Timothy C. Hain, MD
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The Chiari malformation, also known as the Arnold-Chairi malformation, is a relatively common syndrome involving displacement of the cerebellar tonsils below the level of the foramen magnum. Associated with the Chiari malformation may be hydrocephalus, spina bifida, and syringomyelia. In most instances, symptoms present in middle age.
The Chiari malformation is generally thought to be present from birth. However, in some instances, mild Chiari malformations may result from low CSF pressure. A closely related condition, basilar invagination may also be congenital or acquired (from arthritis).
|Figure 1: Saggital midline MRI scan of person with Chiari-I malformation showing displacement of cerebellar tonsil below the upper margin of the foramen magnum|
The definitive method of diagnosis is with a T1 MRI scan of the posterior fossa, which documents the typical downward herniation of the cerebellar tonsils. A displacement of greater than 5 mm below the foramen magnum is considered significant. Occasionally Chiari symptoms occur in persons with lessened displacement of the tonsils (Milhorat et al, 1999).
Symptoms suggestive of Chiari include posterior headaches, dizziness and ataxia (especially associated with straining), fainting with a cough, and weakness or numbness. A recent review of the otologic manifestations of Chiari in 16 patients indicated that 81% reported episodic aural fullness, 81% tinnitus, 69% vertigo, and 56% flutuating hearing. Headache was also common (about 80%). These symptoms overlap with those of Meniere's disease as well as to lesser extent, migraine (Sperling et al, 2001)
Signs of a significant Chiari malformation often include downbeat nystagmus, poor pursuit for age, and alternating skew deviation. Occasionally patients will have sensorineural hearing loss (Hendrix, 1992). Positional nystagmus is common. It is most commonly downbeating or lateral beating. Persons with Chiari may develop vertigo after spending some time with their head inclined on their trunk. In the Dix-Hallpike maneuver for BPPV, this may be recognized by seeing a delayed onset positional nystagmus.
A few individuals with the Chiari malformation develop progresive neurological symptoms.
Surgical treatment is suboccipital decompression. This treatment is best deferred until symptoms are significant.
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