March 31, 2008 -- For the first time, researchers at the Mayo Clinic
and the National Institutes of Health (Wetjen, Heiss, Oldfield) have used
serialized MRIs to create a time line for when syrinxes resolve after decompression
surgery. Specifically, the group prospectively studied the syrinxes of 29 Chiari
patients before and after surgery. They published their findings in the February,
2008 issue of the Journal of Neurosurgery: Pediatrics.
A syrinx, which is a collection of fluid in the spinal cord, is potentially the most damaging symptom of Chiari. As fluid collects in the cord, the syrinx expands and stretches and damages the spinal tissue. This can result in painful sensations (dysesthesia), loss of sensation, muscle weakness and atrophy, urinary and bowel problems, and in some cases even paralysis.
Although syrinxes can be caused by tumors, trauma, and infection, by far the leading cause is Chiari. There are several theories on how Chiari leads to syrinx formation, with one of the most prominent being the Piston Theory (which was developed by the same NIH researchers as this study). The Piston Theory holds that with Chiari, the herniated cerebellar tonsils act like a piston and drive CSF into the spinal cord through small spaces around arteries. In turn, decompression surgery removes the piston effect of the cerebellar tonsils and allows the fluid inside the syrinx to naturally drain out over a period of time.
While it has been known for quite some time that decompression surgery can result in syrinxes decreasing in size, and even collapsing completely, until now, no one has documented how long this takes and what effect, if any, it has on the final clinical outcome.
As stated earlier, in order to study this, the surgeons looked at 29 adult Chiari and syringomyelia patients. The patient group was comprised of 21 women and 8 men with an average age of 37. All had MRI confirmed Chiari and syringomyelia and had been suffering from symptoms anywhere from a few months to many years.
Before undergoing decompression surgery, each patient was evaluated clinically and with MRI. Clinically, their symptoms and neurological signs - headache, dysesthesia, extremity weakness, loss of sensation, ataxia, and atrophy - were categorized as absent, mild, moderate, or severe. The MRI was used to measure the maximum width of the syrinx and its length in vertebral segments. Each patient underwent decompression surgery which included duraplasty, but not intradural exploration. Patients were evaluated, both clinically and with MRI, one week, 3-6 months, one year, and then annually after surgery.
Clinically, the surgeries were very successful, with 96% of the patients improving within six months of surgery. By the one year mark, all but one patient had improved symptom wise. However, the majority of the patients also suffered from some residual symptoms. Specifically, at the three month mark, more than 3 out of 4 patients still had some symptoms (Figure 1). This dropped to 68% two years after surgery, which of course still means that more than half of the group still suffered from some symptoms. The most common residual symptoms were painful dysesthesia and objectively measured loss of sensation.
In looking at the MRIs, the surgeons found that all the syrinxes got smaller after surgery. To study this further however, they went on to define narrowing of the syrinx as a reduction of at least 50% of the maximum width. Using this criteria, 86% of the syrinxes had narrowed 3-6 months after surgery (Figure 2) and all of them, 100%, had narrowed by the 2 year mark. In fact the average maximum width decreased from 6.9 mm before surgery to less than 1.5 mm after surgery. This clearly shows that decompression surgery not only halted the progression of the syrinxes, but reduced their size to the point where they were no longer stretching and distending the cord. The median time (the authors chose to use median rather than mean because there were a couple syrinxes which took a long time to narrow) to get to this narrowed state was 3.6 months. This means that half the syrinxes took less time to narrow, and half took longer.
Interestingly, a significant number of syrinxes, 41%, never resolved completely. However, there was no relationship between whether a syrinx collapsed completely or not and the clinical, symptom based outcome. The surgeons believe that in these cases, the syrinx has caused so much tissue damage that an actual cavity, or hole, was formed which remains filled with fluid even after decompression surgery. However this does not mean the surgery was a failure, or that the remaining fluid is a problem.
Finally, the researchers were unable to find any factors that were related to how long it took for the syrinxes to narrow. They looked at age, sex, symptom duration, syrinx length, syrinx width, syrinx location, and amount of herniation, but none of these were statistically related to how long it took for a syrinx to shrink.
In the end, this study provided a mixed bag for Chiari patients. It is good to establish a timeline for how long it can take some syrinxes to reduce in size to the point they are no longer putting pressure on the spine; but it is also discouraging that so many patients continued to suffer from symptoms even after successful surgery. This shows that damage caused by a syrinx is often permanent, and highlights the need for rapid diagnosis and treatment.
-- Rick Labuda