POSTERIOR-FOSSA RECONSTRUCTION - A SURGICAL TECHNIQUE FOR THE TREATMENT OF CHIARI-I MALFORMATION AND CHIARI-I-SYRINGOMYELIA COMPLEX - PRELIMINARY-RESULTS AND MAGNETIC-RESONANCE-IMAGING QUANTITATIVE ASSESSMENT OF HINDBRAIN MIGRATION

EXPERIMENTAL MODELS HAVE shown that Chiari I malformation is a primary paraaxial mesodermal insufficiency occurring after the closure of the neural folds takes place. According to these hypotheses, a small post erior fossa caused by an underdeveloped occipital bone would be the pr imary factor in the formation of the hindbrain hernia. The main object ive in the surgical treatment of Chiari I malformation and related syr ingomyelia is directed to restore normal cerebrospinal fluid dynamics at the craniovertebral junction. The most widely accepted surgical app roach is to perform a craniovertebral decompression of the posterior f ossa contents with or without a dural graft. It has been emphasized th at suboccipital craniectomy should be small enough to avoid downward m igration of the hindbrain into the craniectomy. This slump of the hind brain has been verified by studies using postoperative assessment by m agnetic resonance imaging. Our aim in this study is to present a modif ication of the conventional surgical technique, which we have called p osterior fossa reconstruction (PFR). Ten patients were operated on usi ng this technique and compared with a historical control group operate d on with the classic approach of making a small suboccipital craniect omy, opening the arachnoid, and closing the dura with a graft. To eval uate the morphological results in both groups objectively, preoperativ e and postoperative measurements of the relative positions of the fast igium and upper pens above a basal line in the midsagittal T1-weighted magnetic resonance images were obtained. In those cases with syringom yelia, syringe-to-cord ratios were calculated. The mean age of the PFR group was 35 +/- 16 years (mean +/- SD); in the control group it was 35.2 +/- 12 years. In the PFR group, the formation of an artificial ci sterna magna was observed in every case; it was observed in only one c ase in the control group. An upward migration of the cerebellum was se en in all cases in the PFR group, with a mean ascent of the fastigium of 6.2 mm. A significant downward migration of the cerebellum was obse rved in seven cases in the control group. No significant differences w ere found in both groups when comparing syringe-to-cord ratios. This l eads us to conclude that PFR is more effective than conventional surgi cal approaches in restoring the normal morphology of the craniovertebr al junction. This allows cranial ascent of the hindbrain verified by m agnetic resonance imaging and good short-term clinical results. Becaus e PFR is mainly an extraarachnoidal approach, complications related to surgery using this technique can be kept to a minimum.