Diagnosis is made through a combination of patient history, neurological examination, and medical imaging. Magnetic resonance imaging (MRI) is considered the preferred imaging modality for<unk> . The MRI visualizes neural tissue such as the cerebellar tonsils and spinal cord as well as bone and other soft tissues. CT and CT myelography are other options and were used prior to the advent of MRI, unfortunately the resolution of CT based modalities do not characterize syringomyelia and other neural abnormalities as well. By convention, the cerebellar tonsil position is measured relative to the basion-opisthion line, using sagittal T1 MRI images or sagittal CT images. The selected cutoff distance for abnormal tonsil position is somewhat arbitrary, as not every person will be symptomatic at a certain amount of tonsil displacement, and the probability of symptoms and syrinx increases with greater displacement; however, greater than 5 mm is the most frequently cited cutoff number, though some consider 3–5 mm to be "borderline,"; pathological signs and syrinx may occur beyond that distance. One study showed little difference in cerebellar tonsil position between standard recumbent MRI and upright MRI for patients without a history of whiplash injury. Neuroradiological investigation is used to first rule out any intracranial condition that could be responsible for tonsillar herniation. Neuroradiological diagnostics evaluate the severity of crowding of the neural structures within the posterior cranial fossa and their pressure against the foramen magnum.<unk>  1.5 is a term used when both brainstem and tonsillar herniation through the foramen magnum are present. The diagnosis of a<unk>  II malformation can be made prenatally, through ultrasound. Classification In the late 19th century, Austrian pathologist Hans<unk>  described seemingly related anomalies of the hindbrain, the so-called<unk> s I, II and III. Later, other investigators added a fourth<unk>  IV) malformation. The scale of severity is rated I – IV, with IV being the most severe. Types III and IV are very rare. Since Dr.<unk> 's original descriptions<unk>  0, 1.5, 3.5, and 5 have been described in the medical literature. Types of<unk>  Type Presentation Clinical Features 0 Syringohydromyelia in the absence of cerebellar tonsillar herniation. Back pain, leg pain, hand numbness/weakness I Herniation of cerebellar tonsils . Tonsillar ectopia below the foramen magnum, with greater than 5 mm below as the most commonly cited cutoff value for abnormal position (although this is considered somewhat controversial). Syringomyelia of cervical or cervicothoracic spinal cord can be seen. Sometimes the medullary kink and brainstem elongation can be seen. Can be congenital, or acquired through trauma. When congenital, may be asymptomatic during childhood, but often manifests with headaches and cerebellar symptoms. Syndrome of occipitoatlantoaxial hypermobility is an acquired<unk>  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; thus they are at risk for acquiring a<unk> . Headache, neck pain, unsteady gait usually during childhood 1.5 In addition to tonsillar ectopia, patients with this entity also have caudal descent of the brainstem. A significant proportion of these patients require a second surgery as a result of persistent syringomyelia. Headache and neck pain, similar to<unk>  I II This is the only type also known as an "Arnold-Chiari" malformation. As opposed to the less-pronounced tonsillar herniation seen with<unk>  I, there is a larger cerebellar vermian displacement. Low-lying torcular herophili (confluence of sinuses), tectal beaking, and hydrocephalus with consequent clival hypoplasia are classic anatomic associations. Usually accompanied by a lumbar or lumbosacral myelomeningocele, with tonsillar herniation below the foramen magnum. Colpocephaly may be seen due to the associated neural tube defect. Paralysis below the spinal bifida defect III Associated with an occipital encephalocele containing a variety of abnormal neuroectodermal tissues as well as possible herniation of elements of cerebellum, brainstem, and occipital lobe. Syringomyelia, tethered cord, and hydrocephalus may also be seen. Abundant neurological deficits 3.5 In 1894, Giuseppe Muscatello described what is believed to be the only case of an occipitocervical encephalocele with a communication to the stomach. This is an interesting historic finding; however, has not been seen nor described since then. Not compatible with life. IV Characterized by a lack of cerebellar development, in which the cerebellum and brain stem lie within the posterior fossa with no relation to the foramen magnum. Equivalent to primary cerebellar agenesis. Not compatible with life V Characterized by absence of cerebellar development and occipital lobe ectopia into the foramen magnum. This rare entity has two described cases in the medical literature, both associated with myelomeningocele. This has sparked controversy, given the proposed mechanism of this syndrome. Experts in neurosurgery disagree if this is truly a separate entity or just a part of the spectrum of<unk>  2 malformations. Syringomyelia associated with<unk>  Other conditions sometimes causally associated with<unk>  include hydrocephalus, syringomyelia, spinal curvature, tethered spinal cord syndrome, and connective tissue disorders such as Ehlers-Danlos syndrome and Marfan syndrome.<unk>  is the most frequently used term for this set of conditions. 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<unk> " to describe its four specific types, reserving the term "Arnold-Chiari" for type II only. Some sources still use "Arnold-Chiari" for all four types.<unk>  or Arnold–Chiari malformation should not be confused with Budd-Chiari syndrome, a hepatic condition also named for Hans<unk> . In Pseudo-Chiari Malformation, leaking of CSF may cause displacement of the cerebellar tonsils and similar symptoms sufficient to be mistaken for a<unk>  I malformation.
Chiari malformation
Glial tumor
Central nervous system cyst
Dandy–Walker malformation
Arachnoid cyst
Subdural hygroma
Cavernous venous malformation
Meningism