Chiari Malformations

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  1. Chiari I
    1. Definition: caudal displacement of cerebellar tonsils below the plane of the foramen magnum
      1. First decade – 6 mm or less
      2. Second and third decade – 5 mm or less
      3. Fourth through eighth decade – 4 mm or less
      4. Ninth decade – 3 mm or less
      5. However, all those over 12 mm are symptomatic while only 30% of those 5-10 mm are symptomatic
    2. adult
      1. syringomyelia with sensory changes and numbness, nystagmus, headache worse with straining, vertigo, hypo or areflexia, deep burning pain in the neck, shoulders, or arms, upper exremity and hand atrophy, spastic paraperesis
    3. adolescence
      1. spastic weakness of the lower extremities, sensory loss, hand atrophy, truncal ataxia, CN 12 palsy
  2. Chiari II
    1. Definition: caudal displacement of the cerebellar vermis, fourth ventricle, and lower brain stem below the plane of the foramen magnum (associated with myelodysplasia)
      1. Radiographic features of Chiari II malformations include skull and dura anomalies such as luckenshadel, small posterior fossa with low-lying transverse sinus, fenestrated falx, heart-shaped incisura with hypoplastic tentorium, gaping foramen magnum, concave clivus, and petrous ridges. Intracranial abnormalities are best visualized by MRI, such as inferiorly displaced vermis, medullary spur and kink, beaked tectum, interdigitated gyri, and cerebellar “creeping” around brainstem. Hydrocephalus often accompanies Chiari II, with associated ventricular anomalies like elongated, tubelike and iferiorly displaced fourth, lage massa intermeda within third, and lateral colpocephaly with scalloped, pointed walls. Myelomeningocele, syringohydomyelia, and diastematomyelia are other characteristics.
      2. Almost all myelomeningocele patients have a Chiari II malformation
    2. childhood
      1. nystagmus (usually presents first); spastic quadriparesis, recurrent pneumonia due to aspiration
    3. infancy
      1. stridor, episodic apnea, absent gag reflex, nystagmus, weak cry; spastic upper extremity weakness
  3. Chiari III
    1. definition: caudal displacement of the cerebellum and brain stem into a high cervical meningocele
  4. Chiari IV
    1. definition: cerebellar hypoplasia (not a type of cerebellar herniation)
    2. NOTE: Dandy-Walker malformation differs from Chiari malformation in that Dandy Walker malformations have a hypoplastic vermis, high transverse sinus and cystic dilation of the fourth ventricle
  5. Chiari anatomy
    1. suboccipital craniectomy and cervical laminectomy is performed; laminectomy is carried down to the level of the displaced fourth ventricle (additional removal of bony elements over the syrinx is not necessary)
      1. tight extradural band corresponding to the periosteum of C1 is occasionally found in types I and II with sectioning allowing for the expansion of the underlying subarachnoid space
      2. a diffuse venous sinus is normally present in the area of the foramen magnum
      3. options after opening the dura include placing a plug of tissue into the enlarged opening into the central canal at the obex, resecting the atrophic caudally displaced cerebellar tonsils, performing a myelotomy if syringomyelia exists, or placing a stent into the foramen of Magendie
        1. must watch for branch of PICA adherent to the posterior surface of the cerebellum
        2. foramen of Magendie is found in the midline by lifting the tonsil and vermis (uvula) up and away (a layer of taenia surrounds the foramen)
      4. placement of a graft into a ‘Y’ shaped dural incision is a common method of expanding the posterior fossa
      5. for patients with a syrinx, ventriculoperitoneal shunt in addition to posterior fossa expansion is the treatment of choice, followed by syringosubarachnoid shunt, aspiration of the cyst or syringostomy
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