Chiari Malformations, Clinical Presentation and MRI.

Chiari malformations (Arnold-Chiari malformations) described by a German pathologist referring to congenital anomalies of the cerebellum and brainstem. It has 4 different subtypes and was described based on increasing severity from I to IV which was believed of the same pathogenesis; pressure exerted from above on hindbrain by congenital hydrocephalus.

Type 1 Chiari malformation [ref1-ref7]
The main anatomical feature is the caudal displacement of the cerebellar tonsils into the cervical canal, below the foramen magnum. Such displacement has variable extend. As a result, this leads to occlusion of the subarachnoid
spaces at the level of the foramen magnum which subsequently intervene with the cerebrospinal fluid (CSF) circulation in both cranial and spinal compartment. Chiari Malformation type I may lead to the development of syringomyelia or spinal cord cavitation, thus further neurologic deficits.

Type 1 Chiari can be asymptomatic, but mostly presented with a variety of symptoms of neurological origin. The cerebellar tonsil rarely descend beyong 3mm below the foramen magnum, and patient with clinical presentation usually has cerebellar tonsil descend more than 5mm. This value has been used widely as cut off point for diagnosis in neuroimaging.

Presentation of Type 1 Chiari malformation is as below:
1. The most common complaint of Chiari malformation is occipital or cervical pain, and is often exacerbated by sneezing,
coughing, or the Valsalva maneuver.
2. Other painful manifestations including the shoulder, back or limb pain, without radicular distribution.
3. Most common signs are motor or sensory deficits in the extremities (>70%), which are an expression of the associated spinal cavitation.
4. Truncal and appendicular ataxia, & cerebellar dysfunction, is the second most common sign (in 30–40%).
5. Clumsiness, nystagmus, diplopia, dysphagia, and dysarthria, due to lower cranial nerve deficits. (in 15–25%),
6. Apneic spells are seen in up to 10% of cases, presenting in infants and young children.
7. Occasionally, presented in children and adolescents is progressive scoliosis (in 30%).

Neuroimaging of Type 1 Chiari malformation.

Chiari malformation is best viewed by MRI. The expected finding in Type 1 Chiari malformation is as followed:
1. Herniation of one or both cerebellar tonsils below the foramen magnum (>5 mm)
2. Cervico-medullary kinking may be present
3. Absence of supratentorial anomalies (apart from sporadic cases of mild ventricular dilation)
4. Fourth ventricle in the usual location (this is important, because if 4th ventricle is herniated it is type2)
5. Syringomyelia may be present (50–60%)
6. Some skeletal abnormalities may be present; such as small posterior cranial fossa, platybasia.

pediatric chiari type 1
Image above shows a T1 weighted MRI of patient with Type 1 Chiari malformation: Cerebellar tonsils are displaced
through the foramen magnum (white bar) to the lower aspect of C2. A syrinx (asterisk) is visible extending from C3 to T2. Image obtained from [ref6]

Type 2 Chiari malformation [ref1][ref8]
Type 2 is somehow quite similar to type 1 Chiari. However, it’s relatively more severe and presence of myelomeningocele. Most references stated that myelomeningocele is presence virtually in type 2 chiari malformation but some textbook does not used it as a definition but rather an “almost exclusive” association. This type of chiari malformation does not only has infratentorial abnormalities but also consist of supratentorial presentation.

Infratentotial anatomical defects often detected by MRI is as below:
1. It consist of cerebellar herniation of tonsil and the vermis associated with 4th ventricles.
2. There is upward herniation of cerebellum above the tentorium.
3. Cerebellum is usually appearing small with the folia shallow or absent. Though the cerebellum is small, the posterior cranial fossa appears “crowded” dues to the abnormal relatively small size of the fossa.
4. Typically, the tentorium is hypoplastic with abnormally low position and may cause confluence of sinus insertion just above the foramen magnum.
5. One finding in Type 2 Chiari malformation is cervico-medullary kinking. This is secondary to the dorsal displacement of the relatively mobile medulla with the spinal cord being anatomically fixed by the dentate ligament.
6. Kinking or forking of the aqueduct.
7. Fusion of the colliculi forming a single mass appearing as “beaking” of the quadrigeminal plate.

Supratentorial anatomical defects includes:
1. Enlarged massa intermedia in majority of patients.
2. varying degrees of callosal agenesis, from partial to complete.
3. Hydrocephalus is present in up to 90% of cases; it is often asymmetrical, with prominent occipital horns and a small 3rd ventricle.
4. The cerebral gyri are abnormally numerous and small; known as polygyria.

Multiple skeletal abnormalities exist as shown below:
1. small posterior cranial fossa
2. scalloping of the petrous bone
3. shortening of the clivus
4. enlargement of the foramen magnum

type 2 chiari malformation 2

Image above shows the T1 weighted MRI of type 2 Chiari malformation of a child. Noticed the herniation of the vermis extending to C-4. There is also partial agenesis of corpus callosum (white arrow) and beaking of the tectum (yellow arrow). Image obtained and modified from [ref8]

type 2 chiari malformation medullary kink

Image above shows the cervico-medullary kinking (left), notice also the presence of syringomyelia (left and right). Image obtained from [ref1].

type 2 chiari malformation tentorium

Image above shows the MRI of child with type 2 Chiari malformation. The tentorium (arrows) inserts abnormally low, placing the confluence of sinus just inside the rim of the foramen magnum. Polygyria is also seen throughout the hemisphere. Image obtained from [ref8].

Clinical presentation of type 2 Chiari malformation.

The presentation depend on the age of onset. If present during early infancy, usually the key features is of brainstem compression. This include:
1) These include stridor secondary to vocal cord paralysis
2) Central obstructive apnea
3) Swallowing difficulty
4) Breath-holding spells
5) Hypotonia

In older children usually presented with spinal, cerebellar and ophthalmologic signs as below:
1) Occipital and cervical pain
2) Myelopathy with weakness of the upper extremities
3) ataxia
4) strabismus
5) nystagmus;
6) defects of smooth pursuit and optokinetic movements
7) defect of convergence
8) scoliosis

Type 3 Chiari malformation. [ref1][ref9][ref10]
This is extremely rare. It is a combination of abnormalities found in type 2 and high cervival/occipital meningoencephalocele containing herniated cerebellar tissue. Clinical presentation includes evidence of encephalocele. Hence, it should be detected early.

Neuroimaging of type 3 Chiari malformation
Type 3 Chiari malformation should have, in addition to the basic anatomical defect of type 2:
1) occipito-cervical meningoencephalocele (herniated cerebellar tissue usually hypoplastic) , protruding through a bony defect involving the lower occipital
squama and/or the posterior arch of the first cervical vertebrae.
2) scalloping of the clivus.

type 3 chiari axial CT

Image above shows an axial brain CT image showing herniation of cerebellum through in a midline occipital bone defect in a type 3 Chiari malformation patient. Image obtained from ref [9].

type 3 chiari sagital mri t2
Image above shows sagittal T2-weighted MR image showing encephalocele in the occipital region and fusion of C4-5 and C5-6 vertebrae. Note the syringomyelia in craniocervical junction. Image obtained from [ref9].

Type 4 Chiari malformation. [ref5][ref11][ref12]
This type of chiari malformation has no hindbrain herniation. It consist of cerebellar hypoplasia or aplasia, a small or relatively normal size posterior fossa with filled with large CSF filled posterior fossa. It can be misdiagnosed as Dandy-Walker syndrome. Dues to the fact that it does not consist of hindbrain herniation, it is not an entity of Chiari malformation, but rather a variation of cerebellar hypoplasia. Hence, this will not be discussed further here.

References
1. Christianto B. Lumenta et al, eds. Neurosurgery. European manual of medicine. First edition, 2010. Springer. Verlag Berlin Heidelberg.
2. Leslie A. Aitken et al. Chiari Type I Malformation in a Pediatric Population. Pediatr Neurol 2009;40:449-454.
3. Sabri Aydin et al. Chiari type I malformations in adults: a morphometric analysis of the posterior cranial fossa. Surgical Neurology 2005;64:237-241.
4. Robert B. Daroff et al, eds. Bradley’s Neurology in clinical practice. Sixth edition, 2012. Elsevier Saunders. Philadelphia, PA.
5. M. I. Levene and F. A. Chervenak. Fetal and Neonatal Neurology and Neurosurgery. Fourth edition, 2009. Elsevier Churchill Livingstone. Philadelphia, PA.
6. Reza Yassari and David Frim. Evaluation and management of the Chiari malformation type 1 for the primary care pediatrician. Pediatr Clin N Am, 2004;51:477–490
7. Nahin Hussain and Sushil Beri. Chiari malformation type 1 in children, varied clinical manifestations. Journal of Pediatric Sciences, 2010;2(2):1-5.
8. Kevin L. Stevenson. Chiari Type II malformation: past, present, and future. Neurosurgical focus, 2004;16(2):1-7.
9. Fatih Serhat et al. The Association of Chiari Type III Malformation and Klippel-Feil Syndrome with Mirror Movement: A Case report. Turkish Neurosurgery, 2011;21(4):655-658.
10. M. Castillo et al. Chiari III malformation: imaging features. AJNR, 1992; 13:107-113.
11. Jarrah A. A. and Maximilian F. R., eds. Congenital Diseases and syndrome: an illustrated radiological guide. First edition, 2009. Springer. Verlag Berlin Heidelberg.
12. L. M. Ketonen et al, eds. Pediatric brain and spine: an atlas of MRI and spectroscopy. First edition, 2005. Springer. Verlag Berlin Heidelberg.

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