【PURPOSE】Chiari type I malformation (CM-I) is a condition where cerebellar tonsil extends through the skull's base, foramen magnum. Approximately 50% of patients with CM-I is associated with syringomyelia. Among them, 30% may develop scoliosis. Foramen magnum decompression (FMD) is a surgical procedure that can reduce the pressure by tonsillar descent to spinal cord. The clinical significance of FMD, however, remains to be clarified in terms of the management of scoliosis. The purpose of the current study was to review the patients with scoliosis associated with CM-I and to discuss the real necessity of FMD.
【SUBJECTS AND METHODS】A total of 1025 adolescent patients with scoliosis were referred to our institute from local clinics from 2013 to 2023. Among them, MRI was taken in 68 patients (8 males, 60 females; age 8 - 14 years with an average of 12.4 years) to evaluate the intracanal abnormalities including CM-I and syringomyelia. CM-I was defined as the cerebellar tonsil extending 5 mm or more below the foramen magnum on MRI. The following findings were investigated; existence of syringomyelia and any neurological deficits.
【RESULTS】CM-I was identified in 5 cases (7.4%). In 2 patients, there were no syringomyelia and no neurological deficits, followed by the observation only. No progress of scoliosis was observed. In 2 of other 3 patients, FMD was performed. The clinical course of the other three patients is described as the following.
Case 1: Nine-year-old girl with left convex thoracic curve of 25 degrees. FMD was performed because of syringomyelia, curve progression, and sensory disturbance in the left upper and lower limbs. After 7 years postoperatively, syringomyelia diminished in size and no progress of scoliosis occurred.
Case 2: Eleven-year-old girl with right convex thoracic curve of 45 degrees without any neurological deficits. FMD was performed to address the presence of syringomyelia and to improve scoliosis. Although the tonsillar descent improved, scoliosis progressed to 50° three years after FMD. PSF (posterior spinal fusion) was performed from T2 to L3. Two-year follow-up after PSF showed no complications.
Case 3: Thirteen-year-old girl with right thoracic curve of 56 degrees without any neurological deficits and syringomyelia. Without preceding FMD, PSF was performed under intraoperative neuro-monitoring (IONM). The scoliosis improved to 15 degrees and no neurological complications occurred.
【DISCUSSION】The indications for FMD in scoliosis associated with CM-I are 1) prevention of scoliosis progression and 2) decrease the risk of spinal cord injury from PSF. Thus, the prophylactic FMD has been recommended before PSF. There are, however, articles suggesting that FMD preceding PSF is not always necessary if performed carefully under IOMN. We have also encountered the case with favorable outcome following PSF without prior FMD. Although further research is needed to clarify the real indication for FMD, we believe that FMD preceding PSF is not always necessary in the patients with CM-I.