Relevant Embryology

Syringomyelia is commonly associated with the spinal abnormality integral to the complex of basilar invagination and Chiari malformation. From our previous study, we concluded that Chiari malformation and related pathological events could be primarily attributable to the maldevelopment of the occipital bone and overcrowding of the normally developed cerebellum within a smaller posterior cranial fossa.3 Neural development is initiated and completed early in embryonic life. Almost all named arteries and cranial nerves and much of the substance of the cerebral and cerebellar hemisphere are formed before any spicule of bone develops. Bone formation is almost an afterthought. Musculoskeletal development of the craniovertebral junction occurs in the latter third of fetal life when the cervical flexure of the fetus is reversed. This development precedes the delivery of the child, and the basic formation of the craniocervical junction is completed when the infant starts holding his or her head at 3 months. The cerebellar mass continues to grow in bulk and texture at least until 1 year of age.

Pathophysiology

Basilar invagination and associated basal maldevelopment, with resultant reduction in the volume of the posterior cranial fossa, are the primary events in syringomyelia pathology. There is usually no demonstrable structural abnormality of the brainstem, cerebellar hemisphere, or fourth ventricle, suggesting that the neural development in these patients remained unaffected in the embryonic dys-genesis. The presence of the normal bulk of the cerebellum in the reduced-volume posterior cranial fossa results in the herniation of part of the cerebellum into the foramen magnum, an entity labeled Chiari I malformation. Essentially, it appears that syringomyelia is a tertiary event to primary basilar invagination and secondary Chiari malformation. Long-standing pulsatile pressure of the herniated tonsil into the foramen magnum can result in compression of We observed that the craniovertebral maldevelopment results in reduced length of the clivus (i.e., the sphenoid part of the clivus is formed relatively normally, whereas the occipital part is formed incompletely) and platybasia. Occipital condylar hypoplasia, nonformation or inadequate formation of the occipitoaxial joint, occipitalization of the atlas, fusion of the atlantoaxial joint and C2–C3 spinal elements, and a range of Klippel-Feil spinal anomalies are also frequently associated. The entire complex of the odontoid process, atlas, and clivus is located rostrally so that the volume of the posterior cranial fossa is effectively reduced. Basal mesodermal maldevelopment results in rostral positioning of the plane of the foramen magnum and eventually in group II basilar invagination.3 the cervicomedullary cord. Apart from physical attempts to increase the volume of the foramen magnum by bone erosion and membranous thinning and expansion, the body probably provides a cushioning counterbalance of fluid from the spinal end in the form of syringomyelia so that the pressure of the tonsils on the cervicomedullary cord is minimized. The signs and symptoms related to the syrinx are more predominant than those related to brainstem compression by tonsillar herniation. The primary aim of the entire process of syrinx formation is to reduce the compression of the brainstem at the level of the foramen magnum by the herniating cerebellum.

We analyzed cases from our departmental archives (Seth G.S. Medical College and K.E.M. Hospital, Parel, Mumba, India) and observed that there was a group of such cases that were treated by syringosubarachnoid shunt surgery as the primary modality of therapy.1 Although symptoms related to spinal cord dysfunction improved after surgery, symptoms related to brainstem and upper cervical cord compression worsened. In a way, a syrinx protects the vital neural structures at the level of the craniovertebral junction at the cost of the spinal cord.

The mechanism of formation of a syrinx within the cord could be similar to the formation of hydrocephalus within the brain. In our earlier studies, we suggested that tumor obstructive hydrocephalus could be a natural protective phenomenon of the body.4–7 The hydrocephalus and an increase in supratumoral intracranial pressure are attempts to reduce the pressure of the tumor over vital brain structures, such as the brainstem and hypothalamus. Similarly, it appears that syringomyelia could be a phenomenon acting as a counterbalance to the relentless pressure exerted by a Chiari malformation.