A 15-year-old girl has had progressive difficulty speak- ing during the past 6 months. She becomes dizzy and falls frequently. She complains of headache and facial and neck pain. During the past month, she has had decreasing bladder and bowel control. On physical examination, there is loss of pain and temperature sensation over the nape of the neck, shoulders, and upper arms, but vibration and position sensation are preserved. She has muscle wasting in the lower neck and shoulders. MRI of the spinal cord shows cervical and thoracic enlargement with a CSF collection dilating the central canal. MRI of the brain shows gross findings similar to those shown in the figure. Which of the following is the most likely diagnosis?
A Arnold-Chiari II malformation
B Cerebral palsy
C Corpus callosal agenesis
D Holoprosencephaly
E Polymicrogyria
[Robbins and Cotran Review of Pathology, fourth edition,p434]

the answer is A. I could choose it from these alternatives by excluding other options but I failed to figure out the rationale for it. The symptoms of loss of sensation and muscle wasting seem to coordinate with syringomyelia (UptoDate: In late infancy and childhood, progressive hydrocephalus is a common problem in CM-II [1]. In addition, CM-II may be associated with one or more of the syndromes associated with CM-I, such as syringomyelia and scoliosis.), but why did the young girl have facial and neck pain? and the loss of sensation is limited to upper body?