Arnold-Chiari refers to the physician who first described the condition. In the case of syringomyelia with Arnold-Chiari malformation, the lower part of the cerebellum protrudes from its normal location in the back of the head.
The word cerebellum means little brain. It is a separate, smaller part of the brain located at the base of the skull just above the cervical spine. It slides down into the cervical or neck portion of the spinal canal.
It sounds like you have developed syrinx in the cervical spine. Neck and shoulder pain with loss of cervical flexion are common with this condition. Surgery to remove the bone pressing on the spinal cord can help relieve the painful symptoms. The procedure is called foramen magnum decompression (FMD).
There isn't much information available about the effect of syringomyelia on cervical spine (neck) motion. Researchers in Japan explored the question of whether cervical motion is affected by the severity of the syringomyelia.
The study was small (30 patients). All had FMD for syringomyelia associated with Chiari malformation. Motion was measured before and after the operation. There was no significant difference pre- and postoperatively.
It does not appear that the loss of motion is linked with how severe the deformity is. More studies are needed to find out what restricts neck motion. With more information, treatment to improve motion may be possible.