Journal of Postgraduate Medicine Medknow Publications and Staff Society of Seth GS Medical College and KEM Hospital, Mumbai, India ISSN: 0022-3859 EISSN: 0972-2823 Vol. 47, Num. 2, 2001, pp. 87-88

Journal of Postgraduate Medicine, Vol. 47, Issue 2, 2001 pp.87-88

Editorial

Is Syringomyelia Pathology or A Natural Protective Phenomenon?

Atul Goel

Department of Neurosurgery, Seth G. S. Medical College and King Edward Memorial Hospital, Parel, Mumbai - 400 012, India.

Code Number: jp01027

Syringomyelia, or ‘hydrocephalus of the cord’ is commonly associated with spinal abnormality complex of basilar invagination and Chiari malformation. Syringomyelia is less commonly ‘secondary’ to a ‘primary’ mass lesion either in the posterior cranial fossa or in the spinal canal, an acute spinal deformity or other such pathology. Rarely primary cause of syringomyelia is not identifiable. Understanding the pathophysiology of syringomyelia is crucially important to design a suitable treatment strategy and prognosticate the ultimate outcome.

Currently the well-accepted premise is that Chiari malformation and related pathological events could be primarily attributable to maldevelopment of the occipital bone and overcrowding of the normally developed cerebellum within a smaller posterior cranial fossa. On the basis of the literature review and our own study,(1) we have observed that the maldevelopment results in a 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, non-formation or inadequate formation of the occipitoaxial joint, and, frequently, occipitalisation of the atlas. Fusion of the atlantoaxial joint, and C(2-3) spinal elements and a range of Klippel-Feil spinal abnormalities are also frequently associated. The entire complex of the odontoid process, the atlas, and clivus is rostrally located, and effectively the volume of the posterior cranial fossa is reduced. Rostral repositioning or platybasia and reduction in the length of the clivus is almost constantly associated with reduced posterior cranial fossa volume. Partial or complete assimilation of the atlas is an important and frequent component of mesodermal maldevelopment. Basal mesodermal maldeveloment will result in rostral positioning of the plane of the foramen magnum and significantly severe basilar invagination if the measurements are taken on the basis of parameters laid down by Chamberlain,(2) McGregor,(3) and Fischgold et al.(4) However, the tip of the odontoid process will remain below both Wackenheim’s clival line(5) and McRae’s line(6) of the foramen magnum in a large percentage of cases.

We have observed that platybasia was as important as invagination of the odontoid process in causing the anterior ‘concavity’ of the brainstem and in reducing the volume of the posterior cranial fossa. In our series, the anterior ‘concavity’ of the brainstem was usually in the form of a smooth curvature or a ‘hump’ and not a ‘kink’, and the anteroposterior dimension of the brainstem was only marginally reduced or unaffected in these cases, thereby indicating that there was no direct brainstem compression by the odontoid process. We observed that although the dimensions of the foramen magnum were large, and in the majority of cases larger than normally found, the volume of its contents and probably the longstanding ‘pulsatile’ compression of the structures at the foramen magnum resulted in the neurological symptoms. Basilar invagination and the associated basal maldevelopment and the resultant reduction in the posterior cranial fossa volume are the primary events in pathology. There is usually no demonstrable structural abnormality of the brainstem, cerebellar hemisphere or fourth ventricle suggesting that the neural development in these patients was unaffected in the embryonic dysgenesis. Chiari malformation or the tonsillar herniation appears to be the result of the presence of normal cerebellar mass in a smaller posterior fossa and thus a secondary event. Long standing pulsatile pressure of the herniating tonsil to the brain stem as a cause of formation of syringomyelia cavitation has been discussed. Essentially it appears that syringomyelia is a tertiary event to the primary basilar invagination and secondary Chiari malformation. In our series, the signs and symptoms related to the syrinx were more predominant than those related to basilar invagination, Chiari malformation or brainstem and upper spinal cord compression. We analysed cases from our departmental archives and observed that there was a group of such cases, which were treated by syringo-subarachnoid shunt surgery as the primary modality of therapy.(7) Although symptoms related to spinal cord dysfunction improved after surgery, symptoms related to brainstem and upper cervical cord compression worsened after surgery. Syrinx was in a way protecting the vital neural structures at the level of craniovertebral junction at the cost of spinal cord. Syringomyelia could thus be a natural protective phenomenon of the body helping the cord to counter balance the long-standing pressure of the tonsils. The mechanism of formation of syrinx within the cord could be similar to formation of hydrocephalus within the brain. In our earlier studies we had suggested that tumour obstructive hydrocephalus could be a natural protective phenomenon of the body.(8) On the similar line, it appears that syringomyelia could be a natural protective phenomenon acting as counter balance to check relentless pressure imposed by the Chiari malformation. Our observations suggest that the treatment strategy in such cases should be directed towards increasing the posterior cranial fossa volume by foramen magnum decompression, which is the primary pathology. Opening of the posterior cranial fossa dura, resection of the cerebellar tonsils, arachnoidal sectioning and syringostomy can be avoided. It appears that if the primary problem is dealt with early in the course of the disease the secondary and the tertiary events will spontaneously regress.

There are occasions when there is syringomyelia and Chiari malformation but no definite basilar invagination. In such cases, it might be possible that the cerebellar mass is larger than normal. Even in such cases, volume expansion of the posterior cranial fossa is required. The theory that dura or dural bands can act as compressive factor appears unacceptable and does not correlate with the pathogenic events. Dura is an expansile structure and can never be a compressive factor.

Syringomyelia secondary to primary cord pathology like a tumour or kyphosis also appears to be a secondary and a ‘protective’ phenomenon. Treatment in such cases should be directed towards treatment of the pathology and not towards the treatment of syringomyelia. Our observation is that if the syringomyelia is treated without dealing with the primary pathology, more often than not the outcome will be poor.

Primary syringomyelia, where the cause is not identifiable, is a relatively complex clinical situation. The syringomyelia in such cases simulates ‘normal pressure hydrocephalus’. The intra-syrinx pressure is relatively low in such cases and the results of treatment are not gratifying. Syrinx drainage is the only available option in such cases.

References

1. Goel A, Bhatjiwale M, Desai K. Basilar invagination: A study based on 190 surgically treated patients. J Neurosurg 1998; 88:962-968. MEDLINE
2. Chamberlain WE. Basilar impression (platybasia): A bizarre developmental anomaly of the occipital bone and upper cervical spine with striking and misleading neurologic manifestation. Yale J Biol Med 1939; 11:487-496.
3. McGregor M. The significance of certain measurements of the skull in the diagnosis of basilar impression. Br J Radiol 1948; 21:171-181.
4. Fischgold H, Lievre JA, Simon J. Indice radiographique de profil de l’impression basilaire. Rev Rhum Ed Fr 1959; 26:72.
5. Wackenheim A. Radiological diagnosis of congenital forms, intermittent forms and progressive forms of stenosis of the spinal canal at the level of the atlas [in French]. Acta Radiol Diagn (Stockh) 1969; 9:481-486.
6. McRae DL. Bony abnormalities in the region of the foramen magnum: Correlation of anatomic and neurologic findings. Acta Radiol 1953; 40:335-354.
7. Goel A, Desai K. Surgery for syringomyelia: An analysis based on 163 surgical cases. Acta Neurochir (Wien) 2000; 142:293-302. MEDLINE
8. Goel A, Nitta J, Kobayashi S. Tumor-obstructive hydrocephalus: a natural defense mechanism. In: Kobayashi S, Goel A, Hongo K, editors. Neurosurgery of complex tumors and vascular lesions. New York; Churchill Livingstone: 1997. pp 373-391.

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