More than a century has elapsed since the first case of ectopia of the cerebellar tonsils through the foramen magnum was described by Hans Chiari (1891) and became known as Chiari Malformation. In the following decades a considerable bulk of clinical research was published on similar conditions along with its variants, giving rise to an entire spectrum of Chiari Malformations with changing definitions over time.
Further research in the last decades has then focused on more pre-clinical data, such as the embryology of the posterior fossa and its genetics determinants, or the mathematical modelling of cerebrospinal fluid (CSF) dynamic across the foramen magnum as well as the best radiological assessment for such a condition. Even more recent works mainly from western countries have used large population-based data as well as pooled analysis on multiple epidemiological studies to look at sociodemographic determinants of Chiari malformations.
It is now clear that the simple ectopia of the cerebellar tonsils and its variants reflect different conditions with different explanations and mechanisms so that some authors have provocatively suggested that Chiari Malformation does not exist as such, and that when facing an ectopia of the cerebellar tonsils we should look at any possible explanation leading to such a condition, and only then look for the right treatment.
Following a “reductionist approach”, that is to say looking at small simple elements of a complex problem, current concepts about the ectopia of cerebellar tonsils can be reduced to three main explanations: posterior fossa disproportion (from mesodermal origin), CSF impairment between cranial and spinal compartments and craniovertebral junction instability.
This framework can account for the heterogenous clinical scenarios observed (from completely asymptomatic to multiple neurological and spinal complaints) as well as for various radiological findings encountered not only in the posterior fossa but also in the cranial compartment (e.g. ventricular abnormalities, dural venous changes, cranial vault malformations, etc.) and in the spinal compartment (e.g. craniovertebral junction abnormalities, thoracolumbar deformities, syringomyelia, tethered cord and dysraphisms, etc.). Finally, this heterogeneous model can account for different surgical options and why sometimes they fail and sometimes succeed.
If we do accept the challenge that Chiari Malformation as such does not exist, we can state that such a framework can constitute a unified practical theory to explain the ectopia of cerebellar tonsils: the choice of the nomenclature then becomes a pure semantic matter.
