In 1998 a study was done on 5 patients that had Syringomyelia (a condition in which a CSF filled cyst forms inside the spinal cord, compromising the spinal cord itself), but no hindbrain herniation (Chiari malformation). The presence of the syrinx was indicative of something obstructing the flow of CSF (and 4 out of the 5 were symptomatic of the syrinx), but because of the lack of tonsillar ectopia, they were not considered Chiari malformations.

• What they discovered was that the blockage of CSF alone causes problems, even serious ones like Syringomyelia.
• This finding alone demanded a reevaluation of all they knew about Chiari.
  http://thejns.org/doi/abs/10.3171/jns.1998.89.2.0212?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub%3Dpubmed

In 2011, a Turkish study again showed the significance of a blockage of CSF alone, when a 38-year-old man with Syringomyelia was shunted and despite the successful draining of the syrinx, his condition returned 9 months later because of the obstruction of fluid (the cause of the syrinx) was allowed to remain. https://www.ncbi.nlm.nih.gov/pubmed/21534216