By John R Adler Jr and Robert Dodd
Intramedullary spinal cord AVMs are rare lesions with an estimated incidence of only 0.02/100,000 person-years. Having a proclivity to affect young adults, this rare condition is associated with progressive spinal cord ischaemia and a significant risk of haemorrhage, often resulting in paralysis. Despite advances in microsurgery and intravascular embolization, many AVMs are not amenable to curative therapies because of their intra-parenchymal spinal cord location and/or a common anterior spinal artery blood supply. Even in very experienced hands, microsurgery and intravascular embolization—both established therapies—are associated with a considerable risk of complications. As a result, there are no good treatment options for many, if not most, intramedullary spinal cord AVMs.
Given the poor prognosis for intramedullary spinal cord AVMs, which is made especially grim by the relative youth of the characteristically late adolescent/early adult presentation, the senior author [Adler] began in 1997 investigating image-guided radiosurgical ablation as a therapeutic tool among selected patients. To date, our Stanford team has published two retrospective reports analysing a now relatively long-term experience with radiosurgery for these challenging lesions.
The current Stanford series includes 31 patients (17 females: 14 males) with a median age of 34 years and intramedullary spinal cord AVMs (19 cervical, six thoracic, and six conus medullaris) that were treated with CyberKnife (Accuray) between 1997 and 2010. By the time of their referral to Stanford, more than two thirds of our patients had moderate-to-severe myelopathies. Meanwhile, a history of haemorrhage was present in three fourths of our cases and not infrequently multiple bleeds had occurred.
An additional 35% presented with a progressive ischaemic myelopathy. Notably, almost half of all the patients in our series had undergone some type of prior non-curative treatment at outside institutions, including microsurgery (19%) or embolization (36%).
The mean AVM volume identified on pre-radiosurgical imaging was 2.8cc (0.3-15cc), which was treated to a mean marginal dose of 20Gy in one to four radiosurgical sessions (median 2) using CyberKnife radiosurgery. Based on the generally favourable outcome that we witnessed over the course of this study, the biologically effective dose (BED) of radiation was escalated gradually. Clinical and magnetic resonance imaging follow-up were carried out annually, and spinal angiography with CT-angiography was repeated at three year intervals.
After a mean follow-up period of 66 months (range 17–170), the treated AVM was completely obliterated in eight cases (26%) and, except for one patient, the lesion was significantly decreased in size in all other patients who had more than three years of follow-up. Pre-radiosurgery neurologic symptoms improved in more than 50% of cases, but worsened in three (10%). Among the latter group, symptoms were relatively minor except for one instance (3%) of radiation-induced myelopathy that occurred within one year of radiosurgery. By far the most notable clinical outcome of all is that after more than 170 years of collective post SRS follow-up, no patient suffered a new haemorrhage.
Although incomplete, our understanding is radiosurgery results in total obliteration or significant shrinkage for intramedullary spinal cord AVM, a phenomenon that appears to be accompanied by a marked decrease in the risk of bleeding. Symptom improvement is also common. Given the significant therapeutic challenges inherent to spinal cord AVM patients, the rate of radiosurgical complications appears acceptable.
Because the history stretches over decades, longer follow-up with additional patients is clearly needed to better define optimal radiation doses, outcomes and shortcomings of using radiosurgery to treat intramedullary spinal cord AVMs. Nevertheless, based on a very favourable experience to date, and the limitations intrinsic to all alternative therapies, we believe that a radiosurgical approach needs to be strongly considered in any patient harbouring one of these challenging lesions. Meanwhile, with even better imaging of spinal cord AVM and understanding of spinal cord radiation tolerance, it is reasonable to expect that future clinical outcomes should improve.
John R Adler Jr is a professor of Neurosurgery in the Neurosurgery Department at Stanford University, Stanford, USA
Robert Dodd is an assistant professor in the Neurosurgery Department at Stanford University, Stanford, USA