A novel technique to treat acquired Chiari I malformation after supratentorial shunting
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The acquired Chiari I malformation with abnormal cranial vault thickening is a rare late complication of supratentorial shunting. It poses a difficult clinical problem, and there is debate about the optimal surgical strategy. Some authors advocate supratentorial skull enlarging procedures while others prefer a normal Chiari decompression consisting of a suboccipital craniectomy, with or without C1 laminectomy and dural patch grafting.
We illustrate three cases of symptomatic acquired Chiari I malformation due to inward cranial vault thickening.
We describe a new surgical approach that appears to be effective in these patients. This approach includes the standard Chiari decompression combined with posterior fossa augmentation by thinning the occipital planum.
Internal volume re-expansion of the posterior fossa by thinning the occipital planum appears to be an effective novel surgical strategy in conjunction with the standard surgical therapy of Chiari decompression.
KeywordsAcquired Chiari I malformation Ventriculoperitoneal shunt Cranial vault thickening
The Chiari I malformation, or primary cerebellar ectopia, is one of the mildest forms of hindbrain herniation and is characterized by a downward displacement of the cerebellar tonsils through the foramen magnum into the cervical canal. The acquired Chiari I malformation is in fact a iatrogenic form of hindbrain herniation, first described after lumboperitoneal shunting [2, 4, 8, 17, 21], but also after multiple lumbar punctures , after baclofen pump placement , and it was later recognized that an acquired Chiari I malformation can also occur after cystoperitoneal and ventriculoperitoneal shunting [1, 3, 4, 6, 7, 9, 10, 11, 12, 14, 20].
In a previous case series, 17 of 1700 patients treated with ventriculoperitoneal shunting developed a Chiari I malformation as a late complication, of which only five patients were symptomatic . It is therefore a rare complication of supratentorial shunting, but it poses a difficult problem. In many cases, the only symptom is headache, which is also a common problem in the general population. In these patients, it may point at overdrainage, slit ventricle syndrome, hydrocephalus, or acquired Chiari I malformation. There are some similarities between the described cases, but the causes of the Chiari I development seem to differ because there is a difference in timing of symptoms. Furthermore, there has been debate about the optimal surgical strategy to alleviate symptoms. We present three illustrative cases of an acquired Chiari I malformation due to cranial vault thickening as a perceived late complication of supratentorial shunting and introduce a novel surgical strategy.
There has been a continuing debate about the optimal surgical treatment of the acquired Chiari I malformation. Some authors prefer supratentorial skull enlarging procedures [1, 4, 11, 12], which can be considered a major surgical procedure with potentially more morbidity than our approach. Others advocate a suboccipital craniectomy, with or without C1 laminectomy and dural patch grafting [9, 13], which can be considered as a standard procedure for the “normal” Chiari I malformation. In our cases, the Chiari I malformation coincides with acquired thickening of the skull with a subsequent decrease in volume of the posterior fossa. Our novel surgical approach to the acquired Chiari I malformation with inward skull thickening and subsequent loss of posterior fossa volume includes a standard Chiari decompression (C0/C1 decompression and dural patch grafting) combined with internal posterior fossa volume augmentation by thinning the occipital planum. In order to improve conditions for proper shunting, we tried to augment the size of the ventricles by inserting gravitational shunts in combination with the local treatment of the Chiari and posterior fossa as described. The combined surgical procedure of Chiari decompression and internal volume expansion of the posterior fossa by thinning of the occipital planum appears to be effective in our three cases, without creating new problems of a symptomatic descending cerebellum.
Based on the literature, three mechanisms of an acquired Chiari I malformation can be differentiated due to a difference in timing of symptoms. For both supratentorial shunting and lumbar shunting, early-onset cases (roughly within a year of placement) without cranial vault thickening have been described [6, 10, 17, 18, 19, 20], while it is also seen as a late complication as was also the case in our patients [1, 3, 4, 12, 13].
An early-onset acquired Chiari I after lumboperitoneal shunting appeals to the pressure gradient theory . A craniospinal pressure gradient is generated which basically sucks the cerebellum into the cervical canal. The successful treatment of an acquired Chiari I malformation caused by lumboperitoneal shunting by converting it to a ventriculoperitoneal shunt supports this theory [4, 17]. An early-onset acquired Chiari I after supratentorial shunting must have a different mechanism because no downward pressure gradient is generated. This may be caused by overdrainage of the ventricles, which leads to sagging of the brain as a whole, possibly in combination with a small (but previously sufficiently sized) posterior fossa. Our three cases had a late-onset development of the Chiari I malformation, characterized by modifications of the skull induced by the shunt. Initially named cephalocranial disproportion , Osuagwu et al. called it posterior cranial fossa disproportion, because they showed that there are normal cerebellar and supratentorial volumes in shunted children but a smaller posterior cranial fossa . They propose that the etiology is an arrested posterior cranial fossa growth, similar to the etiology in the “normal” adult Chiari I malformation [15, 16]. However, they do not describe an abnormally thickened and inward growing cranial vault, as in our patients and those of many others . It is possibly the combination of an arrested skull growth with subsequent inward cranial vault thickening due to chronic overdrainage that gives rise to this kind of acquired Chiari I malformation . This seems evident in our cases, and our surgical approach appeals to this problem.
We illustrate three cases of symptomatic acquired Chiari I malformation with cranial vault thickening as a perceived late complication of supratentorial shunting. Internal volume re-expansion of the posterior fossa by thinning the occipital planum appears to be an effective novel surgical strategy in conjunction with the standard surgical therapy of Chiari decompression.
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Conflict of interest
The authors report no conflict of interest concerning the findings specified in this paper.
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