DEFINITION: heterogeneous group of disorders that are defined by anatomic anomalies of the cerebellum, brainstem, and craniocervical junction, with downward displacement of the cerebellum, either alone or together with the lower medulla, into the spinal canal
- normal cerebellar tonsils may lie up to 3 mm below the foramen magnum in adults.
- tonsils lying 5 mm or more below the foramen magnum on neuroimaging are considered to be consistent with a Chiari malformation
- With infants, however, tonsils as low as 6 mmbelow the foramen magnum can still be normal.
- there is no direct correlation between how low the tonsils are lying and clinical severity.
- Chiari malformations are associated with spinal cord cavitations (ie, syringomyelia).
Table. Classification of Chiari Malformation.
|Chiari 0||anatomic aberration of the brainstem (posterior pontine tilt, downward displacement of the medulla, low lying obex) but with normally placed cerebellar tonsils|
|CM-I||abnormally shaped cerebellar tonsils that are displaced below the level of the foramen magnum|
|Chiari 1.5||CM-II like malformation without spina bifida|
|CM-II||downward displacement of the cerebellar vermis and tonsils, a brainstem malformation with beaked midbrain on neuroimaging, and a spinal myelomeningocele|
|CM-III||small posterior fossa with a high cervical or occipital encephalocele, usually with displacement of cerebellar structures into the encephalocele, and often with inferior displacement of the brainstem into the spinal canal|
|CM-IV||cerebellar hypoplasia unrelated to the other Chiari malformations|
Bony Abnormalities seen in CM:
●Atlas assimilation – common in CMI
●Klippel-Feil anomaly (congenital anomaly consisting of failure of segmentation of any two of the seven cervical vertebrae)
●Platybasia deformity where lower occiput is pushed by upper cervical spine into cranial fossa
●Basilar invagination -(protrusion of the odontoid process through the foramen magnum into theintracranial cavity)
●Luckenschadel, also known as lacunar skull; ossification disorder in which the fetal skullappears fenestrated
Other findings in CM-II
●Inferior displacement of the fourth ventricle into the upper cervical canal
●Elongation and thinning of the lower pons and the medulla
●Beaking of the quadrigeminal plate
●Kinking of medullary spinal cord junction in the cervical canal
●Stenosis or atresia of the cerebral aqueduct
●Upward displacement of the upper cerebellum into the middle fossa
●Colpocephaly (abnormal enlargement) of the posterior lateral ventricles
Theories on Pathogenesis:
- MOLECULAR GENETIC THEORY – genes that program hindbrain segmentation and bone growth is defective
- CROWDING THEORY – growth of posterior fossa is restricted causing compression of brain and squeezes contents through foramen magnum
- HYDRODYNAMIC PULSION THEORY – early hydrocephalus (as fetus) pushes cerebellum and brainstem down
- OLIGO-CSF THEORYneural tube does not close, causing CSF leak; insufficient CSF to distend ventricles which leads to disorganization
- TRACTION BY TETHERED CORD – tethered cord pulls on cerebellar tissue
Pathogenesis of Spinal Cord Cavitations (Syringomyelia)
- CSF forced into central canal due to impaired subarachnoid circulations
- craniospinal pressure dissociation due to blocked CSF flow which leads to pressure backup into venous system, engorgement of Virchow-Robin spaces. Excess fluid leads to spinal cord edema. Fluid accumulation beyond resorptive power of parenchyma and dissipates into central canal and dilates it leading to syrinx formation
Presyrinx – potentially reversible; spinal cord edema due to obstruction of CSF flow, often in cervical region; appears similar to true syrinx on T2 but lacks discrete cavitation on T1
A diagrammatic representation of CSF flow under normal circumstances.
A: Sagittal view of the craniocervical junction and upper cervical spinal cord in an anatomically normal patient shows no obstruction to CSF flow at the foramen magnum. A segment of spinal cord parenchyma (box) is shown in more detail in part B.
B: Magnified view of the box in part A shows CSF flow dynamics in a normal patient with a variably stenotic central canal (CC), as indicated by the horizontal lines. CSF pressure (vertical arrow) is normal. CSF flows from the subarachnoid space (SAS) between the arachnoid (A) and pia (P) to the subpial space, and then enters the perivascular space (PVS). CSF circulates through the cord parenchyma toward the central canal, but may also flow in reverse as these forces are relatively balanced under normal circumstances (double-headed arrows).
Diagrammatic representation of syringomyelia and the “presyrinx” hypothesis in the setting of obstruction to CSF flow.
A: Sagittal view of the craniocervical junction in a patient with a Chiari I malformation shows abnormal descent of the cerebellar tonsil below the level of the foramen magnum(arrow). A segment of spinal cord parenchyma (box) is magnified in parts B to D, which represent views of CSF dynamics at the level of the spinal cord parenchyma in the presence of alterations in normal CSF flow and variable patency of the central canal.
B: Focal noncommunicating syrinx. In the setting of a Chiari I malformation and a variably stenotic central canal (which is a normal variant in many adults), as the tonsils descend rapidly during systole, CSF is driven into the spinal cord parenchyma by increased CSF pressure (thick vertical arrow). Net CSF flow occurs toward the central canal, resulting in focal syringomyelia which is limited in its craniocaudal extent by intervening stenosis of the central canal. CC = central canal, A = arachnoid, P = pia, SAS = subarachnoid space, PVS = perivascular space.
C: Extensive noncommunicating syrinx. This situation is similar to B, but the central canal is more extensively patent. In this situation, a long-segment dilation of the central canal (curved arrows) occurs as CSF is driven into the central canal via the perivascular spaces by the accentuated CSF pulse pressure (thick vertical arrow) that results from downward motion of the low-lying cerebellar tonsils in systole.
D: “Presyrinx”. In the setting of altered CSF flow, as with a Chiari I malformation, fluid in the subarachnoid space is subjected to increased pressure (thick vertical arrow). Net CSF flow is into the spinal cord parenchyma; however, because the central canal is not patent (as indicated by thehorizontal lines), fluid cannot accumulate within the central canal (curved arrows) and therefore diffuses through the cord parenchyma (stippled area),resulting in cord enlargement.
- MRI is best modality
- CT hig res if MRI cannot be performed
- fetal US?
- Cine phase contrast MRI – studies the flow across foramen magnum (determine need for surgery)
- polysomnography – if with sleep apnea, to determine if central or peripheral
Important anatomic / radiologic landmarks:
- posterior foramen magnum decompression with or without dural opening
- anterior foramen magnum decompression – transoral odontoidectomy
- shunting procedures
Medscape,. ‘The ‘Presyrinx’ State’. N.p., 2015. Web. 24 Sept. 2015.
Uptodate.com,. ‘Chiari Malformations’. N.p., 2015. Web. 24 Sept. 2015.