Background Neuroimaging Gross Pathology Histopathology & Immunohistochemistry

BACKGROUND AND CLINICAL INFORMATION: Head

Definition: Chiari II malformation consists of displacement of part of the inferior cerebellar vermis and an elongated and malformed medulla oblongata into the cervical spinal canal. This term is best reserved for cases that have neural tube defect.

Arnold-Chiari malformations: Chiari II malformations associated with meningomyelocele.
Neural tube defects: All the cases of Chiari II malformations originally described are associated with rachischisis. Very few acceptable cases with Chiari II malformations of the cerebellum and brain stem without neural tube defects have been identified. Chronologically, the spinal neural defect forms before the posterior fossa defect. According to the study on fetal surgery on meningomyelocele at the Children’s Hospital of Philadelphia, the Chiari type of malformation can be partially reverted if surgery is performed on the right time.

Posterior fossa: Abnormalities in the posterior fossa occur in structures that develop at a time that overlap with, but slightly later than those structures in the lumbosacal region involved in Chiari II malformation.

Clinical features:

Polyhydraminios: Probably due to impaired fetal swallowing function.
Neural tube defects can vary from complete spinal rachischisis to a meningomyelocele.
Spinal cord: Frequently associated with syringomyelia and hydromyelia. Diastematomyelia can also be seen.
Dysfunction of lower cranial nerves XI to XII: This is one of the most common presentations in infants and includes impaired gag reflex, impaired swallowing, and apnea. Other abnormalities include breath-holding spells, facial paralysis, downbeat nystagmus and periodic alternating nystagmus.
Progressive hydrocephalus: Most commonly the infants have an accelerated rate of head growth during the first weeksof life.
Cerebellar dysfunctions: These may be the presenting symptoms in older children and include ataxia, progressive ataxia, and unsteady gait.
Others neurologic symptoms: Headache and neck pain, more commonly seen in older children
Other malformations: It can also associate with other malformation of the brain, cranium and meninges, cardiovascular, gastrointestinal and genitourinary systems.

Treatment: Closure of the neural tube defect. Hydrocephalus may requre the placement of a ventricular shunt in the first few weeks of life in 85% to 95% of children with Chiari Ii malformation. Surgical decompression of the posterior fossa may be performed, with variable success, in cases that do not response to the shunt.

NEUROIMAGING: Head

Fetal ultrasound: “Banana” and “lemon” sign. The “banana being the cerebellum enveloping the brain stem and the “lemon” being the narrowing of the skull anteriorly. Meningomyelocele and other spinal cord defects are also detected.

In utero MRI: Chiar malformations can be seen as early as 18 weeks.

GROSS PATHOLOGY: Head

Bone and dural abnormalities:

Craniolacunia can be seen in up to 90% of the cases.
Malformations: The clivus is concaved and thinned. The torcula is low. The posterior fossa is shallow and accompanied by an enlarged formen magnum. The dural roof is low and attach at or near the margins of an enlarged foramen magnum. The tentorial hiatus is low.
The falx is short and hypoplastic. Interdigitation of the two hemispherecan be seen in the cases with extreme hyplasia of the falx.

Cerebal hemispheres:

Hydrocephalus of a variable degree is usually present and is communicating in type, possibly due to obstruction of the aqueduct at the level of the foramen magnum. Ascending spinal infections may thus infect the ventricles leading to pyocephalus whilst the cerebral subarachnoid space is spared.
Polygyria and polymicrogyria: The convolutional pattern is usually abnormal even in cases with minimal hydrocephalus. The more hydrocephalic the more abnormal is the gyral pattern. There are often an excessive number of gyri with normal cytoarchitecture. True polymicrogyria with with abnormal cytoarchitecture may also be seen.
Cortical dysplasia may be present.
Heterotopia: Subependymal nodular gray heterotopias in the lateral ventricles and thickening of massa intermedia. There may also be thickening of massa intermedia.

Cerebellum:

Herniation: The herniated cerebellar tissue varies from short to long and can extend as far as the upper dorsal vertebral segments. The herniated tissue involves the nodulus, pyramids, and uvula, often associated with choroid plexus on its dorsal side. Rarely the tonsil is also herniated in addition to the vermis or there is no displacement of cerebellar tissue.
Others: The cerebellum is often asymmetrical and dorsally flattened. The vermis may be burried within the cerebellum and can also extend around the brain stem over its midline.

Brain stem:

Midbrain and pons: The pons is also elongated and appears indistinct from the medulla. The pontomedullary junction is poorly formed. There is often beaking of the tectum. The four colliculi are fused into a mass that has the shape of the beak that point caudally. More often, only the inferior colliculi are fused.
Medulla and upper cord: The salient feature is an elongated medulla oblongata and fourth ventricle into the cervical spinal canal. The gracile and cuneate nuclei may be partially fused and displaced dorsally for a variable distance overlapping the spinal cord. The fourth ventricle may have cystic dilatation.
Nuclei: Hypoplasia or agenesis of cranial nerve nuclei, olivary nuclei and pontine nuclei have also been described.

Spinal cord and vertebral column:

Nerve roots in the upper four to six cervical spinal roots has an upward orientation towards their intervertebral foramina, lower roots are normally placed.
Neural tube defects: The pathology of the spine varies from complete rachischisis to a meningomyelocele. Meinigomyelocele is more common then meningocele and typically occur in lumbosacral region. Spina bifida is invariably present. Different kinds of spinal malformation including hydromyelia, syringomyelia, complete or partial diplomyelia, diastematomyelia, triplomyelia, double canals and multiple canals, and absent of filum terminale.

HISTOPATHOLOGY AND IMMUNOHISTOCHEMISTRY: Head

Aqueduct and central canal: Atresia, forking, and gliosis of the aqueduct can be seen.

Cerebellum: The herniated cerebellar vermis is usually associated with choroid plexus tissue. There is depletion of Purkinje cells and granule cells accompanied by gliosis, shrinkage, and absence of myelin. Cortical dysplasia and heterotopia may also be seen in the cerebellar hemispheres.

Brain stem: It may be difficult to identified normal anatomy in these deformed and damaged brain stems. There may be hypoplasia of cranial nerve nuclei.

NeuroLearn NeuroHelp Malformations For Comment: KarMing-Fung@ouhsc.edu

Background Neuroimaging Gross Pathology Histopathology & Immunohistochemistry