ARNOLD CHIARI MALFORMATION TYPE 2
In this case there is small posterior fossa with towering of cerebellum, inferior tonsillar herniation with beaking of tectum is seen .
Patchy syrinx formation is seen in spinal cord at C5- C7 and D2 -D8 vertebral level.
There is tethering of cord with lower end of conus at L5 vertebral level. open neural tube defect of sacrum with myelo meningomyelocele.
Anatomy: The Chiari II malformation is a complex deformity of the calvarium, dura, and hindbrain, and it is almost always associated with myelomeningocele. The spectrum of abnormalities in Chiari II malformation is broad, with many findings reported.
- Osseous changes
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- Lacunar skull or lückenschädel: Almost all patients with Chiari II malformation manifest dysplasia of the membranous bones of the calvarium. This is called lacunar skull (lückenschädel) and appears as clusters of areas of thinning, pits, and fenestrae that are most prominent near the vertex or torcular herophili . A lacunar skull may be observed in utero in a fetus as early as at 8 months of gestation. The lacunar skull typically persists until the age of 1-3 months and then disappears after approximately 6 months of age, regardless of whether progressive hydrocephalus is present. Subtle calvarial thinning and scalloping may persist into adulthood.
- Concave clivus and petrous ridge: The posterior aspects of the petrous temporal bones often are concave The clivus also develops abnormally and often short, with a concave configuration similar to that of the petrous ridges
- Small posterior fossa and gaping foramen magnum: The posterior fossa is exceptionally small, and the foramen magnum is larger and rounder than usual.
- Low-lying transverse sinuses: The internal occipital protuberance is situated just superior to the foramen magnum. The transverse sinuses and torcular herophili are low lying
- Changes to the dura
- Fenestrated falx: The falx almost always shows partial absence, hypoplasia, and/or fenestrations
- Hypoplastic tentorium: The tentorium is hypoplastic and attaches to the occipital bone far caudally, just above the foramen magnum
- Heart-shaped incisura: The hypoplastic tentorial leaves arise laterally from the low-lying transverse sinuses
- Changes to the cerebellum, medulla, and spinal cord
- Cerebellar peg: Protrusion of vermis and hemispheres through the foramen magnum (90%) results in craniocaudal elongation of cerebellum behind the spinal cord.
- Medullary kink: The medulla is kinked inferiorly (75%) and lies dorsal to the cord, which is unable to descend because of competent dentate ligaments
- Towering cerebellum or vermian pseudotumor: The cerebellar hemispheres and vermis also extend above the incisura of the tentorium
- Corners of the cerebellum are wrapped around the brainstem, pointing anteriorly and laterally
- Tubelike elongated fourth ventricle: The fourth ventricle is elongated craniocaudally, narrowed transversely, and decreased in anteroposterior diameter The cerebellopontine cistern and the cisterna magna are obliterated.
- The combined displacements of the spinal cord, medulla, pons, and cerebellum form a cascade of herniations, each of which compresses all of the tissue in front of it, displacing them anteriorly.
- In older patients, a wide subarachnoid space may be seen behind a vermis that is deeply grooved at the level of cervical nerve C1. The vertebral artery frequently loops on itself within the cervical canal and passes caudally to the level of cervical nerve C3. The possible cause of this groove may be the pulsatile effect of the vertebral artery on the adjacent cerebellum.
- Changes to the midbrain and the third and lateral
- Beaked tectum: Variable degrees of fusion of the colliculi and tectum result in prominent beaking and inferior displacement of the tectal plate Hydrocephalus: Ventricular dilatation is present in 98% of patients, and 90% require a shunt.
- Colpocephaly: The occipital horns and atria are often mildly enlarged because of maldeveloped occipital lobes, especially in the presence of a malformation involving the corpus callosum
- Prominent massa intermedia: A prominent massa intermedia, herniation of the third ventricle into the suprasellar cistern, and an enlarged suprapineal recess often are seen Frequently, a large third ventricle is present as well.
- Approximately one third of infants have a partial block at the level of the ambient cisterns.
- Associated anomalies
- Myelomeningocele (88-100%)
- Dysgenesis of corpus callosum (80-90%)
- Obstructive hydrocephalus following closure of myelomeningocele (50-98%)
- Syringohydromyelia (50-90%)
- Aqueductal stenosis (70%)
- Absence of septum pellucidum (40%)
- Contracted narrow gyria (stenogyria; 50%)
- Heterotopias
- Diastematomyelia
- Segmentation anomalies (<10%), incomplete C1 arch
- Malrotation of the posterior arches of C1 and C2
- Low-lying often-tethered conus medullaris below lumbar nerve L2
- Rare anomalies
- Holoprosencephaly
- Cervical myelocystocele
- Frontometaphyseal dysplasia
- Juvenile distal spinal muscular dystrophy
- Williams syndrome
Clinical symptoms and signs of Chiari II malformation are as follows (in order of decreasing frequency):
- Signs and symptoms in infancy
- Respiratory distress and impaired swallowing (71%)
- Inspiratory stridor (59%)
- Episodic apnea (29%)
- Weak or absent cry (18%)
- Aspiration (12%)
- Nystagmus
- Pain in the upper and lower extremities
- Weakness or spasticity of the upper and lower extremities (53%)
- Depressed or absent gag reflex
- Fixed retrocollis
- Palsy of the seventh cranial nerve
- Scoliosis
- Worsening of bladder and/or bowel function
- Signs and symptoms in childhood
- Syncopal episodes
- Nystagmus (both horizontal and rotatory on lateral gazing)
- Spastic quadriparesis
- Upper extremity weakness with increased tone
- Exaggerated deep tendon reflexes
- Mirror movements
- Appendicular and/or truncal ataxia
- Recurrent pneumonia secondary to aspiration
- Gastroesophageal reflux
- Depressed or absent cough reflex
- Gradual loss of function