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What are the different kinds of Chiari malformations?
CM0
Chiari malformation type 0 (CM0) is a rare type of Chiari malformation that occurs when there is little to no cerebellar tonsil descent and crowding at the base of the skull.
In order to have CM0 you MUST have a syrinx.
Abnormalities associated with CM0:
- Syringomyelia (syrinx)
- Small posterior fossa
- Obex situated lower than normal
- Increased AP diameter of the foramen magnum
- Increased AP distance of the spinomedullary junction to the foramen magnum
- Increased angle between the floor of the fourth ventricle and the clivus
CM0 is likely secondary to a fourth ventricle obstruction such as arachnoid web or something else blocking the flow of the fourth ventricle.
CM1
Chiari Malformation Type 1 (CM1) is the most common type of Chairi. Which occurs when the cerebellar tonsil's herniate into the foramen magnum, the opening between the skull and spinal cord. This can happen when the skull is too small, or the brain grows faster than the skull. Symptoms can appear in any age group, but are often first noticed in adolescence or adulthood.
CM1 has no brain stem or supratentorial anomalies.(Supratentorial refers to the area of the brain above the tentorium cerebelli, a fold of dura mater that separates the cerebrum from the cerebellum.)
Abnormalities Associated with CM1:
- Tonsillar herniation
- Syringomyelia - Syrinx (Common)
- Hydrocephalus (Uncommon)
- Skull anomalies
- Skeletal anomalies
CM1.5
Chiari Malformation Type 1.5 (CM1.5) is a sort of middle ground between CM1 and CM2 with low lying tonsils plus a low brainstem and fourth ventricle obex.
Note: There is no associated neural tube defect.
Abnormalities Associated with CM1.5:
- Low Lying Tonsils
- Craniocervical osseous anomalies
- Syringomyelia
- Hydrocephalus
CM2
Chiari Malformation Type 2 (CM2) is classified by the herniation of the Cerebellar vermis, the brainstem and the fourth ventricle. CM2 almost always has multiple brain anomalies.
Abnormalities associated with CM2:
- Myelomeningocele (A severe form of spinabifida)
- Hydrocephalus
- Syringomyelia
- Tethered Cord
CM3
Chiari Malformation Type 3 (CM3) is a critical condition characterized by some of the cerebellum and the brain stem sticking out or herniating, through an opening in the back of the skull. (Foramen Magnum encephalocele containing herniated cerebellar and brainstem tissue.) Membranes surrounding the brain or spinal cord may also herniate. Symptoms of this very rare form of CM appear in infancy and can cause debilitating and life-threatening complications. Babies with CM3 can have additional severe neurological defects such as seizures and mental and physical delays.
Abnormalities associated with CM3:
- Foramen Magnum encephalocele
- Meningoencephalocele
- Partial absence of the corpus callosum
- Small posterior cranial fossa
- Syringomyelia
- Hydrocephalus
https://pmc.ncbi.nlm.nih.gov/articles/PMC4900115/
CM4
Chiari Malformation Type 4 (CM4) Is the most rare and severe form of Chiari malformation. It is often considered fatal. CM4 involves aplasia or hypoplasia of the cerebellum and tentorium cerebelli.
This can mean the cerebellum is in its normal position, parts of it are missing, and portions of the skull and/or spinal cord may be visible. CM4 Tends to have a normal foramen magnum.
Abnormalities associated with CM4:
- Cerebellar hypoplasia
- Aplasia of the tentorium cerebelli
- Spina bifida
- Hydrocephalus
Aplasia: The failure of an organ or tissue to develop or function normally.
Hypoplasia: The under development of an organ or tissue.
Some sources consider CM4 to be an obsolete diagnosis.