| ||Pathomorphology or Pathophysiology of this disease :|
In 1891, the German pathologist Hans Chiari described congenital hindbrain anomalies in which cerebellar tissue descends into cervical canal, now designated commonly as Chiari type I. According to the name of his professor Dr. Arnold and his name, this hind brain disorder is named as Arnold Chiari malformation, which is divided into four types depending upon its association with other anomalies. Aetiology is unknown so far.
Chiari I malformation, also known as congenital tonsillar ectopia, is a relatively simple anomaly and infrequently associated with other anomalies. In contrast to Chiari II malformation, in this disorder the vermis, 4th ventricle and medulla are normal or minimally deformed. Elongated, “peglike” cerebellar tonsils are displaced inferiorly through the foramen magnum into the upper cervical spinal canal. In general it is not associated with any brain lesion. Mild to moderate hydrocephalus is seen in 20-25% cases. Syringomyelia is seen in 30-60% of all cases. Skeletal anomalies like basilar invagination (25-50%), Klippel-Feil (5-10%) and atlantooccipital assimilation (1-5%) are seen.
Chiari II malformation is a complex anomaly with skull, dura, brain, spine and cord manifestations. Skull and dura abnormalities include calvarial defects (lacunar skull), small posterior fossa with low lying transverse sinuses, fenestrated falx, heart shaped incisura with hypoplastic tentorium, gaping foramen magnum, concave clivus/petrous ridges. Brain anomalies include inferiorly displaced vermis, medullary spur and kink, beaked tectum, interedigitated gyri, cerebellum “creeps” through the brainstem and “towers” through wide tentorial incisura, corpus callosal agenesis, heterotopias, polymicrogyria, stenogyria etc. Hydrocephalus is seen in 90% cases. The 4th ventricle is elongated, tube like, and inferiorly displaced. Large massa intermedia, colpocephaly, scalloped pointed walls. Myelomeningocele is seen in all cases. Syringohydromyelia is seen in 50-90% cases. Diastematomyelia and segmentation anomalies are seen in <10% cases.
Chiari III malformation is a herniation of hind brain into a low occipital and high cervical region in combination with Chiari II malformations. Cerebellum, occipital cortex are common contents of encephalocele. Occasionally medulla and pons are also herniated. The herniated tissue is strikingly abnormal and often non-functioning because of necrosis, gliosis, fibrosis and the presence of heterotopias. On imaging it is particularly important to recognize position of brain stem, medulla and potential anomalies of venous drainage associated with encephalocele. Herniation of 4th and lateral ventricles is also seen.
Chiari IV is severe cerebellar hypoplasia. The main abnormality includes absent or severely hypoplastic cerebellum, small brain stem, and large posterior fossa CSF spaces. Interestingly there is no hydrocephalus in this anomaly. Other CNS anomalies are also usually absent.
Imaging is the mainstay of diagnosis in this malformation.
| ||Radiological findings:|
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MRI 1: Axial T1WI showing dilated posterior horn of lateral ventricles ( colpocephaly). Mild interdigitations of parafalcine gyri.
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MRI 2: Axial T2WI showing the same features as Figure1
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MRI 3: Axial T2WI showing symmetrically dilated ventricles
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MRI 4: Sagittal T1WI showing large massa intermedia. Dilated lateral ventricles.
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MRI 5: Sagittal T1WI showing large massa intermedia, Cerebellar herniation, Tube like 4th ventricle and beaked tectum. Low lying transverse sinus and syrinx seen in spinal cord.
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MRI 6: Sagittal T1WI showing large massa intermedia, beaked tectum, cerebellar herniation, medullary kink and spur, tubelike 4th ventricle, syrinx and myelomeningocele in the sacral region.
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MRI 7: Sagittal T2WI showing herniation with small posterior fossa. Caudally myelomeningocele is seen in the sacral region.
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MRI 8:Sagittal T2WI showing same features as Figure 6.
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MRI 9: Axial T2WI . Sections through lower thorax showing syrinx.