As per your reference I will let you know information of Chiari 1.5 malformation disease, so that you can get idea.

The Chiari 1.5 malformation disease is defined as a tonsillar herniation with additional caudal descent of the brainstem through the foramen magnum, where there are four traditional types of Chiari malformations represent various clinical and anatomical processes.

Radiographic features
MRI
MRI if the best method for the diagnosis with sagittal T1 WI to assess tonsillar herniation:
Descent >6 mm favours Chiari I malformation and >12 mm suggests Chiari 1.5 malformation

Associated findings may include

Posterior angulation of the odontoid process

Hydrocephalus

Crowded small posterior fossa

Syringohydromyelia

Scoliosis

Treatment and prognosis
Posterior fossa decompression surgery for relieving any symptoms
Sometimes repeated surgeries are required

Pathology
The Chiari 1.5 malformation likely results from a Chiari I malformation combined with a smaller posterior that in turn and leads to overcrowding caudal displacement of the medulla.

Aetiology
Congenital: progression of Chiari 1 malformation acquired

Intracranial mass lesion

Lumbar puncture