The Chiari Malformations 3/6

 

Chiari, Part 3 of 6: The Chiari II Malformation

Chiari1Chiari II malformation is virtually always associated with myelomeningocele, MMC. MMC patients are likely to suffer from three separate but related neurosurgical maladies: hydrocephalus (requiring treatment with a ventriculoperitoneal shunt), Chiari II malformation (often requiring decompression surgery), and the myelomeningocele itself (which is surgically repaired at birth but may later present as a tethered cord).

 Patients with Chiari II may have cognitive deficits and be developmentally delayed, though they need not be and the incidence of this is much higher in those who have suffered meningitis. Meningitis is sometimes a complication of the initial MMC closure and/or vp shunt placement for hydrocephalus.

Unlike Chiari I, in which the brain develops normally in utero and afterwards, in Chiari II there appears to be a defect in the in utero development of the brain. The structure of the brain depends on the migration of certain groups of neurons from one place to another during development. In Chiari II, these migration patterns break down to various degrees, leading to a brain which shows characteristic abnormalities anatomically on MRI or CT, but may function normally. If these groups of neurons do not function normally, the patient is likely to demonstrate development delay, which in some cases can be quite severe. If the brainstem is involved in these migration abnormalities, then not only is thinking affected, but the patient may have profound problems with swallowing, gagging, and even breathing. Such patients may well require a tracheostomy and gastric tube in infancy, as well as Chiari decompression in the first year or months of life. Such patients are at increased risk of death early on, and in such a case the true problem is not external compression on the brainstem, but a poorly developed brainstem (intrinsic damage). Chiari decompression cannot help fix a poorly developed brainstem.

Fortunately, most patients with Chiari II do not fall into such an extreme situation, though most patients with Chiari II will need surgical decompression eventually. The exact timing must be individualized as there are many confounding factors. The optimal timing is chosen to preserve neurologic function, while not putting the patient at undue risk. The situation is frequently complicated by the presence of a vp shunt and the possibility of a tethered cord should be suspected and looked for as well.

Symptoms of Chiari II include headache, failure to thrive, coughing, gagging, and feeding problems, changes in gait, decline in previously stable bladder function, loss of or decline in upper extremity function (decreasing finger dexterity—loss of ability to button shirts, manipulate coins, hold a fork, etc).

If there is a history of hydrocephalus and a vp shunt is present, the shunt should be evaluated and revised if necessary. If there is hydrocephalus without a vp shunt, a shunt should be placed prior to any Chiari decompression. If a symptomatic tethered cord is discovered, it should usually be detethered prior to Chiari decompression but only after vp shunt revision. Doing either a tethered cord release (detethering) or a Chiari decompression in the setting of untreated or ineffectively treated hydrocephalus is a recipe for disaster.

Most patients with Chiari II, but not all, will need surgical intervention at some point.

Next time: Chiari Decompression Surgery

 

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