Arnold-Chiari Classification and external resources
A T1-weighted sagittal MRI scan, from a patient with an Arnold-Chiari malformation, demonstrating tonsillar herniation of 7mm
ICD-10 Q07.0 ICD-9 741.0 OMIM 207950 DiseasesDB 899 MeSH D001139
Arnold–Chiari malformation, or often simply Chiari malformation, is a malformation of the brain. It consists of a downward displacement of the cerebellar tonsils through the foramen magnum (the opening at the base of the skull), sometimes causing non-communicating  hydrocephalus as a result of obstruction of cerebrospinal fluid (CSF) outflow. The cerebrospinal fluid outflow is caused by phase difference in outflow and influx of blood in the vasculature of the brain. It can cause headaches, fatigue, muscle weakness in the head and face, difficulty swallowing, dizziness, nausea, impaired coordination, and, in severe cases, paralysis.
The Austrian pathologist Hans Chiari in the late 19th century described seemingly related anomalies of the hindbrain, the so called Chiari malformations I, II and III. Later, other investigators added a fourth (Chiari IV) malformation. The scale of severity is rated I - IV, with IV being the most severe. Types III and IV are very rare.
Type Presentation Other notes I A congenital malformation. Is generally asymptomatic during childhood, but often manifests with headaches and cerebellar symptoms. Herniation of cerebellar tonsils. The most common form. Syndrome of occipitoatlantoaxial hypermobility An acquired Chiari I Malformation in patients with hereditary disorders of connective tissue. Patients who exhibit extreme joint hypermobility and connective tissue weakness as a result of Ehlers-Danlos syndrome or Marfan Syndrome are susceptible to instabilities of the craniocervical junction and thus acquiring a Chiari Malformation. This type is difficult to diagnose and treat. II Usually accompanied by a lumbar myelomeningocele leading to partial or complete paralysis below the spinal defect. As opposed to the less pronounced tonsillar herniation seen with Chiari I, there is a larger cerebellar vermian displacement. Low lying torcular herophili, tectal beaking, and hydrocephalus with consequent clival hypoplasia are classic anatomic associations. The position of the torcular herophili is important for distinction from Dandy-Walker syndrome in which it is classically upturned. This is important because the hypoplastic cerebellum of Dandy-Walker may be difficult to distinguish from a Chiari malformation that has herniated or is ectopic on imaging. Colpocephaly may be seen due to the associated neural tube defect. III Causes severe neurological defects. It is associated with an occipital encephalocele. IV Characterized by a lack of cerebellar development.
Other conditions sometimes associated with Chiari Malformation include hydrocephalus, syringomyelia, spinal curvature, tethered spinal cord syndrome, and connective tissue disorders such as Ehlers-Danlos syndrome and Marfan Syndrome.
Chiari malformation is the most frequently used term for these types of malformations. The use of the term Arnold–Chiari malformation has fallen somewhat out of favor over time, although it is used to refer to the type II malformation. Current sources use "Chiari malformation" to describe four specific types of the condition, reserving the term "Arnold-Chiari" for type II only. Some sources still use "Arnold-Chiari" for all four types. This article uses the latter convention.
Brain Sagging and Pseudo-Chiari Malformation. The displacement of the cerebellar tonsils into the spinal canal may be mistaken for a Chiari I malformation, and some patients with spontaneous intracranial hypotension have undergone decompressive posterior fossa surgery. 
- Headaches aggravated by Valsalva maneuvers, such as yawning, laughing, crying, coughing, sneezing or straining
- Tinnitus (ringing in the ears)
- Dizziness and vertigo
- Nystagmus (irregular eye movements)
- Facial pain
- Muscle weakness
- Impaired gag reflex
- Restless Leg Syndrome
- Sleep Apnea
- Dysphagia (difficulty swallowing)
- Impaired coordination
- intercranial pressure
- pupillary dilatation
- Dysautonomia: tachycardia (rapid heart), syncope (fainting), polydipsia (extreme thirst), chronic fatigue 
The blockage of Cerebro-Spinal Fluid (CSF) flow may also cause a syrinx to form, eventually leading to syringomyelia. Central cord symptoms such as hand weakness, dissociated sensory loss, and, in severe cases, paralysis may occur.
Diagnosis is made through a combination of patient history, neurological examination, and Magnetic Resonance Imaging (MRI). The radiographic criteria for diagnosing a congenital Chiari I Malformation is a downward herniation of the cerebellar tonsils greater than 5 mm below the foramen magnum. Other imaging techniques involve the use of 3-D CT imaging of the brain and cine imaging (a movie of the brain) can be used to determine if the brainstem is being compressed by the pulsating arteries that surround it.
In the Syndrome of Occipitoatlantoaxial Hypermobility, cerebellar tonsillar herniation is typically only evident on an up-right MRI, due to the fact that the Chiari Malformation is gravitationally acquired by means of connective tissue weakness. 3-D CT imaging may aid in the diagnosis of related disorders such as retroflexed odontoid. Invasive cranial traction (lifting of the head off the spine) is often used as a confirmation of the diagnosis.
Once symptomatic onset occurs, a common treatment is decompression surgery, in which a neurosurgeon usually removes the lamina of the first and sometimes the second or even third cervical vertebrae and part of the occipital bone of the skull to relieve pressure. The flow of spinal fluid may be accompanied by a shunt. Since this surgery usually involves the opening of the dura mater and the expansion of the space beneath, a dural graft is usually applied to cover the expanded posterior fossa.
A small number of neurological surgeons believe that detethering the spinal cord as an alternate approach relieves the compression of the brain against the skull opening (foramen magnum), obviating the need for decompression surgery and associated trauma. However, this approach is significantly less documented in the medical literature, with reports on only a handful of patients. It should be noted that the alternative spinal surgery is also not without risk.
On April 24, 2009, a young patient with Type 1 Chiari malformation was successfully treated with a minimally invasive endoscopic transnasal procedure by Dr. Richard Anderson at the Columbia University Medical Center Department of Neurosurgery.
The prognosis differs dependent on the type of malformation (i.e., type I, II, III, or IV). Type I is generally adult-onset and, while not curable, is treatable and rarely fatal. Syndrome of Occipitoatlantoaxial Hypermobility (Ehlers-Danlos syndrome related) is more difficult to treat than the congenital form of the disease. Individuals with this type do not respond well to the decompression surgery and often require an occipitoatlantoaxial fusion for stability. These patients are at risk of experiencing serious heart complications. Types I and II sufferers may also develop syringomyelia. Type II is typically diagnosed at birth or prenatally. Approximately 33% of individuals with Chiari II malformation develop symptoms of brainstem damage within five years; a 1996 study found a mortality rate of 33% or more among symptomatic patients, with death frequently occurring due to respiratory failure. 15% of individuals with Chiari II malformation die within two years of birth. Among children under two who also have myelomeningocele, it is the leading cause of death. Prognosis among children with Chiari II malformation who do not have spina bifida is linked to specific symptoms; the condition may be fatal among symptomatic children when it leads to neurological deterioration, but surgical intervention has shown promise. Types III and IV are extremely rare and patients generally do not survive past the age of two or three.
The prevalence of Chiari I malformation, defined as tonsilar herniations of 3 to 5 mm or greater, is estimated to be in the range of one per 1000 to one per 5000 individuals. The incidence of symptomatic Chiari is less but unknown.
An Austrian pathologist, Hans Chiari, first described these hindbrain malformations in the 1890s. A colleague of Professor Chiari, Dr. Julius Arnold, later contributed to the definition of the condition, and students of Dr. Arnold (Schwalbe and Gredig) suggested the term "Arnold-Chiari malformation" to henceforth refer to the condition.
Society and culture
The condition was brought to the mainstream on the series CSI in the tenth season episode "Internal Combustion" on February 4, 2010. Chiari was briefly mentioned in House, MD on the fifth season episode "House Divided"  and was the focus of the sixth season episode "The Choice." 
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- Duke Chiari Research
- Chari malformation patient information from The Mount Sinai Medical Center
- Chiari Connection International—Chiari and Related Disorders Information
- Chiari & Syringomyelia Foundation
- Conquer Chiari
- World Arnold Chiari Malformation Association
- American Syringomyelia Alliance Project
- The Ann Conroy Trust
- Chiari One
Congenital malformations and deformations of nervous system (Q00–Q07, 740–742) BrainOther Spinal cordOther
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