most proximal site of axonal interruption secondary to axonal disease has been called Wallerian degeneration. On MR, Wallerian degeneration of the pyramidal tract may be seen as a hyperintense signal (HS) on the T2W images. This corresponds to the corticospinal tract, with or without shrinkage of the ipsilateral cerebral peduncle and pons. Altered signal in the ipsilateral brainstem may be seen as early as 4 to5 weeks after the supratentorial ictus. At 4 to 5 weeks, a well-defined band of hypointensity (believed to result from transitory increased lipid-protein ratio) may appear on the T2W images in the topographic distribution of the corticospinal tract. After 10-14 weeks, the signal becomes permanently hyperintense. Signal alterations are usually strongest at about 3-6 months. Shrinkage of the ipsilateral brainstem may appear by 8 months. Depending on the primary site of the infarct, the location of the HS in the cerebral peduncle varies. HS involving the whole cerebral peduncle has a large lesion involving the hemisphere. HS at the center of the cerebral peduncle has a lesion in the paracentral gyrus, precentral gyrus, corona radiata or posterior limb of the internal capsule. HS at the lateral aspect of the cerebral peduncle has a lesion in the parietal or temporal lobes (which spares the corticospinal tract originating from the paracentral gyrus, precentral gyrus, corona radiata or posterior limb of the internal capsule). Hence HS at the center of the cerebral peduncle may be due to Wallerian degeneration of the corticospinal tract and an HS at the lateral side of the cerebral peduncle may be due to Wallerian degeneration of the corticopontine tract.
Cerebral infarction is the most commonly associated primary disorder. Neoplasms, demyelination and post-hemorrhagic conditions may also be associated with white matter tract signal abnormalities
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