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Pathogenesis of Chiari malformation: a morphometric study of the posterior cranial fossa
M Nishikawa1, H Sakamoto, A Hakuba
1Department of Neurosurgery, Osaka City University Medical School, Japan.
Insights
Adult-type Chiari malformation may stem from an underdeveloped occipital bone causing posterior cranial fossa overcrowding. This leads to hindbrain herniation, especially with basilar invagination.
Area of Science:
- Neuroscience
- Developmental Biology
- Anatomy
Background:
- Adult-type Chiari malformation is a condition where brain tissue extends into the spinal canal.
- The pathogenesis, particularly the role of posterior cranial fossa overcrowding, requires further investigation.
- Embryological development of the skull base and hindbrain is crucial for understanding Chiari malformation.
Purpose of the Study:
- To investigate posterior cranial fossa overcrowding as a cause of adult-type Chiari malformation.
- To analyze the morphology of the brainstem, cerebellum, and skull base in relation to embryological development.
- To compare Chiari malformation patients with a control group.
Main Methods:
- Prospective neuroimaging study of 30 Chiari malformation patients and 50 controls.
- Calculation of a "volume ratio" to estimate posterior cranial fossa overcrowding.
- Assessment of occipital bone morphology and tentorial steepness.
Main Results:
- Chiari patients exhibited a significantly larger volume ratio, smaller occipital bone parts (exocciput, supraocciput), and a steeper tentorium compared to controls.
- No significant differences were found in posterior fossa brain volume or hindbrain axial lengths.
- Patients with basilar invagination showed more severe occipital underdevelopment and a larger volume ratio.
Conclusions:
- Underdeveloped occipital bone likely induces posterior cranial fossa overcrowding in adult-type Chiari malformation.
- The normally developed hindbrain is compressed within an underdeveloped posterior cranial fossa.
- Basilar invagination exacerbates overcrowding and hindbrain herniation due to more severe occipital underdevelopment.
Abstract:
To investigate overcrowding in the posterior cranial fossa as the pathogenesis of adult-type Chiari malformation, the authors studied the morphology of the brainstem and cerebellum within the posterior cranial fossa (neural structures consisting of the midbrain, pons, cerebellum, and medulla oblongata) as well as the base of the skull while taking into consideration their embryological development. Thirty patients with Chiari malformation and 50 normal control subjects were prospectively studied using neuroimaging. To estimate overcrowding, the authors used a "volume ratio" in which volume of the posterior fossa brain (consisting of the midbrain, pons, cerebellum, and medulla oblongata within the posterior cranial fossa) was placed in a ratio with the volume of the posterior fossa cranium encircled by bony and tentorial structures. Compared to the control group, in the Chiari group there was a significantly larger volume ratio, the two occipital enchondral parts (the exocciput and supraocciput) were significantly smaller, and the tentorium was pronouncedly steeper. There was no significant difference in the posterior fossa brain volume or in the axial lengths of the hindbrain (the brainstem and cerebellum). In six patients with basilar invagination the medulla oblongata was herniated, all three occipital enchondral parts (the basiocciput, exocciput, and supraocciput) were significantly smaller than in the control group, and the volume ratio was significantly larger than that in the Chiari group without basilar invagination. These results suggest that in adult-type Chiari malformation an underdeveloped occipital bone, possibly due to underdevelopment of the occipital somite originating from the paraxial mesoderm, induces overcrowding in the posterior cranial fossa, which contains the normally developed hindbrain. Basilar invagination is associated with a more severe downward herniation of the hindbrain due to the more severely underdeveloped occipital enchondrium, which further exacerbates overcrowding of the posterior cranial fossa.