May 16, 2017
Johann Conrad Brunner, an Anatomist and a Physician, has been recognized as the first scientist reporting Syringomyelia, in the year 1688, in western literature. A lesser known fact is that the first description of the disorder can actually be found 15 centuries prior to that in an ancient Chinese manual: the Yellow Emperor’s Inner Canon.
In modern Neurosurgery, Syringomyelia shunts are considered a procedure of last resort, to be considered whenever the cause of the syrinx cannot be identified (idiopathic cases), or the cause of the syrinx has been refractory to former surgical treatment.
Syringomyelia cavities are identified in up to 50% of patients affected by Chiari I Malformation. Tumors, vascular malformations, inflammatory disorders, infections, traumatic sequelae, vascular ictal events, and congenital anomalies are the main other causes of Syringomyelia.
The advancement of diagnostic techniques and the increased base of knowledge pertaining to Syringomyelia have contributed to the progressive decline in the quota of true idiopathic cases.
Regional Cerebro-Spinal Fluid (CSF) blockage and CSF pressure differentials have been recognized as key factors in the pathophysiology of Syringomyelia. The understanding of the CSF dynamics in each individual case of idiopathic Syringomyelia is the key in identifying the proper surgical treatment.
In general, the mere presence of a syrinx is not an automatic indication for surgery, unless there is evidence of progressive enlargement on serial MRI’s, or there is a large size on the very first MRI imaging, with the girth of the lesion being a more important factor than its length. Lesser Syringomyelia cavities can become surgically relevant when they cause severe symptoms or focal neurological deficits. In the latter scenario, a syrinx bleb would burst outside of the limits of the central canal, cutting through the spinal cord parenchyma, thus producing the focal deficit.
Whenever the surrounding subarachnoid spaces have stagnating CSF flow and elevated CSF pressure, any attempt to shunt CSF from the syrinx to such space is going to encounter limited or no success. In these cases, a syringopleural or syringoperitoneal shunt would be preferable, given the intermittent negative pressures in the end compartments.
Syrinx cavities located in the lower segments of the spinal cord are often first treated with a terminal ventriculostomy, before entertaining the spinal cord manipulations linked to syrinx shunting.
A 50% failure rate at the 5-year postoperative interval has been described as a late complication of Syringomyelia shunting.
In order to minimize the occurrence of iatrogenic deficits, the surgical access to the syrinx generally occurs through the midline or (less frequently) at the dorsal root entry zone. Large Syringomyelia cavities often deform the standard anatomical landmarks and the perception of where the anatomical midline should be. Other times, the presence of sizeable vessels on the posterior cord surface can force the surgeon to veer away from an ideal entry point, to avoid a vascular insult. In specific situations, the surgeon can intentionally decide on a parasagittal entry point, whenever a syrinx has expanded posteriorly and off the midline, already causing a deep sensory deficit in the posterior columns.
The access to the syrinx cavity can be facilitated by the use of intraoperative ultrasound. Neurophysiological intraoperative monitoring also provides important feedback during these delicate maneuvers.
Syrinx shunting can be performed to the peritoneal cavity, the pleural cavity, or the subarachnoid space. A rare variant of syrinx shunting has the distal catheter ending in the cisterna magna.
Syringosubarachnoid shunting techniques are favored by some surgeons to avoid the risk of CSF leakage at the dural exit point of the distal catheter. This problem can be easily solved by reinforcing the exit point with a small piece of muscle held in place by cross stitching.
Syringo-subarachnoid shunting can be performed by inserting a catheter, or by creating an ostomy through the cord tissue surrounding the syrinx, with the latter modality being complicated by a higher occlusion rate.
Myringotomy (aka tympanostomy) tubes have been used with success in syringostomy surgeries in the past, to maintain the ostomy open.
In conclusion, during the last decades, Syringomyelia shunting has become progressively less used by Neurosurgeons, due to its complication rate and long-term patency problems. In the few instances in which its use is contemplated, individual anatomical and pathophysiological details should be taken into consideration, to choose the most effective surgical strategy.
Paolo A. Bolognese, MD