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Unraveling the riddle of syringomyelia

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Abstract

The pathophysiology of syringomyelia development is not fully understood. Current prevailing theories suggest that increased pulse pressure in the subarachnoid space forces cerebrospinal fluid (CSF) through the spinal cord into the syrinx. It is generally accepted that the syrinx consists of CSF. The here-proposed intramedullary pulse pressure theory instead suggests that syringomyelia is caused by increased pulse pressure in the spinal cord and that the syrinx consists of extracellular fluid. A new principle is introduced implying that the distending force in the production of syringomyelia is a relative increase in pulse pressure in the spinal cord compared to that in the nearby subarachnoid space. The formation of a syrinx then occurs by the accumulation of extracellular fluid in the distended cord. A previously unrecognized mechanism for syrinx formation, the Bernoulli theorem, is also described. The Bernoulli theorem or the Venturi effect states that the regional increase in fluid velocity in a narrowed flow channel decreases fluid pressure. In Chiari I malformations, the systolic CSF pulse pressure and downward motion of the cerebellar tonsils are significantly increased. This leads to increased spinal CSF velocities and, as a consequence of the Bernoulli theorem, decreased fluid pressure in narrow regions of the spinal CSF pathways. The resulting relatively low CSF pressure in the narrowed CSF pathway causes a suction effect on the spinal cord that distends the cord during each systole. Syringomyelia develops by the accumulation of extracellular fluid in the distended cord. In posttraumatic syringomyelia, the downwards directed systolic CSF pulse pressure is transmitted and reflected into the spinal cord below and above the traumatic subarachnoid blockage, respectively. The ensuing increase in intramedullary pulse pressure distends the spinal cord and causes syringomyelia on both sides of the blockage. The here-proposed concept has the potential to unravel the riddle of syringomyelia and affords explanations to previously unanswered clinical and theoretical problems with syringomyelia. It also explains why syringomyelia associated with Chiari I malformations may develop in any part of the spinal cord including the medullary conus. Syringomyelia thus preferentially develops where the systolic CSF flow causes a suction effect on the spinal cord, i.e., at or immediately caudal to physiological or pathological encroachments of the spinal subarachnoid space.

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Acknowledgements

This article was supported by The Foundation for Medical Imaging in Memory of Erik Lysholm (Stiftelsen för Medicinsk Bildering till Erik Lysholms Minne).

The author declares no potential conflicts of interest concerning this article.

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  1. Department of Neuroradiology and MR Research Center, Karolinska University Hospital, S-171 76, Stockholm, Sweden

    Dan Greitz

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Comments

Dieter Hellwig, Marburg, Germany

Since the 18th century different hypotheses about the development of syringomyelia have been proposed, discussed and modified. In 1700 Brunner first described syringomyelia as an dysraphic disorder, which occurs together with spina bifida and meningomyelocele. Ball and Dajan postulated, that due to an increase of pressure in the subarachnoid space CSF is pressed into the spine and forms the syrinx. The association of syringomyelia with malformations of the cranio-cervical junction such as Chiari malformation lead to the assumption that one important etiological factor is the blockage of CSF circulation.

The popular theory of Gardner is based on the hypothesis that a prenatal hydrocephalus with a continuous CSF flow from the fourth ventricle to the central canal creates cervical syringomyelia.

Various forms of syringomyelia are caused by different factors such as ischemic and vascular lesions, the effect of lysosomal enzymes, congenital predisposing influences, tethered cord, and bony stenosis or tumors. They can lead either to blockage of CSF flow, changes in the microcirculation of the spine or obstructions of the flow of extracellular fluid into the perivascular spaces with consecutive dilation. These etiological and differential diagnostic aspects also require logical therapeutic strategies, which make neurosurgical treatment of syringomyelia still a challenge.

Greitz offers an excellent contribution to understanding the fundamental pathophysiological factors in the complex genesis of syringomyelia. For future research to elucidate the etiology of syringomyelia, this outstanding paper is a milestone.

Reference

Hellwig D, Krause M, Rohlfs J, Tirakotai W, Aschoff A, Bertalanffy H (2003) Die nichttraumatische Syringomyelie. In: Grüninger W, Pott M (eds) Nichttraumatische Querschnittlähmungen. Steinkopff, Darmstadt, pp 71–85

Joachim K. Krauss, Hannover, Germany

Dr. Greitz is to be congratulated for this comprehensive overview on the pathophysiology of syringomyelia. He sheds some new light on the complex issue of the development of the formation and the progression of an intramedullary syrinx. The introduction of the Bernouilli theorem is an elegant concept allowing us to come closer to an understanding of some of the mechanisms involved in the priming events leading finally to syringomyelia. In contrast to previous studies, the intramedullary pulse pressure theory is applicable within the frame of different pathological conditions. This work is an important step forward in unraveling the riddle of syringomyelia.

Izumi Koyanagi, Kiyohiro Houkin, Sapporo, Japan

The pathomechanisms of syringomyelia have not been clarified, although a number of experimental and clinical studies have provided hypotheses explaining the formation of the syrinx. The previous studies focused on how the cerebrospinal fluid (CSF) entered from the cerebral ventricles or the subarachnoid space into the syrinx. However, radiological evidence did not support the entrance of CSF from the fourth ventricle via the central canal. Penetration from the spinal subarachnoid space via the perivascular spaces seemed plausible, but did not explain some clinical and experimental evidence. For example, the syringo-subarachnoid shunt was effective in the collapsing of the syringes in Chiari type I malformation and traumatic syringomyelia[1]. Higher pressure of the syrinx fluid compared to the subarachnoid space also has been previously reported. In this article, Dr. Greitz reviewed the recent evidence for a new theory for the pathomechanisms of syringomyelia. With this intramedullary pulse pressure theory, he explains syrinx formation in almost all types of syringomyelia. The pulsated CSF flow in the narrowed subarachnoid space will cause decreased pressure around the spinal cord by the Venturi effect. He explains that an accumulation of the extracellular fluid in the distended spinal cord by this effect is an important mechanism of syrinx formation. This theory is quite agreeable in that the syrinx fluid is derived from the extracellular fluid, not from the CSF in the subarachnoid space or intracranial ventricles. It is also attractive that there is some universal mechanism underlying all types of syringomyelia. The relationship between the extracellular fluid and CSF in the central nervous system may be more interactive and more dynamic than previously considered. However, it is of concern whether the Venturi effect can explain syrinx formation also in Chiari type I malformation, since there are patients with Chiari type I malformation showing a large syrinx at the lower cervical to thoracic levels where no spinal canal stenotic lesions are present. From our experience of syringomyelia with adhesive arachnoiditis[2], the syrinx often originated from the complete subarachnoid block levels. Disturbed absorption of the extracellular fluid from the intramedullary vessels might be an important mechanism of syrinx formation in such cases. Although there are still some unsolved riddles in this disorder, the theory of Dr. Greitz opens a new avenue to the universal principle of syrinx formation.

References

1. Iwasaki Y, Koyanagi I, Hida K, Abe H (1999) Syringo-subarachnoid shunt for syringomyelia using partial hemilaminectomy. Br J Neurosurg 13:41–45

2. Koyanagi I, Iwasaki Y, Hida K, Houkin K (2005) Clinical features and pathomechanisms of syringomyelia associated with spinal arachnoiditis. Surg Neurol 63:350–355

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Greitz, D. Unraveling the riddle of syringomyelia. Neurosurg Rev 29, 251–264 (2006). https://doi.org/10.1007/s10143-006-0029-5

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